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<rfc ipr="pre5378Trust200902" docName="draft-ietf-tls-tls13-07" category="std" obsoletes="3268, 4346, 4366, 5246, 5077" updates="4492">

<?rfc rfcedstyle="yes"?>
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  <front>
    <title abbrev="TLS">The Transport Layer Security (TLS) Protocol Version 1.3</title>

    <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
      <organization>RTFM, Inc.</organization>
      <address>
        <email>ekr@rtfm.com</email>
      </address>
    </author>

    <date year="2015" month="July" day="08"/>

    <area>General</area>
    
    <keyword>Internet-Draft</keyword>

    <abstract>


<t>This document specifies Version 1.3 of the Transport Layer Security
(TLS) protocol.  The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.</t>



    </abstract>


  </front>

  <middle>


<section anchor="introduction" title="Introduction">

<t>DISCLAIMER: This is a WIP draft of TLS 1.3 and has not yet seen significant security analysis.</t>

<t>RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at
https://github.com/tlswg/tls13-spec. Instructions are on that page as
well. Editorial changes can be managed in GitHub, but any substantive
change should be discussed on the TLS mailing list.</t>

<t>The primary goal of the TLS protocol is to provide privacy and data integrity
between two communicating applications. The protocol is composed of two layers:
the TLS Record Protocol and the TLS Handshake Protocol. At the lowest level,
layered on top of some reliable transport protocol (e.g., TCP <xref target="RFC0793"/>), is
the TLS Record Protocol. The TLS Record Protocol provides connection security
that has two basic properties:</t>

<t><list style="symbols">
  <t>The connection is private.  Symmetric cryptography is used for
data encryption (e.g., AES <xref target="AES"/>, etc.).  The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol).  The Record
Protocol can also be used without encryption, i.e., in integrity-only
modes.</t>
  <t>The connection is reliable.  Messages include an authentication
tag which protects them against modification.</t>
  <t>The Record Protocol can operate in an insecure mode but is generally
only used in this mode while another protocol is using the Record
Protocol as a transport for negotiating security parameters.</t>
</list></t>

<t>The TLS Record Protocol is used for encapsulation of various higher- level
protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows
the server and client to authenticate each other and to negotiate an encryption
algorithm and cryptographic keys before the application protocol transmits or
receives its first byte of data. The TLS Handshake Protocol provides connection
security that has three basic properties:</t>

<t><list style="symbols">
  <t>The peer’s identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA <xref target="RSA"/>, DSA <xref target="DSS"/>, etc.).  This
authentication can be made optional, but is generally required for
at least one of the peers.</t>
  <t>The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.</t>
  <t>The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.</t>
</list></t>

<t>One advantage of TLS is that it is application protocol independent.
Higher-level protocols can layer on top of the TLS protocol transparently. The
TLS standard, however, does not specify how protocols add security with TLS;
the decisions on how to initiate TLS handshaking and how to interpret the
authentication certificates exchanged are left to the judgment of the designers
and implementors of protocols that run on top of TLS.</t>

<section anchor="conventions-and-terminology" title="Conventions and Terminology">

<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this
document are to be interpreted as described in RFC 2119 <xref target="RFC2119"/>.</t>

<t>The following terms are used:</t>

<t>client: The endpoint initiating the TLS connection.</t>

<t>connection: A transport-layer connection between two endpoints.</t>

<t>endpoint: Either the client or server of the connection.</t>

<t>handshake: An initial negotiation between client and server that establishes the parameters of their transactions.</t>

<t>peer: An endpoint. When discussing a particular endpoint, “peer” refers to the endpoint that is remote to the primary subject of discussion.</t>

<t>receiver: An endpoint that is receiving records.</t>

<t>sender: An endpoint that is transmitting records.</t>

<t>session: An association between a client and a server resulting from a handshake.</t>

<t>server: The endpoint which did not initiate the TLS connection.</t>

</section>
<section anchor="major-differences-from-tls-12" title="Major Differences from TLS 1.2">

<t>draft-07
- Integration of semi-ephemeral DH proposal.</t>

<t><list style="symbols">
  <t>Add initial 0-RTT support</t>
  <t>Remove resumption and replace with PSK + tickets</t>
  <t>Move ClientKeyShare into an extension.</t>
  <t>Move to HKDF</t>
</list></t>

<t>draft-06</t>

<t><list style="symbols">
  <t>Prohibit RC4 negotiation for backwards compatibility.</t>
  <t>Freeze &amp; deprecate record layer version field.</t>
  <t>Update format of signatures with context.</t>
  <t>Remove explicit IV.</t>
</list></t>

<t>draft-05</t>

<t><list style="symbols">
  <t>Prohibit SSL negotiation for backwards compatibility.</t>
  <t>Fix which MS is used for exporters.</t>
</list></t>

<t>draft-04</t>

<t><list style="symbols">
  <t>Modify key computations to include session hash.</t>
  <t>Remove ChangeCipherSpec</t>
  <t>Renumber the new handshake messages to be somewhat more
consistent with existing convention and to remove a duplicate
registration.</t>
  <t>Remove renegotiation.</t>
  <t>Remove point format negotiation.</t>
</list></t>

<t>draft-03</t>

<t><list style="symbols">
  <t>Remove GMT time.</t>
  <t>Merge in support for ECC from RFC 4492 but without explicit
curves.</t>
  <t>Remove the unnecessary length field from the AD input to AEAD
ciphers.</t>
  <t>Rename {Client,Server}KeyExchange to {Client,Server}KeyShare</t>
  <t>Add an explicit HelloRetryRequest to reject the client’s</t>
</list></t>

<t>draft-02</t>

<t><list style="symbols">
  <t>Increment version number.</t>
  <t>Reworked handshake to provide 1-RTT mode.</t>
  <t>Remove custom DHE groups.</t>
  <t>Removed support for compression.</t>
  <t>Removed support for static RSA and DH key exchange.</t>
  <t>Removed support for non-AEAD ciphers</t>
</list></t>

</section>
</section>
<section anchor="goals" title="Goals">

<t>The goals of the TLS protocol, in order of priority, are as follows:</t>

<t><list style="numbers">
  <t>Cryptographic security: TLS should be used to establish a secure connection
between two parties.</t>
  <t>Interoperability: Independent programmers should be able to develop
applications utilizing TLS that can successfully exchange cryptographic
parameters without knowledge of one another’s code.</t>
  <t>Extensibility: TLS seeks to provide a framework into which new public key
and record protection methods can be incorporated as necessary. This will also
accomplish two sub-goals: preventing the need to create a new protocol (and
risking the introduction of possible new weaknesses) and avoiding the need to
implement an entire new security library.</t>
  <t>Relative efficiency: Cryptographic operations tend to be highly CPU
intensive, particularly public key operations. For this reason, the TLS
protocol has incorporated an optional session caching scheme to reduce the
number of connections that need to be established from scratch. Additionally,
care has been taken to reduce network activity.</t>
</list></t>

</section>
<section anchor="goals-of-this-document" title="Goals of This Document">

<t>This document and the TLS protocol itself have evolved from the SSL 3.0 Protocol
Specification as published by Netscape. The differences between this protocol
and previous versions are significant enough that the various
versions of TLS and SSL 3.0 do not interoperate (although each protocol
incorporates a mechanism by which an implementation can back down to prior
versions). This document is intended primarily for readers who will be
implementing the protocol and for those doing cryptographic analysis of it. The
specification has been written with this in mind, and it is intended to reflect
the needs of those two groups. For that reason, many of the algorithm-dependent
data structures and rules are included in the body of the text (as opposed to
in an appendix), providing easier access to them.</t>

<t>This document is not intended to supply any details of service definition or of
interface definition, although it does cover select areas of policy as they are
required for the maintenance of solid security.</t>

</section>
<section anchor="presentation-language" title="Presentation Language">

<t>This document deals with the formatting of data in an external representation.
The following very basic and somewhat casually defined presentation syntax will
be used. The syntax draws from several sources in its structure. Although it
resembles the programming language “C” in its syntax and XDR <xref target="RFC4506"/> in
both its syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document TLS only; it has no
general application beyond that particular goal.</t>

<section anchor="basic-block-size" title="Basic Block Size">

<t>The representation of all data items is explicitly specified. The basic data
block size is one byte (i.e., 8 bits). Multiple byte data items are
concatenations of bytes, from left to right, from top to bottom. From the byte
stream, a multi-byte item (a numeric in the example) is formed (using C
notation) by:</t>

<figure><artwork><![CDATA[
   value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
           ... | byte[n-1];
]]></artwork></figure>

<t>This byte ordering for multi-byte values is the commonplace network byte order
or big-endian format.</t>

</section>
<section anchor="miscellaneous" title="Miscellaneous">

<t>Comments begin with “/*” and end with “*/”.</t>

<t>Optional components are denoted by enclosing them in “[[ ]]” double
brackets.</t>

<t>Single-byte entities containing uninterpreted data are of type
opaque.</t>

</section>
<section anchor="vectors" title="Vectors">

<t>A vector (single-dimensioned array) is a stream of homogeneous data elements.
The size of the vector may be specified at documentation time or left
unspecified until runtime. In either case, the length declares the number of
bytes, not the number of elements, in the vector. The syntax for specifying a
new type, T’, that is a fixed- length vector of type T is</t>

<figure><artwork><![CDATA[
   T T'[n];
]]></artwork></figure>

<t>Here, T’ occupies n bytes in the data stream, where n is a multiple of the size
of T.  The length of the vector is not included in the encoded stream.</t>

<t>In the following example, Datum is defined to be three consecutive bytes that
the protocol does not interpret, while Data is three consecutive Datum,
consuming a total of nine bytes.</t>

<figure><artwork><![CDATA[
   opaque Datum[3];      /* three uninterpreted bytes */
   Datum Data[9];        /* 3 consecutive 3 byte vectors */
]]></artwork></figure>

<t>Variable-length vectors are defined by specifying a subrange of legal lengths,
inclusively, using the notation &lt;floor..ceiling&gt;. When these are encoded, the
actual length precedes the vector’s contents in the byte stream. The length
will be in the form of a number consuming as many bytes as required to hold the
vector’s specified maximum (ceiling) length. A variable-length vector with an
actual length field of zero is referred to as an empty vector.</t>

<figure><artwork><![CDATA[
   T T'<floor..ceiling>;
]]></artwork></figure>

<t>In the following example, mandatory is a vector that must contain between 300
and 400 bytes of type opaque. It can never be empty. The actual length field
consumes two bytes, a uint16, which is sufficient to represent the value 400
(see <xref target="numbers"/>). On the other hand, longer can represent up to 800 bytes of
data, or 400 uint16 elements, and it may be empty. Its encoding will include a
two-byte actual length field prepended to the vector. The length of an encoded
vector must be an even multiple of the length of a single element (for example,
a 17-byte vector of uint16 would be illegal).</t>

<figure><artwork><![CDATA[
   opaque mandatory<300..400>;
         /* length field is 2 bytes, cannot be empty */
   uint16 longer<0..800>;
         /* zero to 400 16-bit unsigned integers */
]]></artwork></figure>

</section>
<section anchor="numbers" title="Numbers">

<t>The basic numeric data type is an unsigned byte (uint8). All larger numeric
data types are formed from fixed-length series of bytes concatenated as
described in <xref target="basic-block-size"/> and are also unsigned. The following numeric
types are predefined.</t>

<figure><artwork><![CDATA[
   uint8 uint16[2];
   uint8 uint24[3];
   uint8 uint32[4];
   uint8 uint64[8];
]]></artwork></figure>

<t>All values, here and elsewhere in the specification, are stored in network byte
(big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is
equivalent to the decimal value 16909060.</t>

<t>Note that in some cases (e.g., DH parameters) it is necessary to represent
integers as opaque vectors. In such cases, they are represented as unsigned
integers (i.e., leading zero octets are not required even if the most
significant bit is set).</t>

</section>
<section anchor="enumerateds" title="Enumerateds">

<t>An additional sparse data type is available called enum. A field of type enum
can only assume the values declared in the definition. Each definition is a
different type. Only enumerateds of the same type may be assigned or compared.
Every element of an enumerated must be assigned a value, as demonstrated in the
following example. Since the elements of the enumerated are not ordered, they
can be assigned any unique value, in any order.</t>

<figure><artwork><![CDATA[
   enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
]]></artwork></figure>

<t>An enumerated occupies as much space in the byte stream as would its maximal
defined ordinal value. The following definition would cause one byte to be used
to carry fields of type Color.</t>

<figure><artwork><![CDATA[
   enum { red(3), blue(5), white(7) } Color;
]]></artwork></figure>

<t>One may optionally specify a value without its associated tag to force the
width definition without defining a superfluous element.</t>

<t>In the following example, Taste will consume two bytes in the data stream but
can only assume the values 1, 2, or 4.</t>

<figure><artwork><![CDATA[
   enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
]]></artwork></figure>

<t>The names of the elements of an enumeration are scoped within the defined type.
In the first example, a fully qualified reference to the second element of the
enumeration would be Color.blue. Such qualification is not required if the
target of the assignment is well specified.</t>

<figure><artwork><![CDATA[
   Color color = Color.blue;     /* overspecified, legal */
   Color color = blue;           /* correct, type implicit */
]]></artwork></figure>

<t>For enumerateds that are never converted to external representation, the
numerical information may be omitted.</t>

<figure><artwork><![CDATA[
   enum { low, medium, high } Amount;
]]></artwork></figure>

</section>
<section anchor="constructed-types" title="Constructed Types">

<t>Structure types may be constructed from primitive types for convenience. Each
specification declares a new, unique type. The syntax for definition is much
like that of C.</t>

<figure><artwork><![CDATA[
   struct {
       T1 f1;
       T2 f2;
       ...
       Tn fn;
   } [[T]];
]]></artwork></figure>

<t>The fields within a structure may be qualified using the type’s name, with a
syntax much like that available for enumerateds. For example, T.f2 refers to
the second field of the previous declaration. Structure definitions may be
embedded.</t>

<section anchor="variants" title="Variants">

<t>Defined structures may have variants based on some knowledge that is available
within the environment. The selector must be an enumerated type that defines
the possible variants the structure defines. There must be a case arm for every
element of the enumeration declared in the select. Case arms have limited
fall-through: if two case arms follow in immediate succession with no fields in
between, then they both contain the same fields. Thus, in the example below,
“orange” and “banana” both contain V2. Note that this is a new piece of syntax
in TLS 1.2.</t>

<t>The body of the variant structure may be given a label for reference. The
mechanism by which the variant is selected at runtime is not prescribed by the
presentation language.</t>

<figure><artwork><![CDATA[
   struct {
       T1 f1;
       T2 f2;
       ....
       Tn fn;
        select (E) {
            case e1: Te1;
            case e2: Te2;
            case e3: case e4: Te3;
            ....
            case en: Ten;
        } [[fv]];
   } [[Tv]];
]]></artwork></figure>

<t>For example:</t>

<figure><artwork><![CDATA[
   enum { apple, orange, banana } VariantTag;

   struct {
       uint16 number;
       opaque string<0..10>; /* variable length */
   } V1;

   struct {
       uint32 number;
       opaque string[10];    /* fixed length */
   } V2;

   struct {
       select (VariantTag) { /* value of selector is implicit */
           case apple:
             V1;   /* VariantBody, tag = apple */
           case orange:
           case banana:
             V2;   /* VariantBody, tag = orange or banana */
       } variant_body;       /* optional label on variant */
   } VariantRecord;
]]></artwork></figure>

</section>
</section>
<section anchor="constants" title="Constants">

<t>Typed constants can be defined for purposes of specification by declaring a
symbol of the desired type and assigning values to it.</t>

<t>Under-specified types (opaque, variable-length vectors, and structures that
contain opaque) cannot be assigned values. No fields of a multi-element
structure or vector may be elided.</t>

<t>For example:</t>

<figure><artwork><![CDATA[
   struct {
       uint8 f1;
       uint8 f2;
   } Example1;

   Example1 ex1 = {1, 4};  /* assigns f1 = 1, f2 = 4 */
]]></artwork></figure>

</section>
<section anchor="primitive-types" title="Primitive Types">

<t>The following common primitive types are defined and used subsequently:</t>

<figure><artwork><![CDATA[
      enum { false(0), true(1) } Boolean;
]]></artwork></figure>

</section>
<section anchor="cryptographic-attributes" title="Cryptographic Attributes">

<t>The two cryptographic operations — digital signing, and authenticated
encryption with additional data (AEAD) — are designated digitally-signed,
and aead-ciphered, respectively. A field’s cryptographic processing
is specified by prepending an appropriate key word designation before
the field’s type specification.  Cryptographic keys are implied by the
current session state (see <xref target="connection-states"/>).</t>

<section anchor="digital-signing" title="Digital Signing">

<t>A digitally-signed element is encoded as a struct DigitallySigned:</t>

<figure><artwork><![CDATA[
   struct {
      SignatureAndHashAlgorithm algorithm;
      opaque signature<0..2^16-1>;
   } DigitallySigned;
]]></artwork></figure>

<t>The algorithm field specifies the algorithm used (see <xref target="signature-algorithms"/>
for the definition of this field). Note that the algorithm
field was introduced in TLS 1.2, and is not in earlier versions. The signature is a digital signature
using those algorithms over the contents of the element. The contents
themselves do not appear on the wire but are simply calculated. The length of
the signature is specified by the signing algorithm and key.</t>

<t>In previous versions of TLS, the ServerKeyExchange format meant that attackers
can obtain a signature of a message with a chosen, 32-byte prefix. Because TLS
1.3 servers are likely to also implement prior versions, the contents of the
element always start with 64 bytes of octet 32 in order to clear that
chosen-prefix.</t>

<t>Following that padding is a NUL-terminated context string in order to
disambiguate signatures for different purposes. The context string will be
specified whenever a digitally-signed element is used.</t>

<t>Finally, the specified contents of the digitally-signed structure follow the
NUL at the end of the context string. (See the example at the end of this
section.)</t>

<t>In RSA signing, the opaque vector contains the signature generated using the
RSASSA-PKCS1-v1_5 signature scheme defined in <xref target="RFC3447"/>. As discussed in
<xref target="RFC3447"/>, the DigestInfo MUST be DER-encoded <xref target="X680"/> <xref target="X690"/>. For hash
algorithms without parameters (which includes SHA-1), the
DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL, but
implementations MUST accept both without parameters and with NULL parameters.
Note that earlier versions of TLS used a different RSA signature scheme that
did not include a DigestInfo encoding.</t>

<t>In DSA, the 20 bytes of the SHA-1 hash are run directly through the Digital
Signing Algorithm with no additional hashing. This produces two values, r and
s. The DSA signature is an opaque vector, as above, the contents of which are
the DER encoding of:</t>

<figure><artwork><![CDATA[
   Dss-Sig-Value ::= SEQUENCE {
       r INTEGER,
       s INTEGER
   }
]]></artwork></figure>

<t>Note: In current terminology, DSA refers to the Digital Signature Algorithm and
DSS refers to the NIST standard. In the original SSL and TLS specs, “DSS” was
used universally. This document uses “DSA” to refer to the algorithm, “DSS” to
refer to the standard, and it uses “DSS” in the code point definitions for
historical continuity.</t>

<t>All ECDSA computations MUST be performed according to ANSI X9.62 <xref target="X962"/>
or its successors.  Data to be signed/verified is hashed, and the
result run directly through the ECDSA algorithm with no additional
hashing.  The default hash function is SHA-1 <xref target="SHS"/>.  However, an
alternative hash function, such as one of the new SHA hash functions
specified in FIPS 180-2 may be used instead if the certificate
containing the EC public key explicitly requires use of another hash
function.  (The mechanism for specifying the required hash function
has not been standardized, but this provision anticipates such
standardization and obviates the need to update this document in
response.  Future PKIX RFCs may choose, for example, to specify the
hash function to be used with a public key in the parameters field of
subjectPublicKeyInfo.) [[OPEN ISSUE: This needs updating per 4492-bis
https://github.com/tlswg/tls13-spec/issues/59]]</t>

</section>
<section anchor="authenticated-encryption-with-additional-data-aead" title="Authenticated Encryption with Additional Data (AEAD)">

<t>In AEAD encryption, the plaintext is simultaneously encrypted and integrity
protected. The input may be of any length, and aead-ciphered output is
generally larger than the input in order to accommodate the integrity check
value.</t>

<t>In the following example</t>

<figure><artwork><![CDATA[
   struct {
       uint8 field1;
       uint8 field2;
       digitally-signed opaque {
         uint8 field3<0..255>;
         uint8 field4;
       };
   } UserType;
]]></artwork></figure>

<t>Assume that the context string for the signature was specified as “Example”.
The input for the signature/hash algorithm would be:</t>

<figure><artwork><![CDATA[
   2020202020202020202020202020202020202020202020202020202020202020
   2020202020202020202020202020202020202020202020202020202020202020
   4578616d706c6500
]]></artwork></figure>

<t>followed by the encoding of the inner struct (field3 and field4).</t>

<t>The length of the structure, in bytes, would be equal to two
bytes for field1 and field2, plus two bytes for the signature and hash
algorithm, plus two bytes for the length of the signature, plus the length of
the output of the signing algorithm. The length of the signature is known
because the algorithm and key used for the signing are known prior to encoding
or decoding this structure.</t>

</section>
</section>
</section>
<section anchor="the-tls-record-protocol" title="The TLS Record Protocol">

<t>The TLS Record Protocol is a layered protocol. At each layer, messages
may include fields for length, description, and content. The Record
Protocol takes messages to be transmitted, fragments the data into
manageable blocks, protects the records, and transmits the
result. Received data is decrypted and verified, reassembled, and then
delivered to higher-level clients.</t>

<t>Three protocols that use the record protocol are described in this document: the
handshake protocol, the alert protocol, and
the application data protocol. In order to allow extension of the TLS protocol,
additional record content types can be supported by the record protocol. New
record content type values are assigned by IANA in the TLS Content Type
Registry as described in <xref target="iana-considerations"/>.</t>

<t>Implementations MUST NOT send record types not defined in this document unless
negotiated by some extension. If a TLS implementation receives an unexpected
record type, it MUST send an “unexpected_message” alert.</t>

<t>Any protocol designed for use over TLS must be carefully designed to deal with
all possible attacks against it. As a practical matter, this means that the
protocol designer must be aware of what security properties TLS does and does
not provide and cannot safely rely on the latter.</t>

<t>Note in particular that type and length of a record are not protected by
encryption. If this information is itself sensitive, application designers may
wish to take steps (padding, cover traffic) to minimize information leakage.</t>

<section anchor="connection-states" title="Connection States">

<t>[[TODO: I plan to totally rewrite or remove this. IT seems like just cruft.]]</t>

<t>A TLS connection state is the operating environment of the TLS Record
Protocol.  It specifies a record protection algorithm and its
parameters as well as the record protection keys and IVs for the
connection in both the read and the write directions. The security
parameters are set by the TLS Handshake Protocol, which also determines
when new cryptographic keys are installed and used for record
protection.
The initial current state always specifies that records are
not protected.</t>

<t>The security parameters for a TLS Connection read and write state are set by
providing the following values:</t>

