< draft-ietf-tls-cached-info-18.txt   draft-ietf-tls-cached-info-19.txt >
TLS S. Santesson TLS S. Santesson
Internet-Draft 3xA Security AB Internet-Draft 3xA Security AB
Intended status: Standards Track H. Tschofenig Intended status: Standards Track H. Tschofenig
Expires: September 9, 2015 ARM Ltd. Expires: September 24, 2015 ARM Ltd.
March 8, 2015 March 23, 2015
Transport Layer Security (TLS) Cached Information Extension Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-18.txt draft-ietf-tls-cached-info-19.txt
Abstract Abstract
Transport Layer Security (TLS) handshakes often include fairly static Transport Layer Security (TLS) handshakes often include fairly static
information, such as the server certificate and a list of trusted information, such as the server certificate and a list of trusted
certification authorities (CAs). This information can be of certification authorities (CAs). This information can be of
considerable size, particularly if the server certificate is bundled considerable size, particularly if the server certificate is bundled
with a complete certificate chain (i.e., the certificates of with a complete certificate chain (i.e., the certificates of
intermediate CAs up to the root CA). intermediate CAs up to the root CA).
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 9, 2015. This Internet-Draft will expire on September 24, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cached Information Extension . . . . . . . . . . . . . . . . 3 3. Cached Information Extension . . . . . . . . . . . . . . . . 3
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5 4. Exchange Specification . . . . . . . . . . . . . . . . . . . 4
4.1. Omitting the Server Certificate Message . . . . . . . . . 5 4.1. Server Certificate Message . . . . . . . . . . . . . . . 5
4.2. Omitting the CertificateRequest Message . . . . . . . . . 6 4.2. CertificateRequest Message . . . . . . . . . . . . . . . 6
4.3. Omitting the Certificate Status Information (OCSP 5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Stapling and Multi OCSP Stapling) . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7.2. New Registry for CachedInformationType . . . . . . . . . 9
7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7.2. New Registry for CachedInformationType . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 9.2. Informative References . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
Reducing the amount of information exchanged during a Transport Layer Reducing the amount of information exchanged during a Transport Layer
Security handshake to a minimum helps to improve performance in Security handshake to a minimum helps to improve performance in
environments where devices are connected to a network with a low environments where devices are connected to a network with a low
bandwidth, and lossy radio technology. With Internet of Things such bandwidth, and lossy radio technology. With Internet of Things such
environments exist, for example, when devices use IEEE 802.15.4 or environments exist, for example, when devices use IEEE 802.15.4 or
Bluetooth Smart. For more information about the challenges with Bluetooth Smart. For more information about the challenges with
smart object deployments please see [RFC6574]. smart object deployments please see [RFC6574].
This specification defines a TLS extension that allows a client and a This specification defines a TLS extension that allows a client and a
server to exclude transmission information cached in an earlier TLS server to exclude transmission information cached in an earlier TLS
handshake. handshake.
A typical example exchange may therefore look as follows. First, the A typical example exchange may therefore look as follows. First, the
client and the server executes the usual TLS handshake. The client client and the server executes the full TLS handshake. The client
may, for example, decide to cache the certificate provided by the then caches the certificate provided by the server. When the TLS
server. When the TLS client connects to the TLS server some time in client connects to the TLS server some time in the future, without
the future, without using session resumption, it then attaches the using session resumption, it then attaches the cached_info extension
cached_info extension defined in this document to the client hello defined in this document to the client hello message to indicate that
message to indicate that it had cached the certificate, and it it had cached the certificate, and it provides the fingerprint of it.
provides the fingerprint of it. If the server's certificate has not If the server's certificate has not changed then the TLS server does
changed then the TLS server does not need to send its' certificate not need to send its' certificate and the corresponding certificate
and the corresponding certificate list again. In case information chain again. In case information has changed, which can be seen from
has changed, which can be seen from the fingerprint provided by the the fingerprint provided by the client, the certificate payload is
client, the certificate payload is transmitted to the client to allow transmitted to the client to allow the client to update the cache.
the client to update the cache.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This document refers to the TLS protocol but the description is This document refers to the TLS protocol but the description is
equally applicable to DTLS as well. equally applicable to DTLS as well.