<t><list style="hanging">
  <t hangText='connection end'><vspace blankLines='0'/>
  Whether this entity is considered the “client” or the “server” in
this connection.</t>
  <t hangText='Hash algorithm'><vspace blankLines='0'/>
  An algorithm used to generate keys from the appropriate secret (see
<xref target="key-schedule"/> and <xref target="traffic-key-calculation"/>).</t>
  <t hangText='record protection algorithm'><vspace blankLines='0'/>
  The algorithm to be used for record protection. This algorithm must
be of the AEAD type and thus provides integrity and confidentiality
as a single primitive. It is possible to have AEAD algorithms which
do not provide any confidentiality and
<xref target="record-payload-protection"/> defines a special NULL_NULL AEAD
algorithm for use in the initial handshake). This specification
includes the key size of this algorithm and of the nonce for
the AEAD algorithm.</t>
  <t hangText='master secret'><vspace blankLines='0'/>
  A 48-byte secret shared between the two peers in the connection
and used to generate keys for protecting data.</t>
  <t hangText='client random'><vspace blankLines='0'/>
  A 32-byte value provided by the client.</t>
  <t hangText='server random'><vspace blankLines='0'/>
  A 32-byte value provided by the server.</t>
</list></t>

<t>These parameters are defined in the presentation language as:</t>

<figure><artwork><![CDATA[
   enum { server, client } ConnectionEnd;

   enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

   enum { aes_gcm } RecordProtAlgorithm;

   /* The algorithms specified in KDFAlgorithm and
      RecordProtAlgorithm may be added to. */

   struct {
       ConnectionEnd          entity;
       KDFAlgorithm           kdf_algorithm;
       RecordProtAlgorithm    record_prot_algorithm;
       uint8                  enc_key_length;
       uint8                  iv_length;
       opaque                 hs_master_secret[48];
       opaque                 master_secret[48];
       opaque                 client_random[32];
       opaque                 server_random[32];
   } SecurityParameters;
]]></artwork></figure>

<t>[TODO: update this to handle new key hierarchy.]</t>

<t>The connection state will use the security parameters to generate the following four
items:</t>

<figure><artwork><![CDATA[
   client write key
   server write key
   client write iv
   server write iv
]]></artwork></figure>

<t>The client write parameters are used by the server when receiving and
processing records and vice versa. The algorithm used for generating these
items from the security parameters is described in <xref target="traffic-key-calculation"/>.</t>

<t>Once the security parameters have been set and the keys have been generated,
the connection states can be instantiated by making them the current states.
These current states MUST be updated for each record processed. Each connection
state includes the following elements:</t>

<t><list style="hanging">
  <t hangText='cipher state'><vspace blankLines='0'/>
  The current state of the encryption algorithm.  This will consist
of the scheduled key for that connection.</t>
  <t hangText='sequence number'><vspace blankLines='0'/>
  Each connection state contains a sequence number, which is
maintained separately for read and write states.  The sequence
number is set to zero at the beginning of a connection and
incremented by one thereafter.  Sequence numbers are of type uint64 and
MUST NOT exceed 2^64-1.  Sequence numbers do not wrap.  If a TLS
implementation would need to wrap a sequence number, it MUST
terminate the connection.  A sequence number is incremented after
each record: specifically, the first record transmitted under a
particular connection state MUST use sequence number 0.
NOTE: This is a change from previous versions of TLS, where
sequence numbers were reset whenever keys were changed.</t>
</list></t>

</section>
<section anchor="record-layer" title="Record Layer">

<t>The TLS record layer receives uninterpreted data from higher layers in
non-empty blocks of arbitrary size.</t>

<section anchor="fragmentation" title="Fragmentation">

<t>The record layer fragments information blocks into TLSPlaintext records
carrying data in chunks of 2^14 bytes or less. Client message boundaries are
not preserved in the record layer (i.e., multiple client messages of the same
ContentType MAY be coalesced into a single TLSPlaintext record, or a single
message MAY be fragmented across several records).</t>

<figure><artwork><![CDATA[
   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

   enum {
       reserved(20), alert(21), handshake(22),
       application_data(23), early_handshake(25),
       (255)
   } ContentType;

   struct {
       ContentType type;
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       opaque fragment[TLSPlaintext.length];
   } TLSPlaintext;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='type'><vspace blankLines='0'/>
  The higher-level protocol used to process the enclosed fragment.</t>
  <t hangText='record_version'><vspace blankLines='0'/>
  The protocol version the current record is compatible with.
This value MUST be set to { 3, 1 } for all records.
This field is deprecated and MUST be ignored for all purposes.</t>
  <t hangText='length'><vspace blankLines='0'/>
  The length (in bytes) of the following TLSPlaintext.fragment.  The
length MUST NOT exceed 2^14.</t>
  <t hangText='fragment'><vspace blankLines='0'/>
  The application data.  This data is transparent and treated as an
independent block to be dealt with by the higher-level protocol
specified by the type field.</t>
</list></t>

<t>This document describes TLS Version 1.3, which uses the version { 3, 4 }.
The version value 3.4 is historical, deriving from the use of { 3, 1 }
for TLS 1.0 and { 3, 0 } for SSL 3.0. In order to maximize backwards
compatibility, the record layer version identifies as simply TLS 1.0.
Endpoints supporting other versions negotiate the version to use
by following the procedure and requirements in <xref target="backward-compatibility"/>.</t>

<t>Implementations MUST NOT send zero-length fragments of Handshake or Alert
types. Zero-length fragments of Application data MAY
be sent as they are potentially useful as a traffic analysis countermeasure.</t>

</section>
<section anchor="record-payload-protection" title="Record Payload Protection">

<t>The record protection functions translate a TLSPlaintext structure into a
TLSCiphertext. The deprotection functions reverse the process. In TLS 1.3
as opposed to previous versions of TLS, all ciphers are modeled as
“Authenticated Encryption with Additional Data” (AEAD) <xref target="RFC5116"/>.
AEAD functions provide a unified encryption and authentication
operation which turns plaintext into authenticated ciphertext and
back again.</t>

<t>AEAD ciphers take as input a single key, a nonce, a plaintext, and “additional
data” to be included in the authentication check, as described in Section 2.1
of <xref target="RFC5116"/>. The key is either the client_write_key or the server_write_key.</t>

<figure><artwork><![CDATA[
   struct {
       ContentType type;
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       aead-ciphered struct {
          opaque content[TLSPlaintext.length];
       } fragment;
   } TLSCiphertext;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='type'><vspace blankLines='0'/>
  The type field is identical to TLSPlaintext.type.</t>
  <t hangText='record_version'><vspace blankLines='0'/>
  The record_version field is identical to TLSPlaintext.record_version and is always { 3, 1 }.
Note that the handshake protocol including the ClientHello and ServerHello messages authenticates
the protocol version, so this value is redundant.</t>
  <t hangText='length'><vspace blankLines='0'/>
  The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.</t>
  <t hangText='fragment'><vspace blankLines='0'/>
  The AEAD encrypted form of TLSPlaintext.fragment.</t>
</list></t>

<t>The length of the per-record nonce (iv_length) is set to max(8 bytes,
N_MIN) for the AEAD algorithm (see <xref target="RFC5116"/> Section 4). An AEAD
algorithm where N_MAX is less than 8 bytes MUST not be used with TLS.
The per-record nonce for the AEAD construction is formed as follows:</t>

<t><list style="numbers">
  <t>The 64-bit record sequence number is padded to the left with zeroes
to iv_length.</t>
  <t>The padded sequence number is XORed with the static client_write_iv
or server_write_iv, depending on the role.</t>
</list></t>

<t>The resulting quantity (of length iv_length) is used as the per-record
nonce. </t>

<t>Note: This is a different construction from that in TLS 1.2, which
specified a partially explicit nonce.</t>

<t>The plaintext is the TLSPlaintext.fragment.</t>

<t>The additional authenticated data, which we denote as additional_data, is
defined as follows:</t>

<figure><artwork><![CDATA[
   additional_data = seq_num + TLSPlaintext.type +
                     TLSPlaintext.record_version
]]></artwork></figure>

<t>where “+” denotes concatenation.</t>

<t>Note: In versions of TLS prior to 1.3, the additional_data included a
length field. This presents a problem for cipher constructions with
data-dependent padding (such as CBC). TLS 1.3 removes the length
field and relies on the AEAD cipher to provide integrity for the
length of the data.</t>

<t>The AEAD output consists of the ciphertext output by the AEAD encryption
operation. The length will generally be larger than TLSPlaintext.length, but
by an amount that varies with the AEAD cipher. Since the ciphers might
incorporate padding, the amount of overhead could vary with different
TLSPlaintext.length values. Each AEAD cipher MUST NOT produce an expansion of
greater than 1024 bytes. Symbolically,</t>

<figure><artwork><![CDATA[
   AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext,
                                additional_data)
]]></artwork></figure>

<t>[[OPEN ISSUE: Reduce these values?
https://github.com/tlswg/tls13-spec/issues/55]]</t>

<t>In order to decrypt and verify, the cipher takes as input the key, nonce, the
“additional_data”, and the AEADEncrypted value. The output is either the
plaintext or an error indicating that the decryption failed. There is no
separate integrity check. That is:</t>

<figure><artwork><![CDATA[
   TLSPlaintext.fragment = AEAD-Decrypt(write_key, nonce,
                                        AEADEncrypted,
                                        additional_data)
]]></artwork></figure>

<t>If the decryption fails, a fatal “bad_record_mac” alert MUST be generated.</t>

<t>As a special case, we define the NULL_NULL AEAD cipher which is simply
the identity operation and thus provides no security. This cipher
MUST ONLY be used with the initial TLS_NULL_WITH_NULL_NULL cipher suite.</t>

</section>
</section>
</section>
<section anchor="the-tls-handshaking-protocols" title="The TLS Handshaking Protocols">

<t>TLS has three subprotocols that are used to allow peers to agree upon security
parameters for the record layer, to authenticate themselves, to instantiate
negotiated security parameters, and to report error conditions to each other.</t>

<t>The Handshake Protocol is responsible for negotiating a session, which consists
of the following items:</t>

<t><list style="hanging">
  <t hangText='peer certificate'><vspace blankLines='0'/>
  X509v3 <xref target="RFC5280"/> certificate of the peer.  This element of the state
may be null.</t>
  <t hangText='cipher spec'><vspace blankLines='0'/>
  Specifies the authentication and key establishment algorithms,
the hash for use with HKDF to generate keying
material, and the record protection algorithm (See
<xref target="the-security-parameters"/> for formal definition.)</t>
  <t hangText='resumption master secret'><vspace blankLines='0'/>
  a secret shared between the client and server that can be used
as a PSK in future connections.</t>
</list></t>

<t>These items are then used to create security parameters for use by the record
layer when protecting application data. Many connections can be instantiated
using the same session using a PSK established in an initial handshake.</t>

<section anchor="alert-protocol" title="Alert Protocol">

<t>One of the content types supported by the TLS record layer is the alert type.
Alert messages convey the severity of the message (warning or fatal) and a
description of the alert. Alert messages with a level of fatal result in the
immediate termination of the connection. In this case, other connections
corresponding to the session may continue, but the session identifier MUST be
invalidated, preventing the failed session from being used to establish new
connections. Like other messages, alert messages are encrypted
as specified by the current connection state.</t>

<figure><artwork><![CDATA[
   enum { warning(1), fatal(2), (255) } AlertLevel;

   enum {
       close_notify(0),
       unexpected_message(10),              /* fatal */
       bad_record_mac(20),                  /* fatal */
       decryption_failed_RESERVED(21),      /* fatal */
       record_overflow(22),                 /* fatal */
       decompression_failure_RESERVED(30),  /* fatal */
       handshake_failure(40),               /* fatal */
       no_certificate_RESERVED(41),         /* fatal */
       bad_certificate(42),
       unsupported_certificate(43),
       certificate_revoked(44),
       certificate_expired(45),
       certificate_unknown(46),
       illegal_parameter(47),               /* fatal */
       unknown_ca(48),                      /* fatal */
       access_denied(49),                   /* fatal */
       decode_error(50),                    /* fatal */
       decrypt_error(51),                   /* fatal */
       export_restriction_RESERVED(60),     /* fatal */
       protocol_version(70),                /* fatal */
       insufficient_security(71),           /* fatal */
       internal_error(80),                  /* fatal */
       user_canceled(90),
       no_renegotiation(100),               /* fatal */
       unsupported_extension(110),          /* fatal */
       (255)
   } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;
]]></artwork></figure>

<section anchor="closure-alerts" title="Closure Alerts">

<t>The client and the server must share knowledge that the connection is ending in
order to avoid a truncation attack. Either party may initiate the exchange of
closing messages.</t>

<t><list style="hanging">
  <t hangText='close_notify'><vspace blankLines='0'/>
  This message notifies the recipient that the sender will not send
any more messages on this connection.  Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed.  This is a change from TLS 1.0 to conform
with widespread implementation practice.</t>
</list></t>

<t>Either party MAY initiate a close by sending a “close_notify” alert. Any data
received after a closure alert is ignored. If a transport-level close is
received prior to a close_notify, the receiver cannot know that all the
data that was sent has been received. </t>

<t>Unless some other fatal alert has been transmitted, each party is required to
send a “close_notify” alert before closing the write side of the connection. The
other party MUST respond with a “close_notify” alert of its own and close down
the connection immediately, discarding any pending writes. It is not required
for the initiator of the close to wait for the responding “close_notify” alert
before closing the read side of the connection.</t>

<t>If the application protocol using TLS provides that any data may be carried
over the underlying transport after the TLS connection is closed, the TLS
implementation must receive the responding “close_notify” alert before indicating
to the application layer that the TLS connection has ended. If the application
protocol will not transfer any additional data, but will only close the
underlying transport connection, then the implementation MAY choose to close
the transport without waiting for the responding “close_notify”. No part of this
standard should be taken to dictate the manner in which a usage profile for TLS
manages its data transport, including when connections are opened or closed.</t>

<t>Note: It is assumed that closing a connection reliably delivers pending data
before destroying the transport.</t>

</section>
<section anchor="error-alerts" title="Error Alerts">

<t>Error handling in the TLS Handshake protocol is very simple. When an error is
detected, the detecting party sends a message to the other party. Upon
transmission or receipt of a fatal alert message, both parties immediately
close the connection. Servers and clients MUST forget any session-identifiers,
keys, and secrets associated with a failed connection. Thus, any connection
terminated with a fatal alert MUST NOT be resumed.</t>

<t>Whenever an implementation encounters a condition which is defined as a fatal
alert, it MUST send the appropriate alert prior to closing the connection. For
all errors where an alert level is not explicitly specified, the sending party
MAY determine at its discretion whether to treat this as a fatal error or not.
If the implementation chooses to send an alert but intends to close the
connection immediately afterwards, it MUST send that alert at the fatal alert
level.</t>

<t>If an alert with a level of warning is sent and received, generally the
connection can continue normally. If the receiving party decides not to proceed
with the connection (e.g., after having received a “no_renegotiation” alert that
it is not willing to accept), it SHOULD send a fatal alert to terminate the
connection. Given this, the sending party cannot, in general, know how the
receiving party will behave. Therefore, warning alerts are not very useful when
the sending party wants to continue the connection, and thus are sometimes
omitted. For example, if a peer decides to accept an expired certificate
(perhaps after confirming this with the user) and wants to continue the
connection, it would not generally send a “certificate_expired” alert.</t>

<t>The following error alerts are defined:</t>

<t><list style="hanging">
  <t hangText='unexpected_message'><vspace blankLines='0'/>
  An inappropriate message was received.  This alert is always fatal
and should never be observed in communication between proper
implementations.</t>
  <t hangText='bad_record_mac'><vspace blankLines='0'/>
  This alert is returned if a record is received which cannot be
deprotected. Because AEAD algorithms combine decryption and
verification, this message is used for all deprotection failures.
This message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).</t>
  <t hangText='decryption_failed_RESERVED'><vspace blankLines='0'/>
  This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode <xref target="CBCATT"/>.  It MUST
NOT be sent by compliant implementations. This message is always fatal.</t>
  <t hangText='record_overflow'><vspace blankLines='0'/>
  A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSPlaintext record
with more than 2^14 bytes.  This message is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).</t>
  <t hangText='decompression_failure_RESERVED'><vspace blankLines='0'/>
  This alert was used in previous versions of TLS. TLS 1.3 does not
include compression and TLS 1.3 implementations MUST NOT send this
alert when in TLS 1.3 mode. This message is always fatal.</t>
  <t hangText='handshake_failure'><vspace blankLines='0'/>
  Reception of a “handshake_failure” alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available.
This message is always fatal.</t>
  <t hangText='no_certificate_RESERVED'><vspace blankLines='0'/>
  This alert was used in SSL 3.0 but not any version of TLS.  It MUST
NOT be sent by compliant implementations.
This message is always fatal.</t>
  <t hangText='bad_certificate'><vspace blankLines='0'/>
  A certificate was corrupt, contained signatures that did not
verify correctly, etc.</t>
  <t hangText='unsupported_certificate'><vspace blankLines='0'/>
  A certificate was of an unsupported type.</t>
  <t hangText='certificate_revoked'><vspace blankLines='0'/>
  A certificate was revoked by its signer.</t>
  <t hangText='certificate_expired'><vspace blankLines='0'/>
  A certificate has expired or is not currently valid.</t>
  <t hangText='certificate_unknown'><vspace blankLines='0'/>
  Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.</t>
  <t hangText='illegal_parameter'><vspace blankLines='0'/>
  A field in the handshake was out of range or inconsistent with
other fields.  This message is always fatal.</t>
  <t hangText='unknown_ca'><vspace blankLines='0'/>
  A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn’t be matched with a known, trusted CA.  This
message is always fatal.</t>
  <t hangText='access_denied'><vspace blankLines='0'/>
  A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation.  This
message is always fatal.</t>
  <t hangText='decode_error'><vspace blankLines='0'/>
  A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect.  This
message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).</t>
  <t hangText='decrypt_error'><vspace blankLines='0'/>
  A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message.
This message is always fatal.</t>
  <t hangText='export_restriction_RESERVED'><vspace blankLines='0'/>
  This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations. This message is always fatal.</t>
  <t hangText='protocol_version'><vspace blankLines='0'/>
  The protocol version the peer has attempted to negotiate is
recognized but not supported.  (For example, old protocol versions
might be avoided for security reasons.)  This message is always
fatal.</t>
  <t hangText='insufficient_security'><vspace blankLines='0'/>
  Returned instead of “handshake_failure” when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client.  This message is always
fatal.</t>
  <t hangText='internal_error'><vspace blankLines='0'/>
  An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue.  This message is always fatal.</t>
  <t hangText='user_canceled'><vspace blankLines='0'/>
  This handshake is being canceled for some reason unrelated to a
protocol failure.  If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
“close_notify” is more appropriate.  This alert should be followed
by a “close_notify”.  This message is generally a warning.</t>
  <t hangText='no_renegotiation'><vspace blankLines='0'/>
  Sent by the client in response to a HelloRequest or by the server
in response to a ClientHello after initial handshaking. Versions
of TLS prior to TLS 1.3 supported renegotiation of a previously
established connection; TLS 1.3 removes this feature. This
message is always fatal.</t>
  <t hangText='unsupported_extension'><vspace blankLines='0'/>
  sent by clients that receive an extended ServerHello containing
an extension that they did not put in the corresponding ClientHello.
This message is always fatal.</t>
</list></t>

<t>New Alert values are assigned by IANA as described in <xref target="iana-considerations"/>.</t>

</section>
</section>
<section anchor="handshake-protocol-overview" title="Handshake Protocol Overview">

<t>The cryptographic parameters of the session state are produced by the
TLS Handshake Protocol, which operates on top of the TLS record
layer. When a TLS client and server first start communicating, they
agree on a protocol version, select cryptographic algorithms,
optionally authenticate each other, and establish shared secret keying
material.</t>

<t>TLS supports three basic key exchange modes:</t>

<t><list style="symbols">
  <t>Diffie-Hellman (of both the finite field and elliptic curve
varieties).</t>
  <t>A pre-shared symmetric key (PSK)</t>
  <t>A combination of a symmetric key and Diffie-Hellman</t>
</list></t>

<t>Which mode is used depends on the negotiated cipher suite. Conceptually,
the handshake establishes two secrets which are used to derive all the
keys.</t>

<t>Ephemeral Secret (ES): A secret which is derived from fresh (EC)DHE
   shares for this connection. Keying material derived from ES is
   intended to be forward secure (with the exception of pre-shared
   key only modes).</t>

<t>Static Secret (SS): A secret which may be derived from static or
   semi-static keying material, such as a pre-shared key or the
   server’s semi-static (EC)DH share.</t>

<t>In some cases, as with the DH handshake shown in <xref target="tls-full"/>, these
secrets are the same, but having both allows for a uniform key
derivation scheme for all cipher modes.</t>

<t>The basic TLS Handshake for DH is shown in <xref target="tls-full"/>:</t>

<figure title="Message flow for full TLS Handshake" anchor="tls-full"><artwork><![CDATA[
     Client                                               Server

     ClientHello
       + ClientKeyShare        -------->
                                                     ServerHello
                                                 ServerKeyShare*
                                           {EncryptedExtensions}
                                          {ServerConfiguration*}
                                                  {Certificate*}
                                           {CertificateRequest*}
                                            {CertificateVerify*}
                               <--------              {Finished}
     {Certificate*}
     {CertificateVerify*}
     {Finished}                -------->
     [Application Data]        <------->      [Application Data]

* Indicates optional or situation-dependent messages that are not always sent.

{} Indicates messages protected using keys derived from the ephemeral secret.

[] Indicates messages protected using keys derived from the master secret.
]]></artwork></figure>

<t>The first message sent by the client is the ClientHello <xref target="client-hello"/> which contains
a random nonce (ClientHello.random), its offered protocol version,
cipher suite, and extensions, and one or more Diffie-Hellman key
shares in the ClientKeyShare extension <xref target="client-key-share"/>.</t>

<t>The server processes the ClientHello and determines the appropriate
cryptographic parameters for the connection. It then responds with
the following messages:</t>

<t><list style="hanging">
  <t hangText='ServerHello'><vspace blankLines='0'/>
  indicates the negotiated connection parameters. [<xref target="server-hello"/>]</t>
  <t hangText='ServerKeyShare'><vspace blankLines='0'/>
  the server’s ephemeral Diffie-Hellman Share which must be in the
same group as one of the shares offered by the client. This
message will be omitted if DH is not in use (i.e., a pure PSK
cipher suite is selected). The ClientKeyShare and ServerKeyShare
are used together to derive the Static Secret and Ephemeral
Secret (in this mode they are the same).  [<xref target="server-key-share"/>]</t>
  <t hangText='ServerConfiguration'><vspace blankLines='0'/>
  supplies a configuration for a future handshake (see <xref target="cached-server-configuration"/>).
[<xref target="server-configuration"/>]</t>
  <t hangText='EncryptedExtensions'><vspace blankLines='0'/>
  responses to any extensions which are not required in order to
determine the cryptographic parameters. [<xref target="encrypted-extensions"/>]</t>
  <t hangText='Certificate '><vspace blankLines='0'/>
  the server certificate. This message will be omitted if the
server is not authenticating via a certificates. [<xref target="server-certificate"/>]</t>
  <t hangText='CertificateRequest'><vspace blankLines='0'/>
  if certificate-based client authentication is desired, the
desired parameters for that certificate. This message will
be omitted if client authentication is not desired.
[[OPEN ISSUE: See https://github.com/tlswg/tls13-spec/issues/184]].
[<xref target="certificate-request"/>]</t>
  <t hangText='CertificateVerify'><vspace blankLines='0'/>
  a signature over the entire handshake using the public key
in the Certificate message. This message will be omitted if the
server is not authenticating via a certificate. [<xref target="server-certificate-verify"/>]</t>
  <t hangText='Finished'><vspace blankLines='0'/>
  a MAC over the entire handshake computed using the Static Secret.
This message provides key confirmation and 
In some modes (see <xref target="cached-server-configuration"/>) it also authenticates the handshake using the
the Static Secret. [<xref target="server-finished"/>]</t>
</list></t>

<t>Upon receiving the server’s messages, the client responds with his final
flight of messages:</t>

<t><list style="hanging">
  <t hangText='Certificate '><vspace blankLines='0'/>
  the client’s certificate. This message will be omitted if the
client is not authenticating via a certificates. [<xref target="client-certificate"/>]</t>
  <t hangText='CertificateVerify'><vspace blankLines='0'/>
  a signature over the entire handshake using the public key
in the Certificate message. This message will be omitted if the
client is not authenticating via a certificate. [<xref target="client-certificate-verify"/>]</t>
  <t hangText='Finished'><vspace blankLines='0'/>
  a MAC over the entire handshake computed using the Static Secret
and providing key confirmation. [<xref target="server-finished"/>]</t>
</list></t>

<t>At this point, the handshake is complete, and the client and server
may exchange application layer data. Application data MUST NOT
be sent prior to sending the Finished message. If client authentication
is requested, the server MUST NOT send application data
before it receives the client’s Finished.</t>

<t>[[TODO: Move this elsewhere?
Note that higher layers should not be overly reliant on whether TLS always
negotiates the strongest possible connection between two peers. There are a
number of ways in which a man-in-the-middle attacker can attempt to make two
entities drop down to the least secure method they support. The protocol has
been designed to minimize this risk, but there are still attacks available. For
example, an attacker could block access to the port a secure service runs on
or attempt to get the peers to negotiate an unauthenticated connection. The
fundamental rule is that higher levels must be cognizant of what their security
requirements are and never transmit information over a channel less secure than
what they require. The TLS protocol is secure in that any cipher suite offers
its promised level of security: if you negotiate AES-GCM <xref target="GCM"/> with
a 255-bit ECDHE key exchange with a host whose certificate
chain you have verified, you can expect that to be reasonably “secure” 
against algorithmic attacks, at least in the year 2015.]]</t>

<section anchor="incorrect-dhe-share" title="Incorrect DHE Share">

<t>If the client has not provided an appropriate ClientKeyShare (e.g. it
includes only DHE or ECDHE groups unacceptable or unsupported by the
server), the server corrects the mismatch with a HelloRetryRequest and
the client will need to restart the handshake with an appropriate
ClientKeyShare, as shown in Figure 2:</t>

<figure title="Message flow for a full handshake with mismatched parameters" anchor="tls-restart"><artwork><![CDATA[
       Client                                               Server

       ClientHello              
         + ClientKeyShare        -------->
                                 <--------       HelloRetryRequest

       ClientHello
         + ClientKeyShare        -------->
                                                       ServerHello
                                                    ServerKeyShare
                                            {EncryptedExtensions*}
                                            {ServerConfiguration*}
                                                    {Certificate*}
                                             {CertificateRequest*}
                                              {CertificateVerify*}
                                 <--------              {Finished}
       {Certificate*}
       {CertificateVerify*}
       {Finished}                -------->
       [Application Data]        <------->     [Application Data]
]]></artwork></figure>

<t>[[OPEN ISSUE: Should we restart the handshake hash?
https://github.com/tlswg/tls13-spec/issues/104.]]
[[OPEN ISSUE: We need to make sure that this flow doesn’t introduce
downgrade issues. Potential options include continuing the handshake
hashes (as long as clients don’t change their opinion of the server’s
capabilities with aborted handshakes) and requiring the client to send
the same ClientHello (as is currently done) and then checking you get
the same negotiated parameters.]]</t>

<t>If no common cryptographic parameters can be negotiated, the server
will send a fatal alert.</t>

<t>TLS also allows several optimized variants of the basic handshake, as
described below.</t>

</section>
<section anchor="cached-server-configuration" title="Cached Server Configuration">

<t>During an initial handshake, the server can provide a ServerConfiguration
message containing a long-term (EC)DH share. On future
connections, the client can indicate to the server that it knows the
server’s configuration and if that configuration is valid the server
can omit both the Certificate or CertificateVerify message (provided
that a new configuration is not supplied in this handshake).</t>

<t>When a known configuration is used, the server’s long-term DHE key is
combined with the client’s ClientKeyShare to produce SS. ES is
computed as above.  This optimization allows the server to amortize
the transmission of these messages and the server’s signature over
multiple handshakes, thus reducing the server’s computational cost for
cipher suites where signatures are slower than key agreement,
principally RSA signatures paired with ECDHE.</t>

</section>
<section anchor="zero-rtt-exchange" title="Zero-RTT Exchange">

<t>When a cached ServerConfiguration is used, the client can also send
application data as well as its Certificate and CertificateVerify
(if client authentication is requested) on its first flight, thus
reducing handshake latency, as shown below.</t>

<figure title="Message flow for a zero round trip handshake" anchor="tls-0-rtt"><artwork><![CDATA[
       Client                                               Server

       ClientHello
         + ClientKeyShare
       (Certificate*)
       (CertificateVerify*)
       (Application Data)        -------->
                                                       ServerHello
                                                    ServerKeyShare
                                 <--------              {Finished}
       {Finished}                -------->

       [Application Data]        <------->      [Application Data]

() Indicates messages protected using keys derived from the static secret.
]]></artwork></figure>

<t>Note: because sequence numbers continue to increment between the
initial (early) application data and the application data sent
after the handshake has complete, an attacker cannot remove
early application data messages.</t>

<t>IMPORTANT NOTE: The security properties for 0-RTT data (regardless of
the cipher suite) are weaker than those for other kinds of TLS data.
Specifically.</t>

<t><list style="numbers">
  <t>This data is not forward secure, because it is encrypted solely
with the server’s semi-static (EC)DH share.</t>
  <t>There are no guarantees of non-replay between connections.
Unless the server takes special measures outside those provided by TLS (See
<xref target="replay-properties"/>), the server has no guarantee that the same
0-RTT data was not transmitted on multiple 0-RTT connections.
This is especially relevant if the data is authenticated either
with TLS client authentication or inside the application layer
protocol. However, 0-RTT data cannot be duplicated within a connection (i.e., the server
will not process the same data twice for the same connection) and also
cannot be sent as if it were ordinary TLS data. </t>
  <t>If the server key is compromised, and client authentication is used, then
the attacker can impersonate the client to the server (as it knows the
traffic key).</t>
</list></t>

</section>
<section anchor="resumption-and-psk" title="Resumption and PSK">

<t>Finally, TLS provides a pre-shared key (PSK) mode which allows a
client and server who share an existing secret (e.g., a key
established out of band) to establish a connection authenticated by
that key. PSKs can also be established in a previous session and
then reused (“session resumption”). Once a handshake has completed, the server can
send the client a PSK identity which corresponds to a key derived from
the initial handshake (See <xref target="new-session-ticket-message"/>). The client
can then use that PSK identity in future handshakes to negotiate use
of the PSK; if the server accepts it, then the security context of the
original connection is tied to the new connection. In TLS 1.2 and
below, this functionality was provided by “session resumption” and
“session tickets” <xref target="RFC5077"/>. Both mechanisms are obsoleted in TLS
1.3.</t>

<t>PSK ciphersuites can either use PSK in combination with
an (EC)DHE exchange in order to provide forward secrecy in combination
with shared keys, or can use PSKs alone, at the cost of losing forward
secrecy.</t>

<t><xref target="tls-resumption-psk"/> shows a pair of handshakes in which the first establishes
a PSK and the second uses it:</t>

<figure title="Message flow for resumption and PSK" anchor="tls-resumption-psk"><artwork><![CDATA[
       Client                                               Server

Initial Handshake:

       ClientHello
         + ClientKeyShare       -------->
                                                       ServerHello
                                                    ServerKeyShare
                                             {EncryptedExtensions}
                                             {ServerConfiguration*}
                                                    {Certificate*}
                                             {CertificateRequest*}
                                              {CertificateVerify*}
                                 <--------              {Finished}
       {Certificate*}
       {CertificateVerify*}
       {Finished}                -------->
                                 <--------      [NewSessionTicket]
       [Application Data]        <------->      [Application Data]


Subsequent Handshake:
       ClientHello
         + ClientKeyShare,
           PreSharedKeyExtension -------->
                                                       ServerHello
                                            +PreSharedKeyExtension
                                 <--------              {Finished}
       {Certificate*}
       {Finished}                -------->
       [Application Data]        <------->      [Application Data]
]]></artwork></figure>

<t>Note that the client supplies a ClientKeyShare to the server as well, which
allows the server to decline resumption and fall back to a full handshake.
However, because the server is authenticating via a PSK, it does not
send a Certificate or a CertificateVerify. PSK-based resumption cannot
be used to provide a new ServerConfiguration.</t>

<t>The contents and significance of each message will be presented in detail in
the following sections.</t>

</section>
</section>
<section anchor="handshake-protocol" title="Handshake Protocol">

<t>The TLS Handshake Protocol is one of the defined higher-level clients of the
TLS Record Protocol. This protocol is used to negotiate the secure attributes
of a session. Handshake messages are supplied to the TLS record layer, where
they are encapsulated within one or more TLSPlaintext or TLSCiphertext structures, which are
processed and transmitted as specified by the current active session state.</t>

<figure><artwork><![CDATA[
   enum {
       reserved(0), client_hello(1), server_hello(2),
       session_ticket(4), hello_retry_request(6),
       server_key_share(7), certificate(11), reserved(12),
       certificate_request(13), server_configuration(14),
       certificate_verify(15), reserved(16), finished(20), (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;    /* handshake type */
       uint24 length;             /* bytes in message */
       select (HandshakeType) {
           case client_hello:        ClientHello;
           case server_hello:        ServerHello;
           case hello_retry_request: HelloRetryRequest;
           case server_key_share:    ServerKeyShare;
           case server_configuration:ServerConfiguration;
           case certificate:         Certificate;
           case certificate_request: CertificateRequest;
           case certificate_verify:  CertificateVerify;
           case finished:            Finished;
           case session_ticket:      NewSessionTicket;
       } body;
   } Handshake;
]]></artwork></figure>

<t>The handshake protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be omitted,
however.</t>

<t>New handshake message types are assigned by IANA as described in
<xref target="iana-considerations"/>.</t>

<section anchor="hello-messages" title="Hello Messages">

<t>The hello phase messages are used to exchange security enhancement capabilities
between the client and server. When a new session begins, the record layer’s
connection state AEAD algorithm is initialized to NULL_NULL.</t>

<section anchor="client-hello" title="Client Hello">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>When a client first connects to a server, it is required to send the
ClientHello as its first message. The client will also send a
ClientHello when the server has responded to its ClientHello with a
ServerHello that selects cryptographic parameters that don’t match the
client’s ClientKeyShare. In that case, the client MUST send the same
ClientHello (without modification) except including a new ClientKeyShare.
[[OPEN ISSUE: New random values? See: 
https://github.com/tlswg/tls13-spec/issues/185]]
If a server receives a ClientHello at any other time, it MUST send
a fatal “no_renegotiation” alert.</t>
</list></t>

<t>Structure of this message:</t>

<t><list style='empty'>
  <t>The ClientHello message includes a random structure, which is used later in
the protocol.</t>
</list></t>

<figure><artwork><![CDATA[
   struct {
       opaque random_bytes[32];
   } Random;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='random_bytes'><vspace blankLines='0'/>
  32 bytes generated by a secure random number generator.</t>
</list></t>

<t>Note: Versions of TLS prior to TLS 1.3 used the top 32 bits of
the Random value to encode the time since the UNIX epoch.</t>

<t>The cipher suite list, passed from the client to the server in the ClientHello
message, contains the combinations of cryptographic algorithms supported by the
client in order of the client’s preference (favorite choice first). Each cipher
suite defines a key exchange algorithm, a record protection algorithm (including
secret key length) and a hash to be used with HKDF. The server will select a cipher
suite or, if no acceptable choices are presented, return a “handshake_failure”
alert and close the connection. If the list contains cipher suites the server
does not recognize, support, or wish to use, the server MUST ignore those
cipher suites, and process the remaining ones as usual.</t>

<figure><artwork><![CDATA[
   uint8 CipherSuite[2];    /* Cryptographic suite selector */

   enum { null(0), (255) } CompressionMethod;

   struct {
       ProtocolVersion client_version = { 3, 4 };    /* TLS v1.3 */
       Random random;
       SessionID session_id;
       CipherSuite cipher_suites<2..2^16-2>;
       CompressionMethod compression_methods<1..2^8-1>;
       select (extensions_present) {
           case false:
               struct {};
           case true:
               Extension extensions<0..2^16-1>;
       };
   } ClientHello;
]]></artwork></figure>

<t>TLS allows extensions to follow the compression_methods field in an extensions
block. The presence of extensions can be detected by determining whether there
are bytes following the compression_methods at the end of the ClientHello. Note
that this method of detecting optional data differs from the normal TLS method
of having a variable-length field, but it is used for compatibility with TLS
before extensions were defined.</t>

<t><list style="hanging">
  <t hangText='client_version'><vspace blankLines='0'/>
  The version of the TLS protocol by which the client wishes to
communicate during this session.  This SHOULD be the latest
(highest valued) version supported by the client.  For this
version of the specification, the version will be 3.4. (See
<xref target="backward-compatibility"/> for details about backward compatibility.)</t>
  <t hangText='random'><vspace blankLines='0'/>
  A client-generated random structure.</t>
  <t hangText='session_id'><vspace blankLines='0'/>
  Versions of TLS prior to TLS 1.3 supported a session resumption
feature which has been merged with Pre-Shared Keys in this version
(see <xref target="resumption-and-psk"/>).
This field MUST be ignored by a server negotiating TLS 1.3 and 
should be set as a zero length vector (i.e., a single zero byte
length field) by clients which do not have a cached session_id
set by a pre-TLS 1.3 server.</t>
  <t hangText='cipher_suites'><vspace blankLines='0'/>
  This is a list of the cryptographic options supported by the
client, with the client’s first preference first.
Values are defined in <xref target="the-cipher-suite"/>.</t>
  <t hangText='compression_methods'><vspace blankLines='0'/>
  Versions of TLS before 1.3 supported compression and the list of
compression methods was supplied in this field. For any TLS 1.3
ClientHello, this field MUST contain only the “null” compression
method with the code point of 0. If a TLS 1.3 ClientHello is
received with any other value in this field, the server MUST
generate a fatal “illegal_parameter” alert. Note that TLS 1.3
servers may receive TLS 1.2 or prior ClientHellos which contain
other compression methods and MUST follow the procedures for
the appropriate prior version of TLS.</t>
  <t hangText='extensions'><vspace blankLines='0'/>
  Clients MAY request extended functionality from servers by sending
data in the extensions field.  The actual “Extension” format is
defined in <xref target="hello-extensions"/>.</t>
</list></t>

<t>In the event that a client requests additional functionality using extensions,
and this functionality is not supplied by the server, the client MAY abort the
handshake. A server MUST accept ClientHello messages both with and without the
extensions field, and (as for all other messages) it MUST check that the amount
of data in the message precisely matches one of these formats; if not, then it
MUST send a fatal “decode_error” alert.</t>

<t>After sending the ClientHello message, the client waits for a ServerHello
or HelloRetryRequest message.</t>

</section>
<section anchor="server-hello" title="Server Hello">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>The server will send this message in response to a ClientHello message when
it was able to find an acceptable set of algorithms and the client’s
ClientKeyShare extension was acceptable. If the client proposed groups are not
acceptable by the server, it will respond with an “insufficient_security” fatal alert.</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   struct {
       ProtocolVersion server_version;
       Random random;
       uint8 session_id_len;  // Must be 0.
       CipherSuite cipher_suite;
       select (extensions_present) {
           case false:
               struct {};
           case true:
               Extension extensions<0..2^16-1>;
       };
   } ServerHello;
]]></artwork></figure>

<t>The presence of extensions can be detected by determining whether there are
bytes following the cipher_suite field at the end of the ServerHello.</t>

<t><list style="hanging">
  <t hangText='server_version'><vspace blankLines='0'/>
  This field will contain the lower of that suggested by the client
in the ClientHello and the highest supported by the server.  For
this version of the specification, the version is 3.4.  (See
<xref target="backward-compatibility"/> for details about backward compatibility.)</t>
  <t hangText='random'><vspace blankLines='0'/>
  This structure is generated by the server and MUST be
generated independently of the ClientHello.random.</t>
  <t hangText='session_id_len'><vspace blankLines='0'/>
  A single 0 value for backward compatible formatting.
[[OPEN ISSUE: Should we remove?]]</t>
  <t hangText='cipher_suite'><vspace blankLines='0'/>
  The single cipher suite selected by the server from the list in
ClientHello.cipher_suites.  For resumed sessions, this field is
the value from the state of the session being resumed.
[[TODO: interaction with PSK.]]</t>
  <t hangText='extensions'><vspace blankLines='0'/>
  A list of extensions.  Note that only extensions offered by the
client can appear in the server’s list. In TLS 1.3 as opposed to
previous versions of TLS, the server’s extensions are split between
the ServerHello and the EncryptedExtensions <xref target="encrypted-extensions"/>
message. The ServerHello
MUST only include extensions which are required to establish
the cryptographic context.</t>
</list></t>

</section>
<section anchor="hello-retry-request" title="Hello Retry Request">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>The server will send this message in response to a ClientHello
message when it was able to find an acceptable set of algorithms and
groups that are mutually supported, but
the client’s ClientKeyShare did not contain an acceptable
offer. If it cannot find such a match, it will respond with a
“handshake_failure” alert.</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   struct {
       ProtocolVersion server_version;
       CipherSuite cipher_suite;
       NamedGroup selected_group;
       Extension extensions<0..2^16-1>;
   } HelloRetryRequest;
]]></artwork></figure>

<t>[[OPEN ISSUE: Merge in DTLS Cookies?]]</t>

<t><list style="hanging">
  <t hangText='selected_group'><vspace blankLines='0'/>
  The group which the client MUST use for its new ClientHello.</t>
</list></t>

<t>The “server_version”, “cipher_suite” and “extensions” fields have the
same meanings as their corresponding values in the ServerHello. The
server SHOULD send only the extensions necessary for the client to
generate a correct ClientHello pair.</t>

<t>Upon receipt of a HelloRetryRequest, the client MUST first verify
that the “selected_group” field does not identify a group which
was not in the original ClientHello. If it was present, then
the client MUST abort the handshake with a fatal “handshake_failure”
alert. Clients SHOULD also abort with “handshake_failure” in response to any second
HelloRetryRequest which was sent in the same connection (i.e.,
where the ClientHello was itself in response to a HelloRetryRequest).</t>

<t>Otherwise, the client MUST send a ClientHello with a new
ClientKeyShare extension to the server. The ClientKeyShare MUST append
a new ClientKeyShareOffer which is consistent with the
“selected_group” field to the groups in the original ClientKeyShare.</t>

<t>Upon re-sending the ClientHello and receiving the
server’s ServerHello/ServerKeyShare, the client MUST verify that
the selected CipherSuite and NamedGroup match that supplied in
the HelloRetryRequest.</t>

</section>
<section anchor="hello-extensions" title="Hello Extensions">

<t>The extension format is:</t>

<figure><artwork><![CDATA[
   struct {
       ExtensionType extension_type;
       opaque extension_data<0..2^16-1>;
   } Extension;

   enum {
       signature_algorithms(13), 
       early_data(TBD),
       supported_groups(TBD),
       known_configuration(TBD),
       pre_shared_key(TBD)
       client_key_shares(TBD)
       (65535)
   } ExtensionType;
]]></artwork></figure>

<t>Here:</t>

<t><list style="symbols">
  <t>“extension_type” identifies the particular extension type.</t>
  <t>“extension_data” contains information specific to the particular
  extension type.</t>
</list></t>

<t>The initial set of extensions is defined in <xref target="RFC6066"/>.
The list of extension types is maintained by IANA as described in
<xref target="iana-considerations"/>.</t>

<t>An extension type MUST NOT appear in the ServerHello or HelloRetryRequest unless the same extension
type appeared in the corresponding ClientHello. If a client receives an
extension type in ServerHello or HelloRetryRequest that it did not request in the associated
ClientHello, it MUST abort the handshake with an “unsupported_extension” fatal
alert.</t>

<t>Nonetheless, “server-oriented” extensions may be provided in the future within
this framework. Such an extension (say, of type x) would require the client to
first send an extension of type x in a ClientHello with empty extension_data to
indicate that it supports the extension type. In this case, the client is
offering the capability to understand the extension type, and the server is
taking the client up on its offer.</t>

<t>When multiple extensions of different types are present in the ClientHello or
ServerHello messages, the extensions MAY appear in any order. There MUST NOT be
more than one extension of the same type.</t>

<t>Finally, note that extensions can be sent both when starting a new session and
when requesting session resumption or 0-RTT mode. Indeed, a client that requests session
resumption does not in general know whether the server will accept this
request, and therefore it SHOULD send the same extensions as it would send if
it were not attempting resumption.</t>

<t>In general, the specification of each extension type needs to describe the
effect of the extension both during full handshake and session resumption. Most
current TLS extensions are relevant only when a session is initiated: when an
older session is resumed, the server does not process these extensions in
ClientHello, and does not include them in ServerHello. However, some
extensions may specify different behavior during session resumption.
[[TODO: update this and the previous paragraph to cover PSK-based resumption.]] </t>

<t>There are subtle (and not so subtle) interactions that may occur in this
protocol between new features and existing features which may result in a
significant reduction in overall security. The following considerations should
be taken into account when designing new extensions:</t>

<t><list style="symbols">
  <t>Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular features. In
general, error alerts should be used for the former, and a field in the
server extension response for the latter.</t>
  <t>Extensions should, as far as possible, be designed to prevent any attack that
forces use (or non-use) of a particular feature by manipulation of handshake
messages. This principle should be followed regardless of whether the feature
is believed to cause a security problem.
Often the fact that the extension fields are included in the inputs to the
Finished message hashes will be sufficient, but extreme care is needed when
the extension changes the meaning of messages sent in the handshake phase.
Designers and implementors should be aware of the fact that until the
handshake has been authenticated, active attackers can modify messages and
insert, remove, or replace extensions.</t>
  <t>It would be technically possible to use extensions to change major aspects
of the design of TLS; for example the design of cipher suite negotiation.
This is not recommended; it would be more appropriate to define a new version
of TLS — particularly since the TLS handshake algorithms have specific
protection against version rollback attacks based on the version number, and
the possibility of version rollback should be a significant consideration in
any major design change.</t>
</list></t>

<section anchor="signature-algorithms" title="Signature Algorithms">

<t>The client uses the “signature_algorithms” extension to indicate to the server
which signature/hash algorithm pairs may be used in digital signatures. The
“extension_data” field of this extension contains a
“supported_signature_algorithms” value.</t>

<figure><artwork><![CDATA[
   enum {
       none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
       sha512(6), (255)
   } HashAlgorithm;

   enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
     SignatureAlgorithm;

   struct {
         HashAlgorithm hash;
         SignatureAlgorithm signature;
   } SignatureAndHashAlgorithm;

   SignatureAndHashAlgorithm
     supported_signature_algorithms<2..2^16-2>;
]]></artwork></figure>

<t>Each SignatureAndHashAlgorithm value lists a single hash/signature pair that
the client is willing to verify. The values are indicated in descending order
of preference.</t>

<t>Note: Because not all signature algorithms and hash algorithms may be accepted
by an implementation (e.g., DSA with SHA-1, but not SHA-256), algorithms here
are listed in pairs.</t>