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This document defines a new extension type (cached_info(TBD)), which This document defines a new extension type (cached_info(TBD)), which
is used in client hello and server hello messages. The extension is used in client hello and server hello messages. The extension
type is specified as follows. type is specified as follows.
enum { enum {
cached_info(TBD), (65535) cached_info(TBD), (65535)
} ExtensionType; } ExtensionType;
The extension_data field of this extension, when included in the The extension_data field of this extension, when included in the
client hello, MUST contain the CachedInformation structure. The client hello, MUST contain the CachedInformation structure. The
client MUST NOT send multiple CachedObjects of the same client MAY send multiple CachedObjects of the same
CachedInformationType. CachedInformationType. This may, for example, be the case when the
client has cached multiple certificates from a server.
enum { enum {
cert(1), cert_req(2) (255) cert(1), cert_req(2) (255)
} CachedInformationType; } CachedInformationType;
struct { struct {
select (type) { select (type) {
case client: case client:
CachedInformationType type; CachedInformationType type;
opaque hash_value<1..255>; opaque hash_value<1..255>;
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For this type the message digest MUST be calculated using SHA-256 For this type the message digest MUST be calculated using SHA-256
[RFC4634]. [RFC4634].
Omitting the CertificateRequest Message Omitting the CertificateRequest Message
With the type set to 'cert_req', the client MUST include the With the type set to 'cert_req', the client MUST include the
message digest of the CertificateRequest message in the hash_value message digest of the CertificateRequest message in the hash_value
field. For this type the message digest MUST be calculated using field. For this type the message digest MUST be calculated using
SHA-256 [RFC4634]. SHA-256 [RFC4634].
Omitting the Certificate Status Information (OCSP Stapling and
Multiple OCSP Stapling) Message
With the type set to 'cert_status', the client MUST include the
message digest of the CertificateStatus message in the hash_value
field. For this type the message digest MUST be calculated using
SHA-256 [RFC4634].
New types can be added following the policy described in the IANA New types can be added following the policy described in the IANA
considerations section, see Section 7. Different message digest considerations section, see Section 7. Different message digest
algorithms for use with these types can also be added by registering algorithms for use with these types can also be added by registering
a new type that makes use of this updated message digest algorithm. a new type that makes use of this updated message digest algorithm.
4. Exchange Specification 4. Exchange Specification
Clients supporting this extension MAY include the "cached_info" Clients supporting this extension MAY include the "cached_info"
extension in the (extended) client hello. If the client includes the extension in the (extended) client hello. If the client includes the
extension then it MUST contain one or more CachedObject attributes. extension then it MUST contain one or more CachedObject attributes.
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Note: If a server is part of a hosting environment then the client Note: If a server is part of a hosting environment then the client
may have cached multiple data items for a single server. To allow may have cached multiple data items for a single server. To allow
the client to select the appropriate information from the cache it is the client to select the appropriate information from the cache it is
RECOMMENDED that the client utilizes the Server Name Indication RECOMMENDED that the client utilizes the Server Name Indication
extension [RFC6066]. extension [RFC6066].
Following a successful exchange of the "cached_info" extension in the Following a successful exchange of the "cached_info" extension in the
client and server hello, the server alters sending the corresponding client and server hello, the server alters sending the corresponding
handshake message. How information is altered from the handshake handshake message. How information is altered from the handshake
messages is defined in Section 4.1, Section 4.2 and Section 4.3 for messages is defined in Section 4.1, and in Section 4.2 for the types
the types defined in this specification. defined in this specification.
4.1. Omitting the Server Certificate Message 4.1. Server Certificate Message
When a ClientHello message contains the "cached_info" extension with When a ClientHello message contains the "cached_info" extension with
a type set to 'cert' then the server MAY omit the Certificate message a type set to 'cert' then the server MAY send the Certificate message
under the following conditions: shown in Figure 2 under the following conditions:
The server software implements the "cached_info" extension defined The server software implements the "cached_info" extension defined
in this specification. in this specification.
The 'cert' cached info extension is enabled (for example, a policy The 'cert' cached info extension is enabled (for example, a policy
allows the use of this extension). allows the use of this extension).
The server compared the value in the hash_value field of the The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of client-provided "cached_info" extension with the fingerprint of
the Certificate message it normally sends to clients. This check the Certificate message it normally sends to clients. This check
ensures that the information cached by the client is current. ensures that the information cached by the client is current.