<t><list style="hanging">
  <t hangText='hash'><vspace blankLines='0'/>
  This field indicates the hash algorithm which may be used.  The
values indicate support for unhashed data, MD5 <xref target="RFC1321"/>, SHA-1,
SHA-224, SHA-256, SHA-384, and SHA-512 <xref target="SHS"/>, respectively.  The
“none” value is provided for future extensibility, in case of a
signature algorithm which does not require hashing before signing.</t>
  <t hangText='signature'><vspace blankLines='0'/>
  This field indicates the signature algorithm that may be used.
The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
<xref target="RFC3447"/> and DSA <xref target="DSS"/>, and ECDSA <xref target="ECDSA"/>, respectively.  The
“anonymous” value is meaningless in this context but used in
<xref target="server-key-share"/>.  It MUST NOT appear in this extension.</t>
</list></t>

<t>The semantics of this extension are somewhat complicated because the cipher
suite indicates permissible signature algorithms but not hash algorithms.
<xref target="server-certificate"/> and <xref target="server-key-share"/> describe the
appropriate rules.</t>

<t>If the client supports only the default hash and signature algorithms (listed
in this section), it MAY omit the signature_algorithms extension. If the client
does not support the default algorithms, or supports other hash and signature
algorithms (and it is willing to use them for verifying messages sent by the
server, i.e., server certificates and server key share), it MUST send the
signature_algorithms extension, listing the algorithms it is willing to accept.</t>

<t>If the client does not send the signature_algorithms extension, the server MUST
do the following:</t>

<t><list style="symbols">
  <t>If the negotiated key exchange algorithm is one of (DHE_RSA, ECDHE_RSA), behave as if client had sent the value
{sha1,rsa}.</t>
  <t>If the negotiated key exchange algorithm is DHE_DSS, behave
as if the client had sent the value {sha1,dsa}.</t>
  <t>If the negotiated key exchange algorithm is ECDHE_ECDSA,
behave as if the client had sent value {sha1,ecdsa}.</t>
</list></t>

<t>Note: This extension is not meaningful for TLS versions prior to 1.2. Clients
MUST NOT offer it if they are offering prior versions. However, even if clients
do offer it, the rules specified in <xref target="RFC6066"/> require servers to ignore
extensions they do not understand.</t>

<t>Servers MUST NOT send this extension. TLS servers MUST support receiving this
extension.</t>

</section>
<section anchor="negotiated-groups" title="Negotiated Groups">

<t>When sent by the client, the “supported_groups” extension indicates
the named groups which the client supports, ordered from most
preferred to least preferred.</t>

<t>Note: In versions of TLS prior to TLS 1.3, this extension was named
“elliptic curves” and only contained elliptic curve groups. See
<xref target="RFC4492"/> and <xref target="I-D.ietf-tls-negotiated-ff-dhe"/>.</t>

<t>The “extension_data” field of this extension SHALL contain a
“NamedGroupList” value:</t>

<figure><artwork><![CDATA[
   enum {
       // Elliptic Curve Groups.
       sect163k1 (1), sect163r1 (2), sect163r2 (3),
       sect193r1 (4), sect193r2 (5), sect233k1 (6),
       sect233r1 (7), sect239k1 (8), sect283k1 (9),
       sect283r1 (10), sect409k1 (11), sect409r1 (12),
       sect571k1 (13), sect571r1 (14), secp160k1 (15),
       secp160r1 (16), secp160r2 (17), secp192k1 (18),
       secp192r1 (19), secp224k1 (20), secp224r1 (21),
       secp256k1 (22), secp256r1 (23), secp384r1 (24),
       secp521r1 (25),

       // Finite Field Groups.
       ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
       ffdhe6144 (259), ffdhe8192 (260),
       ffdhe_private_use (0x01FC..0x01FF),

       // Reserved Code Points.
       reserved (0xFE00..0xFEFF),
       reserved(0xFF01),
       reserved(0xFF02),
       (0xFFFF)
   } NamedGroup;

   struct {
       NamedGroup named_group_list<1..2^16-1>;
   } NamedGroupList;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='sect163k1, etc'><vspace blankLines='0'/>
  Indicates support of the corresponding named curve
The named curves defined here are those specified in SEC 2 [13].
Note that many of these curves are also recommended in ANSI
X9.62 <xref target="X962"/> and FIPS 186-2 <xref target="DSS"/>.  Values 0xFE00 through 0xFEFF are
reserved for private use.  Values 0xFF01 and 0xFF02 were used in
previous versions of TLS but MUST NOT be offered by TLS 1.3
implementations.
[[OPEN ISSUE: Triage curve list.]]</t>
  <t hangText='ffdhe2432, etc'><vspace blankLines='0'/>
  Indicates support of the corresponding finite field
group, defined in <xref target="I-D.ietf-tls-negotiated-ff-dhe"/></t>
</list></t>

<t>Items in named_curve_list are ordered according to the client’s
preferences (favorite choice first).</t>

<t>As an example, a client that only supports secp192r1 (aka NIST P-192;
value 19 = 0x0013) and secp224r1 (aka NIST P-224; value 21 = 0x0015)
and prefers to use secp192r1 would include a TLS extension consisting
of the following octets.  Note that the first two octets indicate the
extension type (Supported Group Extension):</t>

<figure><artwork><![CDATA[
   00 0A 00 06 00 04 00 13 00 15
]]></artwork></figure>

<t>The client MUST supply a “named_groups” extension containing at
least one group for each key exchange algorithm (currently
DHE and ECDHE) for which it offers a cipher suite.
If the client does not supply a “named_groups” extension with a
compatible group, the server MUST NOT negotiate a cipher suite of the
relevant type.  For instance, if a client supplies only ECDHE groups,
the server MUST NOT negotiate finite field Diffie-Hellman.  If no
acceptable group can be selected across all cipher suites, then the
server MUST generate a fatal “handshake_failure” alert.</t>

<t>NOTE: A server participating in an ECDHE-ECDSA key exchange may use
different curves for (i) the ECDSA key in its certificate, and (ii)
the ephemeral ECDH key in the ServerKeyExchange message.  The server
must consider the supported groups in both cases.</t>

<t>[[TODO: IANA Considerations.]]</t>

</section>
</section>
<section anchor="client-key-share" title="Client Key Share">

<t>The client_key_share extension MUST be provided by the client if it
offers any cipher suites that involve non-PSK (currently DHE or
ECDHE) key exchange.  It contains the client’s cryptographic parameters
for zero or more key establishment methods. [[OPEN ISSUE: Would it
be better to omit it if it’s empty?.
https://github.com/tlswg/tls13-spec/issues/190]]</t>

<t>Meaning of this message:</t>

<figure><artwork><![CDATA[
   struct {
       NamedGroup group;
       opaque key_exchange<1..2^16-1>;
   } ClientKeyShareOffer;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='group'><vspace blankLines='0'/>
  The named group for the key share offer.  This identifies the
specific key exchange method that the ClientKeyShareOffer describes.
Finite Field Diffie-Hellman <xref target="DH"/> parameters are described in
<xref target="ffdhe-param"/>; Elliptic Curve Diffie-Hellman parameters are
described in <xref target="ecdhe-param"/>.</t>
  <t hangText='key_exchange'><vspace blankLines='0'/>
  Key exchange information.  The contents of this field are
determined by the value of NamedGroup entry and its corresponding
definition.</t>
</list></t>

<figure><artwork><![CDATA[
   struct {
       ClientKeyShareOffer offers<0..2^16-1>;
   } ClientKeyShare;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='offers'><vspace blankLines='0'/>
  A list of ClientKeyShareOffer values in descending order of
client preference.</t>
</list></t>

<t>Clients may offer an arbitrary number of ClientKeyShareOffer
values, each representing a single set of key agreement parameters;
for instance a client might offer shares for several elliptic curves
or multiple integer DH groups. The shares for each ClientKeyShareOffer
MUST by generated independently. Clients MUST NOT offer multiple
ClientKeyShareOffers for the same parameters. It is explicitly
permitted to send an empty client_key_share extension as this is used
to elicit the server’s parameters if the client has no useful
information.
[TODO: Recommendation about what the client offers. Presumably which integer
DH groups and which curves.]</t>

<section anchor="ffdhe-param" title="Diffie-Hellman Parameters">

<t>Diffie-Hellman <xref target="DH"/> parameters for both clients and servers are encoded in
the opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value contains the
Diffie-Hellman public value (dh_Y = g^X mod p),
encoded as a big-endian integer.</t>

<figure><artwork><![CDATA[
   opaque dh_Y<1..2^16-1>;
]]></artwork></figure>

</section>
<section anchor="ecdhe-param" title="ECDHE Parameters">

<t>ECDHE parameters for both clients and servers are encoded in the
opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value conveys the Elliptic
Curve Diffie-Hellman public value (ecdh_Y) represented as a byte
string ECPoint.point.</t>

<figure><artwork><![CDATA[
   opaque point <1..2^8-1>;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='point'><vspace blankLines='0'/>
  This is the byte string representation of an elliptic curve
point following the conversion routine in Section 4.3.6 of ANSI
X9.62 <xref target="X962"/>.</t>
</list></t>

<t>Although X9.62 supports multiple point formats, any given curve
MUST specify only a single point format. All curves currently
specified in this document MUST only be used with the uncompressed
point format.</t>

<t>Note: Versions of TLS prior to 1.3 permitted point negotiation;
TLS 1.3 removes this feature in favor of a single point format
for each curve.</t>

<t>[[OPEN ISSUE: We will need to adjust the compressed/uncompressed point issue
if we have new curves that don’t need point compression. This depends
on the CFRG’s recommendations. The expectation is that future curves will
come with defined point formats and that existing curves conform to
X9.62.]]</t>

</section>
<section anchor="known-configuration-extension" title="Known Configuration Extension">

<t>The known_configuration extension allows the client to indicate that
it wishes to reuse the server’s known configuration and semi-static
(EC)DHE key (see <xref target="server-configuration"/> for how to establish these
configurations. This extension allows the omission of the server
certificate and signature, with three potential benefits:</t>

<t><list style="symbols">
  <t>Shortening the handshake because the certificate may be large.</t>
  <t>Reducing cryptographic burden on the server if the server has
an RSA certificate, as well as on the client if the server has an ECDSA certificate.</t>
  <t>Allowing the client and server to do a 0-RTT exchange (See <xref target="zero-rtt-exchange"/>)</t>
</list></t>

<t>The extension is defined as:</t>

<figure><artwork><![CDATA[
      struct {
        select (Role) {
          case client:
            opaque identifier<0..2^16-1>;

          case server:
            struct {};
        }
      } KnownConfigurationExtension
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='identifier'><vspace blankLines='0'/>
  An opaque label for the configuration in question.</t>
</list></t>

<t>A client which wishes to reuse a known configuration MAY supply a
single KnownConfigurationExtension value which indicates the known
configuration it desires to use. It is a fatal error to supply more
than one extension. A server which wishes to use the key replies with
an empty extension (i.e., with a length field of 0) in its ServerHello.</t>

<t>When the client and server mutually agree upon a known configuration via this
mechanism, then the Static Secret (SS) is computed based on the server’s (EC)DHE
key from the identified configuration and the client’s key found in the
ClientKeyShare. If no key from an acceptable group is in the ClientKeyShare,
the server MUST ignore the known_configuration extension. When this
mechanism is used, the server MUST NOT send a Certificate/CertificateVerify
message unless the ServerConfiguration message is also sent.</t>

<t>When the known_configuration data extension is in use, the handshake hash
is extended to include the server’s configuration data and certificate
(see <xref target="the-handshake-hash"/>) so as to tightly bind them together.</t>

</section>
<section anchor="pre-shared-key-extension" title="Pre-Shared Key Extension">

<t>The pre_shared_key extension is used to indicate the identity of the
pre-shared key to be used with a given handshake in association
with a PSK or (EC)DHE-PSK cipher suite (see <xref target="RFC4279"/> for background).</t>

<figure><artwork><![CDATA[
      opaque psk_identity<0..2^16-1>;

      struct {
        select (Role) {
          case client:           
            psk_identity identities<0..2^16-1>;

          case server:
            psk_identity identity;

      } PreSharedKeyExtension;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='identifier'><vspace blankLines='0'/>
  An opaque label for the pre-shared key.</t>
</list></t>

<t>When the client offers a PSK cipher suite, it MUST also supply a
PreSharedKeyExtension to indicate the PSK(s) to be used. If no
such extension is present, the server MUST NOT negotiate
a PSK cipher suite. If no suitable identity is present, the server
MUST NOT negotiate a PSK cipher suite.</t>

<t>If the server selects a PSK cipher suite, it MUST send a
PreSharedKeyExtension with the identity that it selected.
The client MUST verify that the server has selected one of
the identities that the client supplied. If any other identity
is returned, the client MUST generate a fatal “handshake_failure”
alert.</t>

</section>
<section anchor="early-data-indication" title="Early Data Indication">

<t>In cases where TLS clients have previously interacted with the 
server and the server has supplied a known configuration, the client
can send application data and its Certificate/CertificateVerify
messages (if client authentication is required). If the client
opts to do so, it MUST supply an Early Data Indication
extension. This technique MUST only be used along with
the “known_configuration” extension.</t>

<figure><artwork><![CDATA[
      enum { early_handshake(1), early_data(2),
             early_handshake_and_data(3), (255) } EarlyDataType;
           
      struct {
        select (Role) {
          case client:
            opaque context<0..255>;
            EarlyDataType type;
          case server:
            struct {};                
        }
      } EarlyDataIndication;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='context'><vspace blankLines='0'/>
  An optional context value that can be used for anti-replay
(see below).</t>
  <t hangText='type'><vspace blankLines='0'/>
  The type of early data that is being sent. “early_handshake”
means that only handshake data is being sent. “early_data”
means that only data is being sent. “early_handshake_and_data”
means that both are being sent.</t>
</list></t>

<t>If TLS client authentication is being used, then either
“early_handshake” or “early_handshake_and_data” MUST be indicated in
order to send the client authentication data on the first flight. In
either case, the client Certificate and CertificateVerify (assuming
that the Certificate is non-empty) MUST be sent on the first flight A
server which receives an initial flight with only “early_data” and
which expects certificate-based client authentication MUST not
accept early data.</t>

<t>In order to allow servers to readily distinguish between messages sent
in the first flight and in the second flight (in cases where the
server does not accept the EarlyDataIndication extension), the client MUST
send the handshake messages as content type
“early_handshake”. A server which does not accept the extension
proceeds by skipping all records after the ClientHello and until
the next client message of type “handshake”.
[[OPEN ISSUE: This relies on content types
not being encrypted. If we had content types that were
encrypted, this would basically require trial decryption.]]</t>

<t>A server which receives an EarlyDataIndication extension
can behave in one of two ways:</t>

<t><list style="symbols">
  <t>Ignore the extension and return no response. This indicates
that the server has ignored any early data and an ordinary
1-RTT handshake is required.</t>
  <t>Return an empty extension, indicating that it intends to
process the early data. It is not possible for the server
to accept only a subset of the early data messages.</t>
</list></t>

<t>The server MUST first validate that the client’s “known_configuration”
extension is valid and that the client has suppled a valid
key share in the “client_key_shares” extension. If not, it MUST
ignore the extension and discard the early handshake data
and early data. </t>

<t>[[TODO: How does the client behave if the indication is rejected.]]</t>

<t>[[OPEN ISSUE: This just specifies the signaling for 0-RTT but
not the the 0-RTT cryptographic transforms, including:</t>

<t><list style="symbols">
  <t>What is in the handshake hash (including potentially some
speculative data from the server.)</t>
  <t>What is signed in the client’s CertificateVerify</t>
  <t>Whether we really want the Finished to not include the
server’s data at all.</t>
</list></t>

<t>What’s here now needs a lot of cleanup before it is clear
and correct.]]</t>

<t>[[TODO: We should really allow early_data to be used with
PSKs. In order to make this work, we need to either:</t>

<t>(a) explicitly signal the entire cryptographic parameter set
(b) tie it to the PSK identifier (as is presently done in the
    known_configuration extension).</t>

<t>These two should match.
]]</t>

<section anchor="replay-properties" title="Replay Properties">

<t>As noted in <xref target="zero-rtt-exchange"/>, TLS does not provide any
inter-connection mechanism for replay protection for data sent by the
client in the first flight.  As a special case, implementations where
the server configuration, is delivered out of band (as has been
proposed for DTLS-SRTP <xref target="RFC5763"/>), MAY use a unique server
configuration identifier for each connection, thus preventing
replay. Implementations are responsible for ensuring uniqueness of the
identifier in this case.</t>

</section>
</section>
</section>
</section>
<section anchor="server-key-share" title="Server Key Share">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>This message will be sent immediately after the ServerHello message if
the client has provided a ClientKeyShare extension which is compatible
with the selected cipher suite and group parameters.</t>
</list></t>

<t>Meaning of this message:</t>

<t><list style='empty'>
  <t>This message conveys cryptographic information to allow the client to
compute a shared secret secret: a Diffie-Hellman public key with which the
client can complete a key exchange (with the result being the shared secret)
or a public key for some other algorithm.</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   struct {
       NamedGroup group;
       opaque key_exchange<1..2^16-1>;
   } ServerKeyShare;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='group'><vspace blankLines='0'/>
  The named group for the key share offer.  This identifies the
selected key exchange method from the ClientKeyShare
(<xref target="client-key-share"/>), identifying which value from the
ClientKeyShareOffer the server has accepted as is responding to.</t>
  <t hangText='key_exchange'><vspace blankLines='0'/>
  Key exchange information.  The contents of this field are
determined by the value of NamedGroup entry and its corresponding
definition.</t>
</list></t>

</section>
<section anchor="encrypted-extensions" title="Encrypted Extensions">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>If this message is sent, it MUST be sent immediately after the server’s
ServerKeyShare. This is the first message that is encrypted under keys
derived from ES.</t>
</list></t>

<t>Meaning of this message:</t>

<t><list style='empty'>
  <t>The EncryptedExtensions message simply contains any extensions
which should be protected, i.e., any which are not needed to
establish the cryptographic context. The same extension types
MUST NOT appear in both the ServerHello and EncryptedExtensions.
If the same extension appears in both locations, the client
MUST rely only on the value in the EncryptedExtensions block.
[[OPEN ISSUE: Should we just produce a canonical list of what
goes where and have it be an error to have it in the wrong
place? That seems simpler. Perhaps have a whitelist of which
extensions can be unencrypted and everything else MUST be
encrypted.]]</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   struct {
       Extension extensions<0..2^16-1>;
   } EncryptedExtensions;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='extensions'><vspace blankLines='0'/>
  A list of extensions.</t>
</list></t>

</section>
<section anchor="server-certificate" title="Server Certificate">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>The server MUST send a Certificate message whenever the agreed-upon
key exchange method uses certificates for authentication (this
includes all key exchange methods defined in this document except
DH_anon and PSK), unless the KnownKeyExtension is used. This message will
always immediately follow either the EncryptedExtensions message if
one is sent or the ServerKeyShare message.</t>
</list></t>

<t>Meaning of this message:</t>

<t><list style='empty'>
  <t>This message conveys the server’s certificate chain to the client.</t>
</list></t>

<t><list style='empty'>
  <t>The certificate MUST be appropriate for the negotiated cipher suite’s key
exchange algorithm and any negotiated extensions.</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   opaque ASN1Cert<1..2^24-1>;

   struct {
       ASN1Cert certificate_list<0..2^24-1>;
   } Certificate;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='certificate_list'><vspace blankLines='0'/>
  This is a sequence (chain) of certificates.  The sender’s
certificate MUST come first in the list.  Each following
certificate MUST directly certify the one preceding it.  Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority MAY be omitted from the chain, under the
assumption that the remote end must already possess it in order to
validate it in any case.</t>
</list></t>

<t>The same message type and structure will be used for the client’s response to a
certificate request message. Note that a client MAY send no certificates if it
does not have an appropriate certificate to send in response to the server’s
authentication request.</t>

<t>Note: PKCS #7 <xref target="PKCS7"/> is not used as the format for the certificate vector
because PKCS #6 <xref target="PKCS6"/> extended certificates are not used. Also, PKCS #7
defines a SET rather than a SEQUENCE, making the task of parsing the list more
difficult.</t>

<t>The following rules apply to the certificates sent by the server:</t>

<t><list style="symbols">
  <t>The certificate type MUST be X.509v3 <xref target="RFC5280"/>, unless explicitly negotiated
  otherwise (e.g., <xref target="RFC5081"/>).</t>
  <t>The end entity certificate’s public key (and associated
  restrictions) MUST be compatible with the selected key exchange
  algorithm.</t>
</list></t>

<figure><artwork><![CDATA[
    Key Exchange Alg.  Certificate Key Type

    DHE_RSA            RSA public key; the certificate MUST allow the
    ECDHE_RSA          key to be used for signing (the
                       digitalSignature bit MUST be set if the key
                       usage extension is present) with the signature
                       scheme and hash algorithm that will be employed
                       in the server key exchange message.
                       Note: ECDHE_RSA is defined in [RFC4492].

    DHE_DSS            DSA public key; the certificate MUST allow the
                       key to be used for signing with the hash
                       algorithm that will be employed in the server
                       key exchange message.

    ECDHE_ECDSA        ECDSA-capable public key; the certificate MUST
                       allow the key to be used for signing with the
                       hash algorithm that will be employed in the
                       server key exchange message.  The public key
                       MUST use a curve and point format supported by
                       the client, as described in  [RFC4492].
]]></artwork></figure>

<t><list style="symbols">
  <t>The “server_name” and “trusted_ca_keys” extensions <xref target="RFC6066"/> are used to
guide certificate selection. As servers MAY require the presence of the server_name
extension, clients SHOULD send this extension.</t>
</list></t>

<t>If the client provided a “signature_algorithms” extension, then all
certificates provided by the server MUST be signed by a hash/signature
algorithm pair that appears in that extension. Note that this implies that a
certificate containing a key for one signature algorithm MAY be signed using a
different signature algorithm (for instance, an RSA key signed with a DSA key).</t>

<t>If the server has multiple certificates, it chooses one of them based on the
above-mentioned criteria (in addition to other criteria, such as transport
layer endpoint, local configuration and preferences, etc.). If the server has a
single certificate, it SHOULD attempt to validate that it meets these criteria.</t>

<t>Note that there are certificates that use algorithms and/or algorithm
combinations that cannot be currently used with TLS. For example, a certificate
with RSASSA-PSS signature key (id-RSASSA-PSS OID in SubjectPublicKeyInfo)
cannot be used because TLS defines no corresponding signature algorithm.</t>

<t>As cipher suites that specify new key exchange methods are specified for the
TLS protocol, they will imply the certificate format and the required encoded
keying information.</t>

</section>
<section anchor="certificate-request" title="Certificate Request">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>A non-anonymous server can optionally request a certificate from the client,
if appropriate for the selected cipher suite. This message, if sent, will
immediately follow the server’s Certificate message).</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   enum {
       rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
       rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
       fortezza_dms_RESERVED(20), (255)
   } ClientCertificateType;

   opaque DistinguishedName<1..2^16-1>;

   struct {
       ClientCertificateType certificate_types<1..2^8-1>;
       SignatureAndHashAlgorithm
         supported_signature_algorithms<2..2^16-2>;
       DistinguishedName certificate_authorities<0..2^16-1>;
   } CertificateRequest;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='certificate_types'><vspace blankLines='0'/>
  A list of the types of certificate types that the client may
offer.