The original Certificate handshake message syntax is defined in RFC The original Certificate handshake message syntax is defined in RFC
5246 [RFC5246] and has the following structure: 5246 [RFC5246] and has the structure shown in Figure 1.
opaque ASN.1Cert<1..2^24-1>; opaque ASN.1Cert<1..2^24-1>;
struct { struct {
ASN.1Cert certificate_list<0..2^24-1>; ASN.1Cert certificate_list<0..2^24-1>;
} Certificate; } Certificate;
Certificate Message as defined in RFC 5246. Figure 1: Certificate Message as defined in RFC 5246.
The new structure of the CertificateRequest message is shown in
Figure 2.
struct {
opaque hash_value<1..255>;
} CertificateRequest;
Figure 2: Cached Info Certificate Message.
The fingerprint MUST be computed as follows: hash_value:=SHA- The fingerprint MUST be computed as follows: hash_value:=SHA-
256(Certificate) 256(Certificate)
Note that RFC 7250 [RFC7250] allows the certificate payload to Note that RFC 7250 [RFC7250] allows the certificate payload to
contain only the SubjectPublicKeyInfo instead of the full information contain only the SubjectPublicKeyInfo instead of the full information
typically found in a certificate. Hence, when this specification is typically found in a certificate. Hence, when this specification is
used in combination with [RFC7250] and the negotiated certificate used in combination with [RFC7250] and the negotiated certificate
type is a raw public key then the TLS server omits sending a type is a raw public key then the TLS server omits sending a
Certificate payload that contains an ASN.1 Certificate structure with Certificate payload that contains an ASN.1 Certificate structure with
the included SubjectPublicKeyInfo rather than the full certificate. the included SubjectPublicKeyInfo rather than the full certificate.
As such, this extension is compatible with the raw public key As such, this extension is compatible with the raw public key
extension defined in RFC 7250. extension defined in RFC 7250.
4.2. Omitting the CertificateRequest Message 4.2. CertificateRequest Message
When a fingerprint for an object of type 'cert_req' is provided in When a fingerprint for an object of type 'cert_req' is provided in
the client hello, the server MAY omit the CertificateRequest message the client hello, the server MAY omit the CertificateRequest message
under the following conditions: under the following conditions:
The server software implements the "cached_info" extension defined The server software implements the "cached_info" extension defined
in this specification. in this specification.
The 'cert_req' cached info extension is enabled (for example, a The 'cert_req' cached info extension is enabled (for example, a
policy allows the use of this extension). policy allows the use of this extension).
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opaque DistinguishedName<1..2^16-1>; opaque DistinguishedName<1..2^16-1>;
struct { struct {
ClientCertificateType certificate_types<1..2^8-1>; ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>; supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>; DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest; } CertificateRequest;
The fingerprint MUST be computed as follows: hash_value:=SHA- Figure 3: CertificateRequest Message as defined in RFC 5246.
256(CertificateRequest)
4.3. Omitting the Certificate Status Information (OCSP Stapling and
Multi OCSP Stapling)
When a fingerprint for an object of type 'cert_status' is provided in
the client hello, the server MAY omit the CertificateStatus message
under the following conditions:
The server software implements the "cert_status" extension defined
in this specification.
The 'cert_status' cached info extension is enabled (for example, a
policy allows the use of this extension).
The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of
the CertificateStatus message it normally sends to clients. This
check ensures that the information cached by the client is
current.
Both client and server support the use of OCSP Stapling and/or
Multiple OCSP Stapling, as defined in RFC 6066 [RFC6066] and in
[RFC6961].
The CertificateStatus message syntax, defined in [RFC6961], has the
following structure:
struct { The new structure of the CertificateRequest message is shown in
CertificateStatusType status_type; Figure 4.
select (status_type) {
case ocsp: OCSPResponse;
case ocsp_multi: OCSPResponseList;
} response;
} CertificateStatus;
opaque OCSPResponse<0..2^24-1>; struct {
opaque hash_value<1..255>;
} CertificateRequest;
struct { Figure 4: Cached Info CertificateRequest Message.