      <figure><artwork><![CDATA[
    rsa_sign        a certificate containing an RSA key
    dss_sign        a certificate containing a DSA key
    rsa_fixed_dh    a certificate containing a static DH key.
    dss_fixed_dh    a certificate containing a static DH key
]]></artwork></figure>
  </t>
  <t hangText='supported_signature_algorithms'><vspace blankLines='0'/>
  A list of the hash/signature algorithm pairs that the server is
able to verify, listed in descending order of preference.</t>
  <t hangText='certificate_authorities'><vspace blankLines='0'/>
  A list of the distinguished names <xref target="X501"/> of acceptable
certificate_authorities, represented in DER-encoded format.  These
distinguished names may specify a desired distinguished name for a
root CA or for a subordinate CA; thus, this message can be used to
describe known roots as well as a desired authorization space.  If
the certificate_authorities list is empty, then the client MAY
send any certificate of the appropriate ClientCertificateType,
unless there is some external arrangement to the contrary.</t>
</list></t>

<t>The interaction of the certificate_types and
supported_signature_algorithms fields is somewhat complicated.
certificate_types has been present in TLS since SSL 3.0, but was
somewhat underspecified.  Much of its functionality is superseded by
supported_signature_algorithms.  The following rules apply:</t>

<t><list style="symbols">
  <t>Any certificates provided by the client MUST be signed using a
  hash/signature algorithm pair found in
  supported_signature_algorithms.</t>
  <t>The end-entity certificate provided by the client MUST contain a
  key that is compatible with certificate_types.  If the key is a
  signature key, it MUST be usable with some hash/signature
  algorithm pair in supported_signature_algorithms.</t>
  <t>For historical reasons, the names of some client certificate types
  include the algorithm used to sign the certificate.  For example,
  in earlier versions of TLS, rsa_fixed_dh meant a certificate
  signed with RSA and containing a static DH key.  In TLS 1.2, this
  functionality has been obsoleted by the
  supported_signature_algorithms, and the certificate type no longer
  restricts the algorithm used to sign the certificate.  For
  example, if the server sends dss_fixed_dh certificate type and
  {{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
  with a certificate containing a static DH key, signed with RSA-
  SHA1.</t>
</list></t>

<t>New ClientCertificateType values are assigned by IANA as described in
<xref target="iana-considerations"/>.</t>

<t>Note: Values listed as RESERVED MUST NOT be used. They were used in SSL 3.0.</t>

<t>Note: It is a fatal “handshake_failure” alert for an anonymous server to request
client authentication.</t>

</section>
<section anchor="server-configuration" title="Server Configuration">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>This message is used to provide a server configuration which
the client can use in future to skip handshake negotiation and
(optionally) to allow 0-RTT handshakes. The ServerConfiguration
message is sent as the last message before the CertificateVerify.</t>
</list></t>

<t>Structure of this Message:</t>

<figure><artwork><![CDATA[
      struct {
          opaque configuration_id<1..2^16-1>;
          uint32 expiration_date;
          NamedGroup group;
          opaque server_key<1..2^16-1>;
          Boolean early_data_allowed;
      } ServerConfiguration;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='configuration_id'><vspace blankLines='0'/>
  The configuration identifier to be used with the known configuration
extension <xref target="known-configuration-extension"/>.</t>
  <t hangText='group'><vspace blankLines='0'/>
  The group for the long-term DH key that is being established
for this configuration.</t>
  <t hangText='expiration_date'><vspace blankLines='0'/>
  The last time when this configuration is expected to be valid
(in seconds since the Unix epoch). Servers MUST NOT use any value
more than 604800 seconds (7 days) in the future. Clients MUST
not cache configurations for longer than 7 days, regardless of
the expiration_date. [[OPEN ISSUE: Is this the right value?
The idea is just to minimize exposure.]]</t>
  <t hangText='server_key'><vspace blankLines='0'/>
  The long-term DH key that is being established for this configuration.</t>
  <t hangText='early_data_allowed'><vspace blankLines='0'/>
  Whether the client may send data in its first flight (see <xref target="early-data-indication"/>).</t>
</list></t>

<t>The semantics of this message are to establish a shared state between
the client and server for use with the “known_configuration” extension
with the key specified in key and with the handshake parameters negotiated
by this handshake. [[OPEN ISSUE: Should this allow some sort of parameter
negotiation?]]</t>

<t>When the ServerConfiguration message is sent, the server MUST also
send a Certificate message and a CertificateVerify message, even
if the “known_configuration” extension was used for this handshake,
thus requiring a signature over the configuration before it can
be used by the client.</t>

</section>
<section anchor="server-certificate-verify" title="Server Certificate Verify">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>This message is used to provide explicit proof that the server
possesses the private key corresponding to its certificate
and also provides integrity for the handshake up
to this point. This message is only sent when the server is
authenticated via a certificate. When sent, it MUST be the
last server handshake message prior to the Finished.</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   struct {
        digitally-signed struct {
           opaque handshake_hash[hash_length];
        }
   } CertificateVerify;
]]></artwork></figure>

<t><list style='empty'>
  <t>Where session_hash is as described in {{the-handshake-hash} and
includes the messages sent or received, starting at ClientHello and up
to, but not including, this message, including the type and length
fields of the handshake messages. This is a digest of the
concatenation of all the Handshake structures (as defined in
<xref target="handshake-protocol"/>) exchanged thus far. The digest MUST be the
Hash used as the basis for HKDF.</t>
</list></t>

<t><list style='empty'>
  <t>The context string for the signature is “TLS 1.3, server CertificateVerify”. A
hash of the handshake messages is signed rather than the messages themselves
because the digitally-signed format requires padding and context bytes at the
beginning of the input. Thus, by signing a digest of the messages, an
implementation need only maintain one running hash per hash type for
CertificateVerify, Finished and other messages.</t>
</list></t>

<t><list style='empty'>
  <t>If the client has offered the “signature_algorithms” extension, the signature
algorithm and hash algorithm MUST be a pair listed in that extension. Note that
there is a possibility for inconsistencies here. For instance, the client might
offer DHE_DSS key exchange but omit any DSA pairs from its
“signature_algorithms” extension. In order to negotiate correctly, the server
MUST check any candidate cipher suites against the “signature_algorithms”
extension before selecting them. This is somewhat inelegant but is a compromise
designed to minimize changes to the original cipher suite design.</t>
</list></t>

<t><list style='empty'>
  <t>In addition, the hash and signature algorithms MUST be compatible with the key
in the server’s end-entity certificate. RSA keys MAY be used with any permitted
hash algorithm, subject to restrictions in the certificate, if any.</t>
</list></t>

<t><list style='empty'>
  <t>Because DSA signatures do not contain any secure indication of hash
algorithm, there is a risk of hash substitution if multiple hashes may be used
with any key. Currently, DSA <xref target="DSS"/> may only be used with SHA-1. Future
revisions of DSS <xref target="DSS-3"/> are expected to allow the use of other digest
algorithms with DSA, as well as guidance as to which digest algorithms should
be used with each key size. In addition, future revisions of <xref target="RFC5280"/> may
specify mechanisms for certificates to indicate which digest algorithms are to
be used with DSA.
[[TODO: Update this to deal with DSS-3 and DSS-4.
https://github.com/tlswg/tls13-spec/issues/59]]</t>
</list></t>

</section>
<section anchor="server-finished" title="Server Finished">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>The Server’s Finished message is the final message sent by the
server and is essential for providing authentication of the server
side of the handshake and computed keys.</t>
</list></t>

<t>Meaning of this message:</t>

<t><list style='empty'>
  <t>Recipients of Finished messages MUST verify that the contents are
correct. Once a side has sent its Finished message and received and
validated the Finished message from its peer, it may begin to send and
receive application data over the connection. This data will be
protected under keys derived from the ephemeral secret (see
<xref target="cryptographic-computations"/>).</t>
</list></t>

<t>Structure of this message:</t>

<figure><artwork><![CDATA[
   struct {
       opaque verify_data[verify_data_length];
   } Finished;
]]></artwork></figure>

<t>The verify_data value is computed as follows:</t>

<t><list style="hanging">
  <t hangText='verify_data'><vspace blankLines='0'/>
  HMAC(finished_secret, finished_label + ‘\0’ + handshake_hash)
where HMAC uses the Hash algorithm for the handshake.
See <xref target="the-handshake-hash"/> for the definition of
handshake_hash.</t>
  <t hangText='finished_label'><vspace blankLines='0'/>
  For Finished messages sent by the client, the string
“client finished”.  For Finished messages sent by the server,
the string “server finished”.</t>
</list></t>

<t><list style='empty'>
  <t>In previous versions of TLS, the verify_data was always 12 octets long. In
the current version of TLS, it is the size of the HMAC output for the
Hash used for the handshake.</t>
</list></t>

<t>Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations. Also, HelloRequest
messages and the Finished message are omitted from handshake hashes.
The input to the client and server Finished messages may not be
the same because the server’s Finished does not include the client’s
Certificate and CertificateVerify message.</t>

</section>
<section anchor="client-certificate" title="Client Certificate">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>This message is the first handshake message the client can send
after receiving the server’s Finished. This message is only sent if the server requests a
certificate. If no suitable certificate is available, the client MUST send a
certificate message containing no certificates. That is, the certificate_list
structure has a length of zero. If the client does not send any certificates,
the server MAY at its discretion either continue the handshake without client
authentication, or respond with a fatal “handshake_failure” alert. Also, if some
aspect of the certificate chain was unacceptable (e.g., it was not signed by a
known, trusted CA), the server MAY at its discretion either continue the
handshake (considering the client unauthenticated) or send a fatal alert.</t>
</list></t>

<t><list style='empty'>
  <t>Client certificates are sent using the Certificate structure defined in
<xref target="server-certificate"/>.</t>
</list></t>

<t>Meaning of this message:</t>

<t><list style='empty'>
  <t>This message conveys the client’s certificate chain to the server; the server
will use it when verifying the CertificateVerify message (when the client
authentication is based on signing). The certificate MUST be appropriate for the
negotiated cipher suite’s key exchange algorithm, and any negotiated extensions.</t>
</list></t>

<t>In particular:</t>

<t><list style="symbols">
  <t>The certificate type MUST be X.509v3 <xref target="RFC5280"/>, unless explicitly negotiated
otherwise (e.g., <xref target="RFC5081"/>).</t>
  <t>The end-entity certificate’s public key (and associated
restrictions) has to be compatible with the certificate types
listed in CertificateRequest:  <vspace blankLines='1'/>
    <figure><artwork><![CDATA[
 Client Cert. Type   Certificate Key Type

 rsa_sign            RSA public key; the certificate MUST allow the
                     key to be used for signing with the signature
                     scheme and hash algorithm that will be
                     employed in the certificate verify message.

 dss_sign            DSA public key; the certificate MUST allow the
                     key to be used for signing with the hash
                     algorithm that will be employed in the
                     certificate verify message.

 ecdsa_sign          ECDSA-capable public key; the certificate MUST
                     allow the key to be used for signing with the
                     hash algorithm that will be employed in the
                     certificate verify message; the public key
                     MUST use a curve and point format supported by
                     the server.

 rsa_fixed_dh        Diffie-Hellman public key; MUST use the same
 dss_fixed_dh        parameters as server's key.

 rsa_fixed_ecdh      ECDH-capable public key; MUST use the
 ecdsa_fixed_ecdh    same curve as the server's key, and MUST use a
                     point format supported by the server.
]]></artwork></figure>
  </t>
  <t>If the certificate_authorities list in the certificate request
message was non-empty, one of the certificates in the certificate
chain SHOULD be issued by one of the listed CAs.</t>
  <t>The certificates MUST be signed using an acceptable hash/
signature algorithm pair, as described in <xref target="certificate-request"/>.  Note
that this relaxes the constraints on certificate-signing
algorithms found in prior versions of TLS.</t>
</list></t>

<t>Note that, as with the server certificate, there are certificates that use
algorithms/algorithm combinations that cannot be currently used with TLS.</t>

</section>
<section anchor="client-certificate-verify" title="Client Certificate Verify">

<t>When this message will be sent:</t>

<t><list style='empty'>
  <t>This message is used to provide explicit verification of a client
certificate. This message is only sent following a client certificate that has
signing capability (i.e., all certificates except those containing fixed
Diffie-Hellman parameters). When sent, it MUST immediately follow the client’s
Certificate message. The contents of the message are computed as described
in <xref target="server-certificate-verify"/>, except that the context string is
“TLS 1.3, client CertificateVerify”.</t>
</list></t>

<t><list style='empty'>
  <t>The hash and signature algorithms used in the signature MUST be one of those
present in the supported_signature_algorithms field of the CertificateRequest
message. In addition, the hash and signature algorithms MUST be compatible with
the key in the client’s end-entity certificate. RSA keys MAY be used with any
permitted hash algorithm, subject to restrictions in the certificate, if any.</t>
</list></t>

<t><list style='empty'>
  <t>Because DSA signatures do not contain any secure indication of hash
algorithm, there is a risk of hash substitution if multiple hashes may be used
with any key. Currently, DSA <xref target="DSS"/> may only be used with SHA-1. Future
revisions of DSS <xref target="DSS-3"/> are expected to allow the use of other digest
algorithms with DSA, as well as guidance as to which digest algorithms should
be used with each key size. In addition, future revisions of <xref target="RFC5280"/> may
specify mechanisms for certificates to indicate which digest algorithms are to
be used with DSA.</t>
</list></t>

</section>
<section anchor="new-session-ticket-message" title="New Session Ticket Message">

<t>After the server has received the client Finished message, it MAY send
a NewSessionTicket message. This message MUST be sent before the server
sends any application data traffic, and is encrypted under the application
traffic key. This message creates a pre-shared key
(PSK) binding between the resumption master secret and the ticket
label. The client MAY use this PSK for future handshakes by including
it in the pre_shared_key extension in its ClientHello
(<xref target="pre-shared-key-extension"/>) and supplying a suitable PSK cipher
suite.</t>

<figure><artwork><![CDATA[
  struct {
      uint32 ticket_lifetime_hint;
      opaque ticket<0..2^16-1>;
  } NewSessionTicket;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='ticket_lifetime_hint'><vspace blankLines='0'/>
  Indicates the lifetime
in seconds as a 32-bit unsigned integer in network byte order.  A
value of zero is reserved to indicate that the lifetime of the ticket
is unspecified.</t>
  <t hangText='ticket'><vspace blankLines='0'/>
  The value of the ticket to be used as the PSK identifier.</t>
</list></t>

<t>The ticket lifetime hint is informative only.
A client SHOULD delete the ticket and associated
state when the time expires.  It MAY delete the ticket earlier based
on local policy.  A server MAY treat a ticket as valid for a shorter
or longer period of time than what is stated in the
ticket_lifetime_hint.</t>

<t>The ticket itself is an opaque label. It MAY either be a database
lookup key or a self-encrypted and self-authenticated value. Section 4 of <xref target="RFC5077"/>
describes a recommended ticket construction mechanism.</t>

<t>[[TODO: Should we require that tickets be bound to the existing
symmetric cipher suite. See the TODO above about early_data and
PSK.??]</t>

</section>
</section>
</section>
<section anchor="cryptographic-computations" title="Cryptographic Computations">

<t>In order to begin connection protection, the TLS Record Protocol
requires specification of a suite of algorithms, a master secret, and
the client and server random values. The authentication, key
agreement, and record protection algorithms are determined by the
cipher_suite selected by the server and revealed in the ServerHello
message. The random values are exchanged in the hello messages. All
that remains is to calculate the key schedule.</t>

<section anchor="key-schedule" title="Key Schedule">

<t>The TLS handshake establishes secret keying material which is then used
to protect traffic. This keying material is derived from the two
input secret values: Static Secret (SS) and Ephemeral Secret (ES).</t>

<t>The exact source of each of these secrets depends on the operational
mode (DHE, ECDHE, PSK, etc.) and is summarized in the table below:</t>

<figure><artwork><![CDATA[
    Key Exchange            Static Secret (SS)    Ephemeral Secret (ES)
    ------------            ------------------    ---------------------
    (EC)DHE                   Client ephemeral         Client ephemeral
    (full handshake)       w/ server ephemeral      w/ server ephemeral
    
    (EC)DHE                   Client ephemeral         Client ephemeral
    (w/ known_configuration)      w/ Known Key      w/ server ephemeral
    
    PSK                         Pre-Shared Key           Pre-shared key

    PSK + (EC)DHE               Pre-Shared Key         Client ephemeral
                                                    w/ server ephemeral
]]></artwork></figure>

<t>These shared secret values are used to generate cryptographic keys as
shown below. </t>

<t>The derivation process is as follows, where L denotes the length of
the underlying hash function for HKDF.</t>

<figure><artwork><![CDATA[
  HKDF-Expand-Label(Secret, Label, HashValue, Length) =
       HKDF-Expand(Secret, Label + '\0' + HashValue, Length)

  1. xSS = HKDF(0, SS, "extractedSS", L)
  
  2. xES = HKDF(0, ES, "extractedES", L)
  
  3. master_secret= HKDF(xSS, xES, "master secret", L)
  
  4. finished_secret = HKDF-Expand-Label(xSS,
                                         "finished secret",
                                         handshake_hash, L)
  
  Where handshake_hash includes all the messages in the
  client's first flight and the server's flight, excluding
  the Finished messages (which are never included in the
  hashes).
  
  5. resumption_secret = HKDF-Expand-Label(master_secret,
                                           "resumption master secret"
                                           session_hash, L)
  
  Where session_hash is as defined in {{the-handshake-hash}}.
  
  6. exporter_secret = HKDF-Expand-Label(master_secret,
                                         "exporter master secret",
                                         session_hash, L)
  
  Where session_hash is the session hash as defined in
  {{the-handshake-hash}} (i.e., the entire handshake except
  for Finished).
]]></artwork></figure>

<t>The traffic keys are computed from xSS, xES, and the master_secret
as described in <xref target="traffic-key-calculation"/> below.</t>

</section>
<section anchor="traffic-key-calculation" title="Traffic Key Calculation">

<t>[[OPEN ISSUE: This needs to be revised. Most likely we’ll extract each
  key component separately. See https://github.com/tlswg/tls13-spec/issues/5]]</t>

<t>The Record Protocol requires an algorithm to generate keys required by the
current connection state (see <xref target="the-security-parameters"/>) from the security
parameters provided by the handshake protocol.</t>

<t>The traffic key computation takes four input values and returns a key block
of sufficient size to produce the needed traffic keys:</t>

<t><list style="symbols">
  <t>A secret value</t>
  <t>A string label that indicates the purpose of keys being generated.</t>
  <t>The current handshake hash.</t>
  <t>The total length in octets of the key block.</t>
</list></t>

<t>The keying material is computed using:</t>

<figure><artwork><![CDATA[
   key_block = HKDF-Expand-Label(Secret, Label,
                                 handshake_hash,
                                 total_length)
]]></artwork></figure>

<t>The key_block is partitioned as follows:</t>

<figure><artwork><![CDATA[
   client_write_key[SecurityParameters.enc_key_length]
   server_write_key[SecurityParameters.enc_key_length]
   client_write_IV[SecurityParameters.iv_length]
   server_write_IV[SecurityParameters.iv_length]
]]></artwork></figure>

<t>The following table describes the inputs to the key calculation for
each class of traffic keys:</t>

<figure><artwork><![CDATA[
  Record Type Secret  Label                              Handshake Hash
  ----------- ------  -----                             ---------------
  Early data     xSS  "early data key expansion"            ClientHello

  Handshake      xES  "handshake key expansion"          ClientHello...
                                                         ServerKeyShare

  Application  master "application data key expansion"    All handshake
               secret                                      messages but
                                                               Finished
                                                         (session_hash)
]]></artwork></figure>

<section anchor="the-handshake-hash" title="The Handshake Hash">

<figure><artwork><![CDATA[
   handshake_hash = Hash(
                         Hash(handshake_messages) ||
                         Hash(configuration)
                        )
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='handshake_messages'><vspace blankLines='0'/>
  All handshake messages sent or
received, starting at ClientHello up to the present time, with the
exception of the Finished message, including the type and length
fields of the handshake messages. This is the concatenation of all the
exchanged Handshake structures in plaintext form (even if they
were encrypted on the wire).</t>
  <t hangText='configuration'><vspace blankLines='0'/>
  When the known_configuration extension is in use (<xref target="known-configuration-extension"/>,
this contains the concatenation of the ServerConfiguration and Certificate
messages from the handshake where the configuration was established. Note that
this requires the client and server to memorize these values.</t>
</list></t>

<t>This final value of the handshake hash is referred to as the “session
hash” because it contains all the handshake messages required to
establish the session. Note that if client authentication is not used,
then the session hash is complete at the point when the server has
sent its first flight. Otherwise, it is only complete when the client
has sent its first flight, as it covers the client’s Certificate and
CertificateVerify.</t>

</section>
<section anchor="diffie-hellman" title="Diffie-Hellman">

<t>A conventional Diffie-Hellman computation is performed. The negotiated key (Z)
is used as the shared_secret, and is used in the key schedule as
specified above. Leading bytes of Z that contain all zero bits are stripped
before it is used as the input to HKDF.</t>

</section>
<section anchor="elliptic-curve-diffie-hellman" title="Elliptic Curve Diffie-Hellman">

<t>All ECDH calculations (including parameter and key generation as well
as the shared secret calculation) are performed according to [6]
using the ECKAS-DH1 scheme with the identity map as key derivation
function (KDF), so that the shared secret is the x-coordinate of
the ECDH shared secret elliptic curve point represented as an octet
string.  Note that this octet string (Z in IEEE 1363 terminology) as
output by FE2OSP, the Field Element to Octet String Conversion
Primitive, has constant length for any given field; leading zeros
found in this octet string MUST NOT be truncated.</t>

<t>(Note that this use of the identity KDF is a technicality.  The
complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use this secret for anything
other than for computing other secrets.)</t>

</section>
</section>
</section>
<section anchor="mandatory-cipher-suites" title="Mandatory Cipher Suites">

<t>In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the cipher suite <eref target="https://github.com/tlswg/tls13-spec/issues/32">TODO:Needs to be selected</eref>. (See <xref target="the-cipher-suite"/> for the definition.)</t>

</section>
<section anchor="application-data-protocol" title="Application Data Protocol">

<t>Application data messages are carried by the record layer and are fragmented
and encrypted based on the current connection state. The messages
are treated as transparent data to the record layer.</t>

</section>
<section anchor="security-considerations" title="Security Considerations">

<t>Security issues are discussed throughout this memo, especially in Appendices C,
D, and E.</t>

</section>
<section anchor="iana-considerations" title="IANA Considerations">

<t>[[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]</t>

<t>This document uses several registries that were originally created in
<xref target="RFC4346"/>. IANA has updated these to reference this document. The registries
and their allocation policies (unchanged from <xref target="RFC4346"/>) are listed below.</t>

<t><list style="symbols">
  <t>TLS ClientCertificateType Identifiers Registry: Future values in
  the range 0-63 (decimal) inclusive are assigned via Standards
  Action <xref target="RFC2434"/>.  Values in the range 64-223 (decimal) inclusive
  are assigned via Specification Required <xref target="RFC2434"/>.  Values from
  224-255 (decimal) inclusive are reserved for Private Use
  <xref target="RFC2434"/>.</t>
  <t>TLS Cipher Suite Registry: Future values with the first byte in
  the range 0-191 (decimal) inclusive are assigned via Standards
  Action <xref target="RFC2434"/>.  Values with the first byte in the range 192-254
  (decimal) are assigned via Specification Required <xref target="RFC2434"/>.
  Values with the first byte 255 (decimal) are reserved for Private
  Use <xref target="RFC2434"/>.</t>
  <t>TLS ContentType Registry: Future values are allocated via
  Standards Action <xref target="RFC2434"/>.</t>
  <t>TLS Alert Registry: Future values are allocated via Standards
  Action <xref target="RFC2434"/>.</t>
  <t>TLS HandshakeType Registry: Future values are allocated via
  Standards Action <xref target="RFC2434"/>.</t>
</list></t>

<t>This document also uses a registry originally created in <xref target="RFC4366"/>. IANA has
updated it to reference this document. The registry and its allocation policy
(unchanged from <xref target="RFC4366"/>) is listed below:</t>

<t><list style="symbols">
  <t>TLS ExtensionType Registry: Future values are allocated via IETF
  Consensus <xref target="RFC2434"/>.  IANA has updated this registry to include
  the signature_algorithms extension and its corresponding value
  (see <xref target="hello-extensions"/>).</t>
</list></t>

<t>This document also uses two registries originally created in <xref target="RFC4492"/>. IANA
[should update/has updated] it to reference this document. The registries
and their allocation policies are listed below.</t>

<t><list style="symbols">
  <t>TLS NamedCurve registry: Future values are allocated via IETF Consensus
<xref target="RFC2434"/>.</t>
  <t>TLS ECPointFormat Registry: Future values are allocated via IETF Consensus
<xref target="RFC2434"/>.</t>
</list></t>

<t>In addition, this document defines two new registries to be maintained by IANA:</t>

<t><list style="symbols">
  <t>TLS SignatureAlgorithm Registry: The registry has been initially
  populated with the values described in <xref target="signature-algorithms"/>.  Future
  values in the range 0-63 (decimal) inclusive are assigned via
  Standards Action <xref target="RFC2434"/>.  Values in the range 64-223 (decimal)
  inclusive are assigned via Specification Required <xref target="RFC2434"/>.
  Values from 224-255 (decimal) inclusive are reserved for Private
  Use <xref target="RFC2434"/>.</t>
  <t>TLS HashAlgorithm Registry: The registry has been initially
  populated with the values described in <xref target="signature-algorithms"/>.  Future
  values in the range 0-63 (decimal) inclusive are assigned via
  Standards Action <xref target="RFC2434"/>.  Values in the range 64-223 (decimal)
  inclusive are assigned via Specification Required <xref target="RFC2434"/>.
  Values from 224-255 (decimal) inclusive are reserved for Private
  Use <xref target="RFC2434"/>.</t>
</list></t>

</section>


  </middle>

  <back>

    <references title='Normative References'>





<reference anchor='RFC2104'>

<front>
<title abbrev='HMAC'>HMAC: Keyed-Hashing for Message Authentication</title>
<author initials='H.' surname='Krawczyk' fullname='Hugo Krawczyk'>
<organization>IBM, T.J. Watson Research Center</organization>
<address>
<postal>
<street>P.O.Box 704</street>
<city>Yorktown Heights</city>
<region>NY</region>
<code>10598</code>
<country>US</country></postal>
<email>hugo@watson.ibm.com</email></address></author>
<author initials='M.' surname='Bellare' fullname='Mihir Bellare'>
<organization>University of California at San Diego, Dept of Computer Science and Engineering</organization>
<address>
<postal>
<street>9500 Gilman Drive</street>
<street>Mail Code 0114</street>
<city>La Jolla</city>
<region>CA</region>
<code>92093</code>
<country>US</country></postal>
<email>mihir@cs.ucsd.edu</email></address></author>
<author initials='R.' surname='Canetti' fullname='Ran Canetti'>
<organization>IBM T.J. Watson Research Center</organization>
<address>
<postal>
<street>P.O.Box 704</street>
<city>Yorktown Heights</city>
<region>NY</region>
<code>10598</code>
<country>US</country></postal>
<email>canetti@watson.ibm.com</email></address></author>
<date year='1997' month='February' />
<abstract>
<t>This document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key.  The cryptographic strength of HMAC depends on the properties of the underlying hash function.</t></abstract></front>

<seriesInfo name='RFC' value='2104' />
<format type='TXT' octets='22297' target='http://www.rfc-editor.org/rfc/rfc2104.txt' />
</reference>



<reference anchor='RFC2119'>

<front>
<title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='Scott Bradner'>
<organization>Harvard University</organization>
<address>
<postal>
<street>1350 Mass. Ave.</street>
<street>Cambridge</street>
<street>MA 02138</street></postal>
<phone>- +1 617 495 3864</phone>
<email>sob@harvard.edu</email></address></author>
<date year='1997' month='March' />
<area>General</area>
<keyword>keyword</keyword>
<abstract>
<t>
   In many standards track documents several words are used to signify
   the requirements in the specification.  These words are often
   capitalized.  This document defines these words as they should be
   interpreted in IETF documents.  Authors who follow these guidelines
   should incorporate this phrase near the beginning of their document:

<list>
<t>
      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
      "OPTIONAL" in this document are to be interpreted as described in
      RFC 2119.
</t></list></t>
<t>
   Note that the force of these words is modified by the requirement
   level of the document in which they are used.
</t></abstract></front>

<seriesInfo name='BCP' value='14' />
<seriesInfo name='RFC' value='2119' />
<format type='TXT' octets='4723' target='http://www.rfc-editor.org/rfc/rfc2119.txt' />
<format type='HTML' octets='17970' target='http://xml.resource.org/public/rfc/html/rfc2119.html' />
<format type='XML' octets='5777' target='http://xml.resource.org/public/rfc/xml/rfc2119.xml' />
</reference>



<reference anchor='RFC2434'>

<front>
<title abbrev='Guidelines for IANA Considerations'>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials='T.' surname='Narten' fullname='Thomas Narten'>
<organization>IBM Corporation</organization>
<address>
<postal>
<street>3039 Cornwallis Ave.</street>
<street>PO Box 12195 - BRQA/502</street>
<street>Research Triangle Park</street>
<street>NC 27709-2195</street></postal>
<phone>919-254-7798</phone>
<email>narten@raleigh.ibm.com</email></address></author>
<author initials='H.T.' surname='Alvestrand' fullname='Harald Tveit Alvestrand'>
<organization>Maxware</organization>
<address>
<postal>
<street>Pirsenteret</street>
<street>N-7005 Trondheim</street>
<country>Norway</country></postal>
<phone>+47 73 54 57 97</phone>
<email>Harald@Alvestrand.no</email></address></author>
<date year='1998' month='October' />
<area>General</area>
<keyword>Internet Assigned Numbers Authority</keyword>
<keyword>IANA</keyword>
<abstract>
<t>
   Many protocols make use of identifiers consisting of constants and
   other well-known values. Even after a protocol has been defined and
   deployment has begun, new values may need to be assigned (e.g., for a
   new option type in DHCP, or a new encryption or authentication
   algorithm for IPSec).  To insure that such quantities have consistent
   values and interpretations in different implementations, their
   assignment must be administered by a central authority. For IETF
   protocols, that role is provided by the Internet Assigned Numbers
   Authority (IANA).
</t>
<t>
   In order for the IANA to manage a given name space prudently, it
   needs guidelines describing the conditions under which new values can
   be assigned. If the IANA is expected to play a role in the management
   of a name space, the IANA must be given clear and concise
   instructions describing that role.  This document discusses issues
   that should be considered in formulating a policy for assigning
   values to a name space and provides guidelines to document authors on
   the specific text that must be included in documents that place
   demands on the IANA.
</t></abstract></front>

<seriesInfo name='BCP' value='26' />
<seriesInfo name='RFC' value='2434' />
<format type='TXT' octets='25092' target='http://www.rfc-editor.org/rfc/rfc2434.txt' />
<format type='XML' octets='27060' target='http://xml.resource.org/public/rfc/xml/rfc2434.xml' />
</reference>



<reference anchor='RFC1321'>

<front>
<title abbrev='MD5 Message-Digest Algorithm'>The MD5 Message-Digest Algorithm</title>
<author initials='R.' surname='Rivest' fullname='Ronald L. Rivest'>
<organization>Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<street>NE43-324</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139-1986</code>
<country>US</country></postal>
<phone>+1 617 253 5880</phone>
<email>rivest@theory.lcs.mit.edu</email></address></author>
<date year='1992' month='April' /></front>

<seriesInfo name='RFC' value='1321' />
<format type='TXT' octets='35222' target='http://www.rfc-editor.org/rfc/rfc1321.txt' />
</reference>



<reference anchor='RFC3447'>

<front>
<title>Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1</title>
<author initials='J.' surname='Jonsson' fullname='J. Jonsson'>
<organization /></author>
<author initials='B.' surname='Kaliski' fullname='B. Kaliski'>
<organization /></author>
<date year='2003' month='February' />
<abstract>
<t>This memo represents a republication of PKCS #1 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process.  The body of this document is taken directly from the PKCS #1 v2.1 document, with certain corrections made during the publication process.  This memo provides information for the Internet community.</t></abstract></front>

<seriesInfo name='RFC' value='3447' />
<format type='TXT' octets='143173' target='http://www.rfc-editor.org/rfc/rfc3447.txt' />
</reference>



<reference anchor='RFC5280'>

<front>
<title>Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile</title>
<author initials='D.' surname='Cooper' fullname='D. Cooper'>
<organization /></author>
<author initials='S.' surname='Santesson' fullname='S. Santesson'>
<organization /></author>
<author initials='S.' surname='Farrell' fullname='S. Farrell'>
<organization /></author>
<author initials='S.' surname='Boeyen' fullname='S. Boeyen'>
<organization /></author>
<author initials='R.' surname='Housley' fullname='R. Housley'>
<organization /></author>
<author initials='W.' surname='Polk' fullname='W. Polk'>
<organization /></author>
<date year='2008' month='May' />
<abstract>
<t>This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet.  An overview of this approach and model is provided as an introduction.  The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms.  Standard certificate extensions are described and two Internet-specific extensions are defined.  A set of required certificate extensions is specified.  The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions.  An algorithm for X.509 certification path validation is described.  An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='5280' />
<format type='TXT' octets='352580' target='http://www.rfc-editor.org/rfc/rfc5280.txt' />
</reference>



<reference anchor='RFC5288'>

<front>
<title>AES Galois Counter Mode (GCM) Cipher Suites for TLS</title>
<author initials='J.' surname='Salowey' fullname='J. Salowey'>
<organization /></author>
<author initials='A.' surname='Choudhury' fullname='A. Choudhury'>
<organization /></author>
<author initials='D.' surname='McGrew' fullname='D. McGrew'>
<organization /></author>
<date year='2008' month='August' />
<abstract>
<t>This memo describes the use of the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS) authenticated encryption operation.  GCM provides both confidentiality and data origin authentication, can be efficiently implemented in hardware for speeds of 10 gigabits per second and above, and is also well-suited to software implementations.  This memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and Diffie-Hellman-based key exchange mechanisms. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='5288' />
<format type='TXT' octets='16468' target='http://www.rfc-editor.org/rfc/rfc5288.txt' />
</reference>



<reference anchor='RFC5289'>

<front>
<title>TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)</title>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
<organization /></author>
<date year='2008' month='August' />
<abstract>
<t>RFC 4492 describes elliptic curve cipher suites for Transport Layer Security (TLS).  However, all those cipher suites use HMAC-SHA-1 as their Message Authentication Code (MAC) algorithm.  This document describes sixteen new cipher suites for TLS that specify stronger MAC algorithms.  Eight use Hashed Message Authentication Code (HMAC) with SHA-256 or SHA-384, and eight use AES in Galois Counter Mode (GCM).  This memo provides information for the Internet community.</t></abstract></front>

<seriesInfo name='RFC' value='5289' />
<format type='TXT' octets='12195' target='http://www.rfc-editor.org/rfc/rfc5289.txt' />
</reference>



<reference anchor='RFC5869'>

<front>
<title>HMAC-based Extract-and-Expand Key Derivation Function (HKDF)</title>
<author initials='H.' surname='Krawczyk' fullname='H. Krawczyk'>
<organization /></author>
<author initials='P.' surname='Eronen' fullname='P. Eronen'>
<organization /></author>
<date year='2010' month='May' />
<abstract>
<t>This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications.  The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions.  This document is not an Internet Standards Track specification; it is published for informational purposes.</t></abstract></front>

<seriesInfo name='RFC' value='5869' />
<format type='TXT' octets='25854' target='http://www.rfc-editor.org/rfc/rfc5869.txt' />
</reference>


<reference anchor="AES" >
  <front>
    <title>Specification for the Advanced Encryption Standard (AES)</title>
    <author >
      <organization>National Institute of Standards and Technology</organization>
    </author>
    <date year="2001" month="November" day="26"/>
  </front>
  <seriesInfo name="NIST" value="FIPS 197"/>
</reference>
<reference anchor="DSS" >
  <front>
    <title>Digital Signature Standard</title>
    <author >
      <organization>National Institute of Standards and Technology, U.S. Department of Commerce</organization>
    </author>
    <date year="2000"/>
  </front>
  <seriesInfo name="NIST" value="FIPS PUB 186-2"/>
</reference>
<reference anchor="SHS" >
  <front>
    <title>Secure Hash Standard</title>
    <author >
      <organization>National Institute of Standards and Technology, U.S. Department of Commerce</organization>
    </author>
    <date year="2002" month="August"/>
  </front>
  <seriesInfo name="NIST" value="FIPS PUB 180-2"/>
</reference>
<reference anchor="X680" >
  <front>
    <title>Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation</title>
    <author >
      <organization>ITU-T</organization>
    </author>
    <date year="2002"/>
  </front>
  <seriesInfo name="ISO/IEC" value="8824-1:2002"/>
</reference>
<reference anchor="X690" >
  <front>
    <title>Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)</title>
    <author >
      <organization>ITU-T</organization>
    </author>
    <date year="2002"/>
  </front>
  <seriesInfo name="ISO/IEC" value="8825-1:2002"/>
</reference>
<reference anchor="X962" >
  <front>
    <title>Public Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)</title>
    <author >
      <organization>ANSI</organization>
    </author>
    <date year="1998"/>
  </front>
  <seriesInfo name="ANSI" value="X9.62"/>
</reference>
<reference anchor="DH" >
  <front>
    <title>New Directions in Cryptography</title>
    <author initials="W." surname="Diffie">
      <organization></organization>
    </author>
    <author initials="M." surname="Hellman">
      <organization></organization>
    </author>
    <date year="1977" month="June"/>
  </front>
  <seriesInfo name="IEEE Transactions on Information Theory, V.IT-22 n.6" value=""/>
</reference>


    </references>

    <references title='Informative References'>





<reference anchor='RFC0793'>

<front>
<title abbrev='Transmission Control Protocol'>Transmission Control Protocol</title>
<author initials='J.' surname='Postel' fullname='Jon Postel'>
<organization>University of Southern California (USC)/Information Sciences Institute</organization>
<address>
<postal>
<street>4676 Admiralty Way</street>
<city>Marina del Rey</city>
<region>CA</region>
<code>90291</code>
<country>US</country></postal></address></author>
<date year='1981' day='1' month='September' /></front>

<seriesInfo name='STD' value='7' />
<seriesInfo name='RFC' value='793' />
<format type='TXT' octets='172710' target='http://www.rfc-editor.org/rfc/rfc793.txt' />
</reference>



<reference anchor='RFC1948'>

<front>
<title abbrev='Sequence Number Attacks'>Defending Against Sequence Number Attacks</title>
<author initials='S.' surname='Bellovin' fullname='Steven M. Bellovin'>
<organization>AT&amp;T Research</organization>
<address>
<postal>
<street>600 Mountain Avenue</street>
<city>Murray Hill</city>
<region>NJ</region>
<code>07974</code>
<country>US</country></postal>
<phone>+1 908 582 5886</phone>
<email>smb@research.att.com</email></address></author>
<date year='1996' month='May' />
<abstract>
<t>IP spoofing attacks based on sequence number spoofing have become a serious threat on the Internet (CERT Advisory CA-95:01).  While ubiquitous crypgraphic authentication is the right answer, we propose a simple modification to TCP implementations that should be a very substantial block to the current wave of attacks.</t></abstract></front>

<seriesInfo name='RFC' value='1948' />
<format type='TXT' octets='13074' target='http://www.rfc-editor.org/rfc/rfc1948.txt' />
</reference>



<reference anchor='RFC2246'>

<front>
<title>The TLS Protocol Version 1.0</title>
<author initials='T.' surname='Dierks' fullname='Tim Dierks'>
<organization>Certicom</organization>
<address>
<email>tdierks@certicom.com</email></address></author>
<author initials='C.' surname='Allen' fullname='Christopher Allen'>
<organization>Certicom</organization>
<address>
<email>callen@certicom.com</email></address></author>
<date year='1999' month='January' />
<abstract>
<t>This document specifies Version 1.0 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery.</t></abstract></front>

<seriesInfo name='RFC' value='2246' />
<format type='TXT' octets='170401' target='http://www.rfc-editor.org/rfc/rfc2246.txt' />
</reference>



<reference anchor='RFC3268'>

<front>
<title>Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)</title>
<author initials='P.' surname='Chown' fullname='P. Chown'>
<organization /></author>
<date year='2002' month='June' />
<abstract>
<t>This document proposes several new ciphersuites.  At present, the symmetric ciphers supported by Transport Layer Security (TLS) are RC2, RC4, International Data Encryption Algorithm (IDEA), Data Encryption Standard (DES), and triple DES.  The protocol would be enhanced by the addition of Advanced Encryption Standard (AES) ciphersuites. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='3268' />
<format type='TXT' octets='13530' target='http://www.rfc-editor.org/rfc/rfc3268.txt' />
</reference>



<reference anchor='RFC4086'>

<front>
<title>Randomness Requirements for Security</title>
<author initials='D.' surname='Eastlake' fullname='D. Eastlake'>
<organization /></author>
<author initials='J.' surname='Schiller' fullname='J. Schiller'>
<organization /></author>
<author initials='S.' surname='Crocker' fullname='S. Crocker'>
<organization /></author>
<date year='2005' month='June' />
<abstract>
<t>Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space.&lt;/t>&lt;t> Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract></front>

<seriesInfo name='BCP' value='106' />
<seriesInfo name='RFC' value='4086' />
<format type='TXT' octets='114321' target='http://www.rfc-editor.org/rfc/rfc4086.txt' />
</reference>



<reference anchor='RFC4279'>

<front>
<title>Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)</title>
<author initials='P.' surname='Eronen' fullname='P. Eronen'>
<organization /></author>
<author initials='H.' surname='Tschofenig' fullname='H. Tschofenig'>
<organization /></author>
<date year='2005' month='December' />
<abstract>
<t>This document specifies three sets of new ciphersuites for the Transport Layer Security (TLS) protocol to support authentication based on pre-shared keys (PSKs).  These pre-shared keys are symmetric keys, shared in advance among the communicating parties.  The first set of ciphersuites uses only symmetric key operations for authentication.  The second set uses a Diffie-Hellman exchange authenticated with a pre-shared key, and the third set combines public key authentication of the server with pre-shared key authentication of the client. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='4279' />
<format type='TXT' octets='32160' target='http://www.rfc-editor.org/rfc/rfc4279.txt' />
</reference>



<reference anchor='RFC4302'>

<front>
<title>IP Authentication Header</title>
<author initials='S.' surname='Kent' fullname='S. Kent'>
<organization /></author>
<date year='2005' month='December' />
<abstract>
<t>This document describes an updated version of the IP Authentication Header (AH), which is designed to provide authentication services in IPv4 and IPv6.  This document obsoletes RFC 2402 (November 1998). [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='4302' />
<format type='TXT' octets='82328' target='http://www.rfc-editor.org/rfc/rfc4302.txt' />
</reference>



<reference anchor='RFC4303'>

<front>
<title>IP Encapsulating Security Payload (ESP)</title>
<author initials='S.' surname='Kent' fullname='S. Kent'>
<organization /></author>
<date year='2005' month='December' />
<abstract>
<t>This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6.  ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality.  This document obsoletes RFC 2406 (November 1998). [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='4303' />
<format type='TXT' octets='114315' target='http://www.rfc-editor.org/rfc/rfc4303.txt' />
</reference>



<reference anchor='RFC4346'>

<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.1</title>
<author initials='T.' surname='Dierks' fullname='T. Dierks'>
<organization /></author>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
<organization /></author>
<date year='2006' month='April' />
<abstract>
<t>This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol.  The TLS protocol provides communications security over the Internet.  The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='4346' />
<format type='TXT' octets='187041' target='http://www.rfc-editor.org/rfc/rfc4346.txt' />
</reference>



<reference anchor='RFC4366'>

<front>
<title>Transport Layer Security (TLS) Extensions</title>
<author initials='S.' surname='Blake-Wilson' fullname='S. Blake-Wilson'>
<organization /></author>
<author initials='M.' surname='Nystrom' fullname='M. Nystrom'>
<organization /></author>
<author initials='D.' surname='Hopwood' fullname='D. Hopwood'>
<organization /></author>
<author initials='J.' surname='Mikkelsen' fullname='J. Mikkelsen'>
<organization /></author>
<author initials='T.' surname='Wright' fullname='T. Wright'>
<organization /></author>
<date year='2006' month='April' />
<abstract>
<t>This document describes extensions that may be used to add functionality to Transport Layer Security (TLS). It provides both generic extension mechanisms for the TLS handshake client and server hellos, and specific extensions using these generic mechanisms.&lt;/t>&lt;t> The extensions may be used by TLS clients and servers. The extensions are backwards compatible: communication is possible between TLS clients that support the extensions and TLS servers that do not support the extensions, and vice versa. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='4366' />
<format type='TXT' octets='66344' target='http://www.rfc-editor.org/rfc/rfc4366.txt' />
</reference>



<reference anchor='RFC4492'>

<front>
<title>Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)</title>
<author initials='S.' surname='Blake-Wilson' fullname='S. Blake-Wilson'>
<organization /></author>
<author initials='N.' surname='Bolyard' fullname='N. Bolyard'>
<organization /></author>
<author initials='V.' surname='Gupta' fullname='V. Gupta'>
<organization /></author>
<author initials='C.' surname='Hawk' fullname='C. Hawk'>
<organization /></author>
<author initials='B.' surname='Moeller' fullname='B. Moeller'>
<organization /></author>
<date year='2006' month='May' />
<abstract>
<t>This document describes new key exchange algorithms based on Elliptic Curve Cryptography (ECC) for the Transport Layer Security (TLS) protocol.  In particular, it specifies the use of Elliptic Curve Diffie-Hellman (ECDH) key agreement in a TLS handshake and the use of Elliptic Curve Digital Signature Algorithm (ECDSA) as a new authentication mechanism.  This memo provides information for the Internet community.</t></abstract></front>

<seriesInfo name='RFC' value='4492' />
<format type='TXT' octets='72231' target='http://www.rfc-editor.org/rfc/rfc4492.txt' />
</reference>



<reference anchor='RFC4506'>

<front>
<title>XDR: External Data Representation Standard</title>
<author initials='M.' surname='Eisler' fullname='M. Eisler'>
<organization /></author>
<date year='2006' month='May' />
<abstract>
<t>This document describes the External Data Representation Standard (XDR) protocol as it is currently deployed and accepted.  This document obsoletes RFC 1832. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='STD' value='67' />
<seriesInfo name='RFC' value='4506' />
<format type='TXT' octets='55477' target='http://www.rfc-editor.org/rfc/rfc4506.txt' />
</reference>



<reference anchor='RFC5077'>

<front>
<title>Transport Layer Security (TLS) Session Resumption without Server-Side State</title>
<author initials='J.' surname='Salowey' fullname='J. Salowey'>
<organization /></author>
<author initials='H.' surname='Zhou' fullname='H. Zhou'>
<organization /></author>
<author initials='P.' surname='Eronen' fullname='P. Eronen'>
<organization /></author>
<author initials='H.' surname='Tschofenig' fullname='H. Tschofenig'>
<organization /></author>
<date year='2008' month='January' />
<abstract>
<t>This document describes a mechanism that enables the Transport Layer Security (TLS) server to resume sessions and avoid keeping per-client session state.  The TLS server encapsulates the session state into a ticket and forwards it to the client.  The client can subsequently resume a session using the obtained ticket.  This document obsoletes RFC 4507. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='5077' />
<format type='TXT' octets='41989' target='http://www.rfc-editor.org/rfc/rfc5077.txt' />
</reference>



<reference anchor='RFC5081'>

<front>
<title>Using OpenPGP Keys for Transport Layer Security (TLS) Authentication</title>
<author initials='N.' surname='Mavrogiannopoulos' fullname='N. Mavrogiannopoulos'>
<organization /></author>
<date year='2007' month='November' />
<abstract>
<t>This memo proposes extensions to the Transport Layer Security (TLS) protocol to support the OpenPGP key format.  The extensions discussed here include a certificate type negotiation mechanism, and the required modifications to the TLS Handshake Protocol.  This memo defines an Experimental Protocol for the Internet community.</t></abstract></front>

<seriesInfo name='RFC' value='5081' />
<format type='TXT' octets='15300' target='http://www.rfc-editor.org/rfc/rfc5081.txt' />
</reference>



<reference anchor='RFC5116'>

<front>
<title>An Interface and Algorithms for Authenticated Encryption</title>
<author initials='D.' surname='McGrew' fullname='D. McGrew'>
<organization /></author>
<date year='2008' month='January' />
<abstract>
<t>This document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms.  The interface and registry can be used as an application-independent set of cryptoalgorithm suites.  This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='5116' />
<format type='TXT' octets='50539' target='http://www.rfc-editor.org/rfc/rfc5116.txt' />
</reference>



<reference anchor='RFC5705'>

<front>
<title>Keying Material Exporters for Transport Layer Security (TLS)</title>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
<organization /></author>
<date year='2010' month='March' />
<abstract>
<t>A number of protocols wish to leverage Transport Layer Security (TLS) to perform key establishment but then use some of the keying material for their own purposes.  This document describes a general mechanism for allowing that. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='5705' />
<format type='TXT' octets='16346' target='http://www.rfc-editor.org/rfc/rfc5705.txt' />
</reference>



<reference anchor='RFC5763'>

<front>
<title>Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS)</title>
<author initials='J.' surname='Fischl' fullname='J. Fischl'>
<organization /></author>
<author initials='H.' surname='Tschofenig' fullname='H. Tschofenig'>
<organization /></author>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
<organization /></author>
<date year='2010' month='May' />
<abstract>
<t>This document specifies how to use the Session Initiation Protocol (SIP) to establish a Secure Real-time Transport Protocol (SRTP) security context using the Datagram Transport Layer Security (DTLS) protocol.  It describes a mechanism of transporting a fingerprint attribute in the Session Description Protocol (SDP) that identifies the key that will be presented during the DTLS handshake.  The key exchange travels along the media path as opposed to the signaling path.  The SIP Identity mechanism can be used to protect the integrity of the fingerprint attribute from modification by intermediate proxies. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='5763' />
<format type='TXT' octets='81546' target='http://www.rfc-editor.org/rfc/rfc5763.txt' />
</reference>



<reference anchor='RFC6066'>

<front>
<title>Transport Layer Security (TLS) Extensions: Extension Definitions</title>
<author initials='D.' surname='Eastlake' fullname='D. Eastlake'>
<organization /></author>
<date year='2011' month='January' />
<abstract>
<t>This document provides specifications for existing TLS extensions.  It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2".  The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='6066' />
<format type='TXT' octets='55079' target='http://www.rfc-editor.org/rfc/rfc6066.txt' />
</reference>



<reference anchor='RFC6176'>

<front>
<title>Prohibiting Secure Sockets Layer (SSL) Version 2.0</title>
<author initials='S.' surname='Turner' fullname='S. Turner'>
<organization /></author>
<author initials='T.' surname='Polk' fullname='T. Polk'>
<organization /></author>
<date year='2011' month='March' />
<abstract>
<t>This document requires that when Transport Layer Security (TLS) clients and servers establish connections, they never negotiate the use of Secure Sockets Layer (SSL) version 2.0.  This document updates the backward compatibility sections found in the Transport Layer Security (TLS). [STANDARDS-TRACK]</t></abstract></front>

<seriesInfo name='RFC' value='6176' />
<format type='TXT' octets='7642' target='http://www.rfc-editor.org/rfc/rfc6176.txt' />
</reference>



<reference anchor='RFC7465'>

<front>
<title>Prohibiting RC4 Cipher Suites</title>
<author initials='A.' surname='Popov' fullname='A. Popov'>
<organization /></author>
<date year='2015' month='February' />
<abstract>
<t>This document requires that Transport Layer Security (TLS) clients and servers never negotiate the use of RC4 cipher suites when they establish connections.  This applies to all TLS versions.  This document updates RFCs 5246, 4346, and 2246.</t></abstract></front>

<seriesInfo name='RFC' value='7465' />
<format type='TXT' octets='8397' target='http://www.rfc-editor.org/rfc/rfc7465.txt' />
</reference>



<reference anchor='RFC7568'>

<front>
<title>Deprecating Secure Sockets Layer Version 3.0</title>
<author initials='R.' surname='Barnes' fullname='R. Barnes'>
<organization /></author>
<author initials='M.' surname='Thomson' fullname='M. Thomson'>
<organization /></author>
<author initials='A.' surname='Pironti' fullname='A. Pironti'>
<organization /></author>
<author initials='A.' surname='Langley' fullname='A. Langley'>
<organization /></author>
<date year='2015' month='June' />
<abstract>
<t>The Secure Sockets Layer version 3.0 (SSLv3), as specified in RFC 6101, is not sufficiently secure. This document requires that SSLv3 not be used. The replacement versions, in particular, Transport Layer Security (TLS) 1.2 (RFC 5246), are considerably more secure and capable protocols.&lt;/t>&lt;t> This document updates the backward compatibility section of RFC 5246 and its predecessors to prohibit fallback to SSLv3.</t></abstract></front>

<seriesInfo name='RFC' value='7568' />
<format type='TXT' octets='13489' target='http://www.rfc-editor.org/rfc/rfc7568.txt' />
</reference>



<reference anchor='I-D.ietf-tls-negotiated-ff-dhe'>
<front>
<title>Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for TLS</title>

<author initials='D' surname='Gillmor' fullname='Daniel Kahn Gillmor'>
    <organization />
</author>

<date month='June' day='1' year='2015' />

<abstract><t>Traditional finite-field-based Diffie-Hellman (DH) key exchange during the TLS handshake suffers from a number of security, interoperability, and efficiency shortcomings.  These shortcomings arise from lack of clarity about which DH group parameters TLS servers should offer and clients should accept.  This document offers a solution to these shortcomings for compatible peers by using a section of the TLS "EC Named Curve Registry" to establish common finite-field DH parameters with known structure and a mechanism for peers to negotiate support for these groups.  This draft updates TLS versions 1.0 [RFC2246], 1.1 [RFC4346], and 1.2 [RFC5246], as well as the TLS ECC extensions [RFC4492].</t></abstract>

</front>

<seriesInfo name='Internet-Draft' value='draft-ietf-tls-negotiated-ff-dhe-10' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-tls-negotiated-ff-dhe-10.txt' />
</reference>



<reference anchor='I-D.ietf-tls-session-hash'>
<front>
<title>Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension</title>

<author initials='K' surname='Bhargavan' fullname='Karthikeyan Bhargavan'>
    <organization />
</author>

<author initials='A' surname='Delignat-Lavaud' fullname='Antoine Delignat-Lavaud'>
    <organization />
</author>

<author initials='A' surname='Pironti' fullname='Alfredo Pironti'>
    <organization />
</author>

<author initials='A' surname='Langley' fullname='Adam Langley'>
    <organization />
</author>

<author initials='M' surname='Ray' fullname='Marsh Ray'>
    <organization />
</author>

<date month='April' day='16' year='2015' />

<abstract><t>The Transport Layer Security (TLS) master secret is not cryptographically bound to important session parameters such as the server certificate.  Consequently, it is possible for an active attacker to set up two sessions, one with a client and another with a server, such that the master secrets on the two sessions are the same.  Thereafter, any mechanism that relies on the master secret for authentication, including session resumption, becomes vulnerable to a man-in-the-middle attack, where the attacker can simply forward messages back and forth between the client and server.  This specification defines a TLS extension that contextually binds the master secret to a log of the full handshake that computes it, thus preventing such attacks.</t></abstract>

</front>

<seriesInfo name='Internet-Draft' value='draft-ietf-tls-session-hash-05' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-tls-session-hash-05.txt' />
</reference>


<reference anchor="CBCATT" target="https://www.openssl.org/~bodo/tls-cbc.txt">
  <front>
    <title>Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures</title>
    <author initials="B." surname="Moeller">
      <organization></organization>
    </author>
    <date year="2004" month="May" day="20"/>
  </front>
</reference>
<reference anchor="DSS-3" >
  <front>
    <title>Digital Signature Standard</title>
    <author >
      <organization>National Institute of Standards and Technology, U.S.</organization>
    </author>
    <date year="2006"/>
  </front>
  <seriesInfo name="NIST" value="FIPS PUB 186-3 Draft"/>
</reference>
<reference anchor="ECDSA" >
  <front>
    <title>Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)</title>
    <author >
      <organization>American National Standards Institute</organization>
    </author>
    <date year="2005" month="November"/>
  </front>
  <seriesInfo name="ANSI" value="ANS X9.62-2005"/>
</reference>
<reference anchor="FI06" target="http://www.imc.org/ietf-openpgp/mail-archive/msg14307.html">
  <front>
    <title>Bleichenbacher's RSA signature forgery based on implementation error</title>
    <author fullname="Hal Finney">
      <organization></organization>
    </author>
    <date year="2006" month="August" day="27"/>
  </front>
</reference>
<reference anchor="GCM" >
  <front>
    <title>Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC</title>
    <author initials="M." surname="Dworkin">
      <organization></organization>
    </author>
    <date year="2007" month="November"/>
  </front>
  <seriesInfo name="NIST" value="Special Publication 800-38D"/>
</reference>
<reference anchor="PKCS6" >
  <front>
    <title>PKCS #6: RSA Extended Certificate Syntax Standard, version 1.5</title>
    <author >
      <organization>RSA Laboratories</organization>
    </author>
    <date year="1993" month="November"/>
  </front>
</reference>
<reference anchor="PKCS7" >
  <front>
    <title>PKCS #7: RSA Cryptographic Message Syntax Standard, version 1.5</title>
    <author >
      <organization>RSA Laboratories</organization>
    </author>
    <date year="1993" month="November"/>
  </front>
</reference>
<reference anchor="RSA" >
  <front>
    <title>A Method for Obtaining Digital Signatures and Public-Key Cryptosystems</title>
    <author initials="R." surname="Rivest">
      <organization></organization>
    </author>
    <author initials="A." surname="Shamir">
      <organization></organization>
    </author>
    <author initials="L.M." surname="Adleman">
      <organization></organization>
    </author>
    <date year="1978" month="February"/>
  </front>
  <seriesInfo name="Communications of the ACM" value="v. 21, n. 2, pp. 120-126."/>
</reference>
<reference anchor="SSL2" >
  <front>
    <title>The SSL Protocol</title>
    <author fullname="Kipp Hickman">
      <organization>Netscape Communications Corp.</organization>
    </author>
    <date year="1995" month="February" day="09"/>
  </front>
</reference>
<reference anchor="SSL3" >
  <front>
    <title>The SSL 3.0 Protocol</title>
    <author initials="A." surname="Freier">
      <organization>Netscape Communications Corp.</organization>
    </author>
    <author initials="P." surname="Karlton">
      <organization>Netscape Communications Corp.</organization>
    </author>
    <author initials="P." surname="Kocher">
      <organization>Netscape Communications Corp.</organization>
    </author>
    <date year="1996" month="November" day="18"/>
  </front>
</reference>
<reference anchor="TIMING" >
  <front>
    <title>Remote timing attacks are practical</title>
    <author initials="D." surname="Boneh">
      <organization></organization>
    </author>
    <author initials="D." surname="Brumley">
      <organization></organization>
    </author>
    <date year="2003"/>
  </front>
  <seriesInfo name="USENIX" value="Security Symposium"/>
</reference>
<reference anchor="X501" >
  <front>
    <title>Information Technology - Open Systems Interconnection - The Directory: Models</title>
    <author >
      <organization></organization>
    </author>
    <date year="1993"/>
  </front>
  <seriesInfo name="ITU-T" value="X.501"/>
</reference>


    </references>


<section anchor="protocol-data-structures-and-constant-values" title="Protocol Data Structures and Constant Values">

<t>This section describes protocol types and constants.</t>

<section anchor="record-layer-1" title="Record Layer">
<figure><artwork><![CDATA[
   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

   enum {
       reserved(20), alert(21), handshake(22),
       application_data(23), early_handshake(25),
       (255)
   } ContentType;

   struct {
       ContentType type;
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       opaque fragment[TLSPlaintext.length];
   } TLSPlaintext;

   struct {
       ContentType type;
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       aead-ciphered struct {
          opaque content[TLSPlaintext.length];
       } fragment;
   } TLSCiphertext;
]]></artwork></figure>

</section>
<section anchor="alert-messages" title="Alert Messages">
<figure><artwork><![CDATA[
   enum { warning(1), fatal(2), (255) } AlertLevel;

   enum {
       close_notify(0),
       unexpected_message(10),              /* fatal */
       bad_record_mac(20),                  /* fatal */
       decryption_failed_RESERVED(21),      /* fatal */
       record_overflow(22),                 /* fatal */
       decompression_failure_RESERVED(30),  /* fatal */
       handshake_failure(40),               /* fatal */
       no_certificate_RESERVED(41),         /* fatal */
       bad_certificate(42),
       unsupported_certificate(43),
       certificate_revoked(44),
       certificate_expired(45),
       certificate_unknown(46),
       illegal_parameter(47),               /* fatal */
       unknown_ca(48),                      /* fatal */
       access_denied(49),                   /* fatal */
       decode_error(50),                    /* fatal */
       decrypt_error(51),                   /* fatal */
       export_restriction_RESERVED(60),     /* fatal */
       protocol_version(70),                /* fatal */
       insufficient_security(71),           /* fatal */
       internal_error(80),                  /* fatal */
       user_canceled(90),
       no_renegotiation(100),               /* fatal */
       unsupported_extension(110),          /* fatal */
       (255)
   } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;
]]></artwork></figure>

</section>
<section anchor="handshake-protocol-1" title="Handshake Protocol">
<figure><artwork><![CDATA[
   enum {
       reserved(0), client_hello(1), server_hello(2),
       session_ticket(4), hello_retry_request(6),
       server_key_share(7), certificate(11), reserved(12),
       certificate_request(13), server_configuration(14),
       certificate_verify(15), reserved(16), finished(20), (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;    /* handshake type */
       uint24 length;             /* bytes in message */
       select (HandshakeType) {
           case client_hello:        ClientHello;
           case server_hello:        ServerHello;
           case hello_retry_request: HelloRetryRequest;
           case server_key_share:    ServerKeyShare;
           case server_configuration:ServerConfiguration;
           case certificate:         Certificate;
           case certificate_request: CertificateRequest;
           case certificate_verify:  CertificateVerify;
           case finished:            Finished;
           case session_ticket:      NewSessionTicket;
       } body;
   } Handshake;
]]></artwork></figure>

<section anchor="hello-messages-1" title="Hello Messages">

<figure><artwork><![CDATA[
   uint8 CipherSuite[2];    /* Cryptographic suite selector */

   enum { null(0), (255) } CompressionMethod;

   struct {
       ProtocolVersion client_version = { 3, 4 };    /* TLS v1.3 */
       Random random;
       SessionID session_id;
       CipherSuite cipher_suites<2..2^16-2>;
       CompressionMethod compression_methods<1..2^8-1>;
       select (extensions_present) {
           case false:
               struct {};
           case true:
               Extension extensions<0..2^16-1>;
       };
   } ClientHello;

   struct {
       ProtocolVersion server_version;
       Random random;
       uint8 session_id_len;  // Must be 0.
       CipherSuite cipher_suite;
       select (extensions_present) {
           case false:
               struct {};
           case true:
               Extension extensions<0..2^16-1>;
       };
   } ServerHello;

   struct {
       ProtocolVersion server_version;
       CipherSuite cipher_suite;
       NamedGroup selected_group;
       Extension extensions<0..2^16-1>;
   } HelloRetryRequest;

   struct {
       ExtensionType extension_type;
       opaque extension_data<0..2^16-1>;
   } Extension;

   enum {
       signature_algorithms(13), 
       early_data(TBD),
       supported_groups(TBD),
       known_configuration(TBD),
       pre_shared_key(TBD)
       client_key_shares(TBD)
       (65535)
   } ExtensionType;

      struct {
        select (Role) {
          case client:
            opaque identifier<0..2^16-1>;

          case server:
            struct {};
        }
      } KnownConfigurationExtension

      opaque psk_identity<0..2^16-1>;

      struct {
        select (Role) {
          case client:           
            psk_identity identities<0..2^16-1>;

          case server:
            psk_identity identity;

      } PreSharedKeyExtension;

      enum { early_handshake(1), early_data(2),
             early_handshake_and_data(3), (255) } EarlyDataType;
           
      struct {
        select (Role) {
          case client:
            opaque context<0..255>;
            EarlyDataType type;
          case server:
            struct {};                
        }
      } EarlyDataIndication;

   struct {
       Extension extensions<0..2^16-1>;
   } EncryptedExtensions;

      struct {
          opaque configuration_id<1..2^16-1>;
          uint32 expiration_date;
          NamedGroup group;
          opaque server_key<1..2^16-1>;
          Boolean early_data_allowed;
      } ServerConfiguration;
]]></artwork></figure>

<section anchor="signature-algorithm-extension" title="Signature Algorithm Extension">
<figure><artwork><![CDATA[
   enum {
       none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
       sha512(6), (255)
   } HashAlgorithm;

   enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
     SignatureAlgorithm;

   struct {
         HashAlgorithm hash;
         SignatureAlgorithm signature;
   } SignatureAndHashAlgorithm;

   SignatureAndHashAlgorithm
     supported_signature_algorithms<2..2^16-2>;
]]></artwork></figure>

</section>
<section anchor="named-group-extension" title="Named Group Extension">
<figure><artwork><![CDATA[
   enum {
       // Elliptic Curve Groups.
       sect163k1 (1), sect163r1 (2), sect163r2 (3),
       sect193r1 (4), sect193r2 (5), sect233k1 (6),
       sect233r1 (7), sect239k1 (8), sect283k1 (9),
       sect283r1 (10), sect409k1 (11), sect409r1 (12),
       sect571k1 (13), sect571r1 (14), secp160k1 (15),
       secp160r1 (16), secp160r2 (17), secp192k1 (18),
       secp192r1 (19), secp224k1 (20), secp224r1 (21),
       secp256k1 (22), secp256r1 (23), secp384r1 (24),
       secp521r1 (25),

       // Finite Field Groups.
       ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
       ffdhe6144 (259), ffdhe8192 (260),
       ffdhe_private_use (0x01FC..0x01FF),

       // Reserved Code Points.
       reserved (0xFE00..0xFEFF),
       reserved(0xFF01),
       reserved(0xFF02),
       (0xFFFF)
   } NamedGroup;

   struct {
       NamedGroup named_group_list<1..2^16-1>;
   } NamedGroupList;
]]></artwork></figure>

</section>
</section>
<section anchor="key-exchange-messages" title="Key Exchange Messages">
<figure><artwork><![CDATA[
   struct {
       NamedGroup group;
       opaque key_exchange<1..2^16-1>;
   } ClientKeyShareOffer;

   struct {
       ClientKeyShareOffer offers<0..2^16-1>;
   } ClientKeyShare;

   opaque dh_Y<1..2^16-1>;

   opaque point <1..2^8-1>;

   struct {
       NamedGroup group;
       opaque key_exchange<1..2^16-1>;
   } ServerKeyShare;
]]></artwork></figure>

</section>
<section anchor="authentication-messages" title="Authentication Messages">
<figure><artwork><![CDATA[
   opaque ASN1Cert<1..2^24-1>;

   struct {
       ASN1Cert certificate_list<0..2^24-1>;
   } Certificate;

   enum {
       rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
       rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
       fortezza_dms_RESERVED(20), (255)
   } ClientCertificateType;

   opaque DistinguishedName<1..2^16-1>;

   struct {
       ClientCertificateType certificate_types<1..2^8-1>;
       SignatureAndHashAlgorithm
         supported_signature_algorithms<2..2^16-2>;
       DistinguishedName certificate_authorities<0..2^16-1>;
   } CertificateRequest;

   struct {
        digitally-signed struct {
           opaque handshake_hash[hash_length];
        }
   } CertificateVerify;
]]></artwork></figure>

</section>
<section anchor="handshake-finalization-messages" title="Handshake Finalization Messages">

<figure><artwork><![CDATA[
   struct {
       opaque verify_data[verify_data_length];
   } Finished;
]]></artwork></figure>

</section>
<section anchor="ticket-establishment" title="Ticket Establishment">
<figure><artwork><![CDATA[
  struct {
      uint32 ticket_lifetime_hint;
      opaque ticket<0..2^16-1>;
  } NewSessionTicket;
]]></artwork></figure>

</section>
</section>
<section anchor="the-cipher-suite" title="The Cipher Suite">

<t>The following values define the cipher suite codes used in the ClientHello and
ServerHello messages.
A cipher suite defines a cipher specification supported in TLS.</t>

<t>TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a TLS
connection during the first handshake on that channel, but MUST NOT be
negotiated, as it provides no more protection than an unsecured connection.</t>

<figure><artwork><![CDATA[
   CipherSuite TLS_NULL_WITH_NULL_NULL = {0x00,0x00};
]]></artwork></figure>

<t>The following cipher suite definitions, defined in <xref target="RFC5288"/>, are
used for server-authenticated (and optionally client-authenticated)
Diffie-Hellman. DHE denotes ephemeral Diffie-Hellman,
where the Diffie-Hellman parameters are signed by a signature-capable
certificate, which has been signed by the CA. The signing algorithm
used by the server is specified after the DHE component of the
CipherSuite name. The server can request any signature-capable
certificate from the client for client authentication.</t>

<figure><artwork><![CDATA[
   CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E};
   CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F};
   CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2};
   CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3};
]]></artwork></figure>

<t>The following cipher suite definitions, defined in <xref target="RFC5289"/>, are
used for server-authenticated (and optionally client-authenticated)
Elliptic Curve Diffie-Hellman. ECDHE denotes ephemeral Diffie-Hellman,
where the Diffie-Hellman parameters are signed by a signature-capable
certificate, which has been signed by the CA. The signing algorithm
used by the server is specified after the DHE component of the
CipherSuite name. The server can request any signature-capable
certificate from the client for client authentication.</t>

<figure><artwork><![CDATA[
   CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xC0,0x2B};
   CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xC0,0x2C};
   CipherSuite TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256   = {0xC0,0x2F};
   CipherSuite TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384   = {0xC0,0x30};
]]></artwork></figure>

<t>The following ciphers, defined in <xref target="RFC5288"/>,
are used for completely anonymous Diffie-Hellman
communications in which neither party is authenticated. Note that this mode is
vulnerable to man-in-the-middle attacks. Using this mode therefore is of
limited use: These cipher suites MUST NOT be used by TLS implementations
unless the application layer has specifically requested to allow anonymous key
exchange. (Anonymous key exchange may sometimes be acceptable, for example, to
support opportunistic encryption when no set-up for authentication is in place,
or when TLS is used as part of more complex security protocols that have other
means to ensure authentication.)</t>

<figure><artwork><![CDATA[
   CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6};
   CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7};
]]></artwork></figure>

<t>[[TODO: Add all the defined AEAD ciphers. This currently only lists
GCM. https://github.com/tlswg/tls13-spec/issues/53]]
Note that using non-anonymous key exchange without actually verifying the key
exchange is essentially equivalent to anonymous key exchange, and the same
precautions apply. While non-anonymous key exchange will generally involve a
higher computational and communicational cost than anonymous key exchange, it
may be in the interest of interoperability not to disable non-anonymous key
exchange when the application layer is allowing anonymous key exchange.</t>

<t>o  For cipher suites ending with _SHA256, HKDF is used
      with SHA-256 as the hash function.</t>

<t>o  For cipher suites ending with _SHA384, HKDF is used
      with SHA-384 as the hash function.</t>

<t>New cipher suite values are assigned by IANA as described in
<xref target="iana-considerations"/>.</t>

<t>Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in
SSL 3.0.</t>

</section>
<section anchor="the-security-parameters" title="The Security Parameters">

<t>These security parameters are determined by the TLS Handshake Protocol and
provided as parameters to the TLS record layer in order to initialize a
connection state. SecurityParameters includes:</t>

<figure><artwork><![CDATA[
   enum { server, client } ConnectionEnd;

   enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

   enum { aes_gcm } RecordProtAlgorithm;

   /* The algorithms specified in KDFAlgorithm and
      RecordProtAlgorithm may be added to. */

   struct {
       ConnectionEnd          entity;
       KDFAlgorithm           kdf_algorithm;
       RecordProtAlgorithm    record_prot_algorithm;
       uint8                  enc_key_length;
       uint8                  iv_length;
       opaque                 hs_master_secret[48];
       opaque                 master_secret[48];
       opaque                 client_random[32];
       opaque                 server_random[32];
   } SecurityParameters;
]]></artwork></figure>

</section>
<section anchor="changes-to-rfc-4492" title="Changes to RFC 4492">

<t>RFC 4492 <xref target="RFC4492"/> adds Elliptic Curve cipher suites to TLS. This document
changes some of the structures used in that document. This section details the
required changes for implementors of both RFC 4492 and TLS 1.2. Implementors of
TLS 1.2 who are not implementing RFC 4492 do not need to read this section.</t>

<t>This document adds a “signature_algorithm” field to the digitally-signed
element in order to identify the signature and digest algorithms used to create
a signature. This change applies to digital signatures formed using ECDSA as
well, thus allowing ECDSA signatures to be used with digest algorithms other
than SHA-1, provided such use is compatible with the certificate and any
restrictions imposed by future revisions of <xref target="RFC5280"/>.</t>

<t>As described in <xref target="server-certificate"/> and <xref target="client-certificate"/>, the
restrictions on the signature algorithms used to sign certificates are no
longer tied to the cipher suite (when used by the server) or the
ClientCertificateType (when used by the client). Thus, the restrictions on the
algorithm used to sign certificates specified in Sections 2 and 3 of RFC 4492
are also relaxed. As in this document, the restrictions on the keys in the
end-entity certificate remain.</t>

</section>
</section>
<section anchor="cipher-suite-definitions" title="Cipher Suite Definitions">

<figure><artwork><![CDATA[
Cipher Suite                          Key        Record
                                      Exchange   Protection   Hash

TLS_NULL_WITH_NULL_NULL               NULL       NULL_NULL    N/A
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256   DHE_RSA    AES_128_GCM  SHA256
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384   DHE_RSA    AES_256_GCM  SHA384
TLS_DHE_DSS_WITH_AES_128_GCM_SHA256   DHE_DSS    AES_128_GCM  SHA256
TLS_DHE_DSS_WITH_AES_256_GCM_SHA384   DHE_DSS    AES_256_GCM  SHA384
TLS_DH_anon_WITH_AES_128_GCM_SHA256   DH_anon    AES_128_GCM  SHA256
TLS_DH_anon_WITH_AES_256_GCM_SHA384   DH_anon    AES_128_GCM  SHA384
]]></artwork></figure>

</section>
<section anchor="implementation-notes" title="Implementation Notes">

<t>The TLS protocol cannot prevent many common security mistakes. This section
provides several recommendations to assist implementors.</t>

<section anchor="random-number-generation-and-seeding" title="Random Number Generation and Seeding">

<t>TLS requires a cryptographically secure pseudorandom number generator (PRNG).
Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash
operations, most notably SHA-1, are acceptable, but cannot provide more
security than the size of the random number generator state.</t>

<t>To estimate the amount of seed material being produced, add the number of bits
of unpredictable information in each seed byte. For example, keystroke timing
values taken from a PC compatible 18.2 Hz timer provide 1 or 2 secure bits
each, even though the total size of the counter value is 16 bits or more.
Seeding a 128-bit PRNG would thus require approximately 100 such timer values.</t>

<t><xref target="RFC4086"/> provides guidance on the generation of random values.</t>

</section>
<section anchor="certificates-and-authentication" title="Certificates and Authentication">

<t>Implementations are responsible for verifying the integrity of certificates and
should generally support certificate revocation messages. Certificates should
always be verified to ensure proper signing by a trusted Certificate Authority
(CA). The selection and addition of trusted CAs should be done very carefully.
Users should be able to view information about the certificate and root CA.</t>

</section>
<section anchor="cipher-suites" title="Cipher Suites">

<t>TLS supports a range of key sizes and security levels, including some that
provide no or minimal security. A proper implementation will probably not
support many cipher suites. For instance, anonymous Diffie-Hellman is strongly
discouraged because it cannot prevent man-in-the-middle attacks. Applications
should also enforce minimum and maximum key sizes. For example, certificate
chains containing keys or signatures weaker than 2048-bit RSA or 224-bit ECDSA
are not appropriate for secure applications.</t>

</section>
<section anchor="implementation-pitfalls" title="Implementation Pitfalls">

<t>Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand, and have been a source of
interoperability and security problems. Many of these areas have been clarified
in this document, but this appendix contains a short list of the most important
things that require special attention from implementors.</t>

<t>TLS protocol issues:</t>

<t><list style="symbols">
  <t>Do you correctly handle handshake messages that are fragmented to
  multiple TLS records (see <xref target="fragmentation"/>)? Including corner cases
  like a ClientHello that is split to several small fragments? Do
  you fragment handshake messages that exceed the maximum fragment
  size? In particular, the certificate and certificate request
  handshake messages can be large enough to require fragmentation.</t>
  <t>Do you ignore the TLS record layer version number in all TLS
  records? (see <xref target="backward-compatibility"/>)</t>
  <t>Have you ensured that all support for SSL, RC4, and EXPORT ciphers
  is completely removed from all possible configurations that support
  TLS 1.3 or later, and that attempts to use these obsolete capabilities
  fail correctly? (see <xref target="backward-compatibility"/>)</t>
  <t>Do you handle TLS extensions in ClientHello correctly, including
  omitting the extensions field completely?</t>
  <t>When the server has requested a client certificate, but no
  suitable certificate is available, do you correctly send an empty
  Certificate message, instead of omitting the whole message (see
  <xref target="client-certificate"/>)?</t>
</list></t>

<t>Cryptographic details:</t>

<t><list style="symbols">
  <t>What countermeasures do you use to prevent timing attacks against
  RSA signing operations <xref target="TIMING"/>.</t>
  <t>When verifying RSA signatures, do you accept both NULL and missing parameters
(see <xref target="cryptographic-attributes"/>)? Do you verify that the RSA padding
doesn’t have additional data after the hash value? <xref target="FI06"/></t>
  <t>When using Diffie-Hellman key exchange, do you correctly strip
  leading zero bytes from the negotiated key (see <xref target="diffie-hellman"/>)?</t>
  <t>Does your TLS client check that the Diffie-Hellman parameters sent
  by the server are acceptable (see
  <xref target="diffie-hellman-key-exchange-with-authentication"/>)?</t>
  <t>Do you use a strong and, most importantly, properly seeded random number
generator (see <xref target="random-number-generation-and-seeding"/>) Diffie-Hellman private values, the
DSA “k” parameter, and other security-critical values?</t>
</list></t>

</section>
</section>
<section anchor="backward-compatibility" title="Backward Compatibility">

<t>The TLS protocol provides a built-in mechanism for version negotiation between
endpoints potentially supporting different versions of TLS.</t>

<t>TLS 1.x and SSL 3.0 use compatible ClientHello messages. Servers can also handle
clients trying to use future versions of TLS as long as the ClientHello format
remains compatible and the client supports the highest protocol version available
in the server.</t>

<t>Prior versions of TLS used the record layer version number for various
purposes. (TLSPlaintext.record_version &amp; TLSCiphertext.record_version)
As of TLS 1.3, this field is deprecated and its value MUST be ignored by all
implementations. Version negotiation is performed using only the handshake versions.
(ClientHello.client_version &amp; ServerHello.server_version)
In order to maximize interoperability with older endpoints, implementations
that negotiate the usage of TLS 1.0-1.2 SHOULD set the record layer
version number to the negotiated version for the ServerHello and all
records thereafter.</t>

<section anchor="negotiating-with-an-older-server" title="Negotiating with an older server">

<t>A TLS 1.3 client who wishes to negotiate with such older servers will send a
normal TLS 1.3 ClientHello containing { 3, 4 } (TLS 1.3) in
ClientHello.client_version. If the server does not support this version it
will respond with a ServerHello containing an older version number. If the
client agrees to use this version, the negotiation will proceed as appropriate
for the negotiated protocol. A client resuming a session SHOULD initiate the
connection using the version that was previously negotiated.</t>

<t>If the version chosen by the server is not supported by the client (or not
acceptable), the client MUST send a “protocol_version” alert message and close
the connection.</t>

<t>If a TLS server receives a ClientHello containing a version number greater than
the highest version supported by the server, it MUST reply according to the
highest version supported by the server.</t>

<t>Some legacy server implementations are known to not implement the TLS
specification properly and might abort connections upon encountering
TLS extensions or versions which it is not aware of. Interoperability
with buggy servers is a complex topic beyond the scope of this document.
Multiple connection attempts may be required in order to negotiate
a backwards compatible connection, however this practice is vulnerable
to downgrade attacks and is NOT RECOMMENDED.</t>

</section>
<section anchor="negotiating-with-an-older-client" title="Negotiating with an older client">

<t>A TLS server can also receive a ClientHello containing a version number smaller
than the highest supported version. If the server wishes to negotiate with old
clients, it will proceed as appropriate for the highest version supported by
the server that is not greater than ClientHello.client_version. For example, if
the server supports TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the
server will proceed with a TLS 1.0 ServerHello. If the server only supports
versions greater than client_version, it MUST send a “protocol_version”
alert message and close the connection.</t>

<t>Note that earlier versions of TLS did not clearly specify the record layer
version number value in all cases (TLSPlaintext.record_version). Servers
will receive various TLS 1.x versions in this field, however its value
MUST always be ignored.</t>

</section>
<section anchor="backwards-compatibility-security-restrictions" title="Backwards Compatibility Security Restrictions">

<t>If an implementation negotiates usage of TLS 1.2, then negotiation of cipher
suites also supported by TLS 1.3 SHOULD be preferred, if available.</t>

<t>The security of RC4 cipher suites is considered insufficient for the reasons
cited in [RFC7465]. Implementations MUST NOT offer or negotiate RC4 cipher suites
for any version of TLS for any reason.</t>

<t>Old versions of TLS permitted the usage of very low strength ciphers.
Ciphers with a strength less than 112 bits MUST NOT be offered or
negotiated for any version of TLS for any reason.</t>

<t>The security of SSL 2.0 <xref target="SSL2"/> is considered insufficient for the reasons enumerated
in [RFC6176], and MUST NOT be negotiated for any reason.</t>

<t>Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-HELLO.
Implementations MUST NOT negotiate TLS 1.3 or later using an SSL version 2.0 compatible
CLIENT-HELLO. Implementations are NOT RECOMMENDED to accept an SSL version 2.0 compatible
CLIENT-HELLO in order to negotiate older versions of TLS.</t>

<t>Implementations MUST NOT send or accept any records with a version less than { 3, 0 }.</t>

<t>The security of SSL 3.0 <xref target="SSL3"/> is considered insufficient for the reasons enumerated
in [RFC7568], and MUST NOT be negotiated for any reason.</t>

<t>Implementations MUST NOT send a ClientHello.client_version or ServerHello.server_version
set to { 3, 0 } or less. Any endpoint receiving a Hello message with
ClientHello.client_version or ServerHello.server_version set to { 3, 0 } MUST respond
with a “protocol_version” alert message and close the connection.</t>

</section>
</section>
<section anchor="security-analysis" title="Security Analysis">

<t>[[TODO: The entire security analysis needs a rewrite.]]</t>

<t>The TLS protocol is designed to establish a secure connection between a client
and a server communicating over an insecure channel. This document makes
several traditional assumptions, including that attackers have substantial
computational resources and cannot obtain secret information from sources
outside the protocol. Attackers are assumed to have the ability to capture,
modify, delete, replay, and otherwise tamper with messages sent over the
communication channel. This appendix outlines how TLS has been designed to
resist a variety of attacks.</t>

<section anchor="handshake-protocol-2" title="Handshake Protocol">

<t>The handshake protocol is responsible for selecting a cipher spec and
generating a master secret, which together comprise the primary cryptographic
parameters associated with a secure session. The handshake protocol can also
optionally authenticate parties who have certificates signed by a trusted
certificate authority.</t>

<section anchor="authentication-and-key-exchange" title="Authentication and Key Exchange">

<t>TLS supports three authentication modes: authentication of both parties, server
authentication with an unauthenticated client, and total anonymity. Whenever
the server is authenticated, the channel is secure against man-in-the-middle
attacks, but completely anonymous sessions are inherently vulnerable to such
attacks. Anonymous servers cannot authenticate clients. If the server is
authenticated, its certificate message must provide a valid certificate chain
leading to an acceptable certificate authority. Similarly, authenticated
clients must supply an acceptable certificate to the server. Each party is
responsible for verifying that the other’s certificate is valid and has not
expired or been revoked.</t>

<t>[[TODO: Rewrite this because the master_secret is not used this way any
more after Hugo’s changes.]]
The general goal of the key exchange process is to create a master_secret
known to the communicating parties and not to attackers (see
<xref target="key-schedule"/>). The master_secret is required to generate the
Finished messages and record protection keys (see <xref target="server-finished"/> and
<xref target="traffic-key-calculation"/>). By sending a correct Finished message, parties thus prove
that they know the correct master_secret.</t>

<section anchor="anonymous-key-exchange" title="Anonymous Key Exchange">

<t>Completely anonymous sessions can be established using Diffie-Hellman for key
exchange. The server’s public parameters are contained in the server key
share message, and the client’s are sent in the client key share message.
Eavesdroppers who do not know the private values should not be able to find the
Diffie-Hellman result.</t>

<t>Warning: Completely anonymous connections only provide protection against
passive eavesdropping. Unless an independent tamper-proof channel is used to
verify that the Finished messages were not replaced by an attacker, server
authentication is required in environments where active man-in-the-middle
attacks are a concern.</t>

</section>
<section anchor="diffie-hellman-key-exchange-with-authentication" title="Diffie-Hellman Key Exchange with Authentication">

<t>When Diffie-Hellman key exchange is used, the client and server use
the client key exchange and server key exchange messages to send
temporary Diffie-Hellman parameters. The signature in the certificate
verify message (if present) covers the entire handshake up to that
point and thus attests the certificate holder’s desire to use the
the ephemeral DHE keys.</t>

<t>Peers SHOULD validate each other’s public key Y (dh_Ys offered by
the server or DH_Yc offered by the client) by ensuring that 1 &lt; Y &lt;
p-1.  This simple check ensures that the remote peer is properly
behaved and isn’t forcing the local system into a small subgroup.</t>

<t>Additionally, using a fresh key for each handshake provides Perfect
Forward Secrecy. Implementations SHOULD generate a new X for each
handshake when using DHE cipher suites.</t>

</section>
</section>
<section anchor="version-rollback-attacks" title="Version Rollback Attacks">

<t>Because TLS includes substantial improvements over SSL Version 2.0, attackers
may try to make TLS-capable clients and servers fall back to Version 2.0. This
attack can occur if (and only if) two TLS- capable parties use an SSL 2.0
handshake.</t>

<t>Although the solution using non-random PKCS #1 block type 2 message padding is
inelegant, it provides a reasonably secure way for Version 3.0 servers to
detect the attack. This solution is not secure against attackers who can
brute-force the key and substitute a new ENCRYPTED-KEY-DATA message containing
the same key (but with normal padding) before the application-specified wait
threshold has expired. Altering the padding of the least-significant 8 bytes of
the PKCS padding does not impact security for the size of the signed hashes and
RSA key lengths used in the protocol, since this is essentially equivalent to
increasing the input block size by 8 bytes.</t>

</section>
<section anchor="detecting-attacks-against-the-handshake-protocol" title="Detecting Attacks Against the Handshake Protocol">

<t>An attacker might try to influence the handshake exchange to make the parties
select different encryption algorithms than they would normally choose.</t>

<t>For this attack, an attacker must actively change one or more handshake
messages. If this occurs, the client and server will compute different values
for the handshake message hashes. As a result, the parties will not accept each
others’ Finished messages. Without the static secret, the attacker cannot
repair the Finished messages, so the attack will be discovered.</t>

</section>
</section>
<section anchor="protecting-application-data" title="Protecting Application Data">

<t>The shared secrets are hashed with the handshake transcript
to produce unique record protection secrets for each connection.</t>

<t>Outgoing data is protected using an AEAD algorithm before transmission. The
authentication data includes the sequence number, message type, message length,
and the message contents. The message type field is necessary to ensure that messages
intended for one TLS record layer client are not redirected to another. The
sequence number ensures that attempts to delete or reorder messages will be
detected. Since sequence numbers are 64 bits long, they should never overflow.
Messages from one party cannot be inserted into the other’s output, since they
use independent keys.</t>

</section>
<section anchor="denial-of-service" title="Denial of Service">

<t>TLS is susceptible to a number of denial-of-service (DoS) attacks. In
particular, an attacker who initiates a large number of TCP connections can
cause a server to consume large amounts of CPU doing asymmetric crypto
operations. However, because TLS is generally used over TCP, it is difficult for the
attacker to hide his point of origin if proper TCP SYN randomization is used
<xref target="RFC1948"/> by the TCP stack.</t>

<t>Because TLS runs over TCP, it is also susceptible to a number of DoS attacks on
individual connections. In particular, attackers can forge RSTs, thereby
terminating connections, or forge partial TLS records, thereby causing the
connection to stall. These attacks cannot in general be defended against by a
TCP-using protocol. Implementors or users who are concerned with this class of
attack should use IPsec AH <xref target="RFC4302"/> or ESP <xref target="RFC4303"/>.</t>

</section>
<section anchor="final-notes" title="Final Notes">

<t>For TLS to be able to provide a secure connection, both the client and server
systems, keys, and applications must be secure. In addition, the implementation
must be free of security errors.</t>

<t>The system is only as strong as the weakest key exchange and authentication
algorithm supported, and only trustworthy cryptographic functions should be
used. Short public keys and anonymous servers should be used with great
caution. Implementations and users must be careful when deciding which
certificates and certificate authorities are acceptable; a dishonest
certificate authority can do tremendous damage.</t>

</section>
</section>
<section anchor="working-group-information" title="Working Group Information">

<t>The discussion list for the IETF TLS working group is located at the e-mail
address <eref target="mailto:tls@ietf.org">tls@ietf.org</eref>. Information on the group and information on how to
subscribe to the list is at <eref target="https://www1.ietf.org/mailman/listinfo/tls">https://www1.ietf.org/mailman/listinfo/tls</eref></t>

<t>Archives of the list can be found at:
<eref target="https://www.ietf.org/mail-archive/web/tls/current/index.html">https://www.ietf.org/mail-archive/web/tls/current/index.html</eref></t>

</section>
<section anchor="contributors" title="Contributors">

<figure><artwork><![CDATA[
Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu

Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com

Steven M. Bellovin
Columbia University
smb@cs.columbia.edu

Benjamin Beurdouche

Karthikeyan Bhargavan (co-author of [I-D.ietf-tls-session-hash])
INRIA
karthikeyan.bhargavan@inria.fr

Simon Blake-Wilson (co-author of RFC4492)
BCI
sblakewilson@bcisse.com

Nelson Bolyard
Sun Microsystems, Inc.
nelson@bolyard.com (co-author of RFC4492)

Ran Canetti
IBM
canetti@watson.ibm.com

Pete Chown
Skygate Technology Ltd
pc@skygate.co.uk

Antoine Delignat-Lavaud (co-author of [I-D.ietf-tls-session-hash])
INRIA
antoine.delignat-lavaud@inria.fr

Tim Dierks (co-editor of TLS 1.0, 1.1, and 1.2)
Independent
tim@dierks.org

Taher Elgamal
Securify
taher@securify.com

Pasi Eronen
Nokia
pasi.eronen@nokia.com

Anil Gangolli
anil@busybuddha.org

David M. Garrett

Vipul Gupta (co-author of RFC4492)
Sun Microsystems Laboratories
vipul.gupta@sun.com

Chris Hawk (co-author of RFC4492)
Corriente Networks LLC
chris@corriente.net

Kipp Hickman

Alfred Hoenes

David Hopwood
Independent Consultant
david.hopwood@blueyonder.co.uk

Daniel Kahn Gillmor
ACLU
dkg@fifthhorseman.net

Phil Karlton (co-author of SSL 3.0)

Paul Kocher (co-author of SSL 3.0)
Cryptography Research
paul@cryptography.com

Hugo Krawczyk
IBM
hugo@ee.technion.ac.il

Adam Langley (co-author of [I-D.ietf-tls-session-hash])
Google
agl@google.com

Ilari Liusvaara
ilari.liusvaara@elisanet.fi

Jan Mikkelsen
Transactionware
janm@transactionware.com

Bodo Moeller (co-author of RFC4492)
Google
bodo@openssl.org

Erik Nygren
Akamai Technologies
erik+ietf@nygren.org

Magnus Nystrom
RSA Security
magnus@rsasecurity.com

Alfredo Pironti (co-author of [I-D.ietf-tls-session-hash])
INRIA
alfredo.pironti@inria.fr

Marsh Ray (co-author of [I-D.ietf-tls-session-hash])
Microsoft
maray@microsoft.com

Robert Relyea
Netscape Communications
relyea@netscape.com

Jim Roskind
Netscape Communications
jar@netscape.com

Michael Sabin

Dan Simon
Microsoft, Inc.
dansimon@microsoft.com

Martin Thomson
Mozilla
mt@mozilla.com

Tom Weinstein

Tim Wright
Vodafone
timothy.wright@vodafone.com
]]></artwork></figure>


</section>


  </back>
</rfc>