OCSPResponse ocsp_response_list<1..2^24-1>;
} OCSPResponseList;
The fingerprint MUST be computed as follows: hash_value:=SHA- The fingerprint MUST be computed as follows: hash_value:=SHA-
256(CertificateStatus) 256(CertificateRequest)
5. Example 5. Example
Figure 1 illustrates an example exchange using the TLS cached info Figure 5 illustrates an example exchange using the TLS cached info
extension. In the normal TLS handshake exchange shown in flow (A) extension. In the normal TLS handshake exchange shown in flow (A)
the TLS server provides its certificate in the Certificate payload to the TLS server provides its certificate in the Certificate payload to
the client, see step [1]. This allows the client to store the the client, see step [1]. This allows the client to store the
certificate for future use. After some time the TLS client again certificate for future use. After some time the TLS client again
interacts with the same TLS server and makes use of the TLS cached interacts with the same TLS server and makes use of the TLS cached
info extension, as shown in flow (B). The TLS client indicates info extension, as shown in flow (B). The TLS client indicates
support for this specification via the "cached_info" extension, see support for this specification via the "cached_info" extension, see
[2], and indicates that it has stored the certificate from the [2], and indicates that it has stored the certificate from the
earlier exchange (by indicating the 'cert' type). With [3] the TLS earlier exchange (by indicating the 'cert' type). With [3] the TLS
server acknowledges the supports of the 'cert' type and by including server acknowledges the supports of the 'cert' type and by including
the value in the server hello informs the client that the certificate the value in the server hello informs the client that the content of
payload has been omitted. the certificate payload contains the fingerprint of the certificate
instead of the RFC 5246-defined payload of the certificate message in
message [4].
(A) Initial (full) Exchange (A) Initial (full) Exchange
ClientHello -> ClientHello ->
<- ServerHello <- ServerHello
Certificate* // [1] Certificate* // [1]
ServerKeyExchange* ServerKeyExchange*
CertificateRequest* CertificateRequest*
ServerHelloDone ServerHelloDone
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Finished Finished
Application Data <-------> Application Data Application Data <-------> Application Data
(B) TLS Cached Extension Usage (B) TLS Cached Extension Usage
ClientHello ClientHello
cached_info=(cert) -> // [2] cached_info=(cert) -> // [2]
<- ServerHello <- ServerHello
cached_info=(cert) [3] cached_info=(cert) [3]
Certificate [4]
ServerKeyExchange* ServerKeyExchange*
ServerHelloDone ServerHelloDone
ClientKeyExchange ClientKeyExchange
CertificateVerify* CertificateVerify*
[ChangeCipherSpec] [ChangeCipherSpec]
Finished -> Finished ->
<- [ChangeCipherSpec] <- [ChangeCipherSpec]
Finished Finished
Application Data <-------> Application Data Application Data <-------> Application Data
Figure 1: Example Message Exchange Figure 5: Example Message Exchange
6. Security Considerations 6. Security Considerations
This specification defines a mechanism to reference stored state This specification defines a mechanism to reference stored state
using a fingerprint. Sending a fingerprint of cached information in using a fingerprint. Sending a fingerprint of cached information in
an unencrypted handshake, as the client and server hello is, may an unencrypted handshake, as the client and server hello is, may
allow an attacker or observer to correlate independent TLS exchanges. allow an attacker or observer to correlate independent TLS exchanges.
While some information elements used in this specification, such as While some information elements used in this specification, such as
server certificates, are public objects and usually do not contain server certificates, are public objects and usually do not contain
sensitive information, other (not yet defined cached info types) may. sensitive information, other (not yet defined cached info types) may.
Those who implement and deploy this specification should therefore Those who implement and deploy this specification should therefore
make an informed decision whether the cached information is inline make an informed decision whether the cached information is inline
with their security and privacy goals. In case of concerns, it is with their security and privacy goals. In case of concerns, it is
advised to avoid sending the fingerprint of the data objects in advised to avoid sending the fingerprint of the data objects in
clear. clear.
The use of the cached info extension allows the server to obmit The use of the cached info extension allows the server to obmit
skipping to change at page 10, line 45 skipping to change at page 9, line 45
7.2. New Registry for CachedInformationType 7.2. New Registry for CachedInformationType
IANA is requested to establish a registry for TLS IANA is requested to establish a registry for TLS
CachedInformationType values. The first entries in the registry are CachedInformationType values. The first entries in the registry are
o cert(1) o cert(1)
o cert_req(2) o cert_req(2)
o cert_status(3)
The policy for adding new values to this registry, following the The policy for adding new values to this registry, following the
terminology defined in RFC 5226 [RFC5226], is as follows: terminology defined in RFC 5226 [RFC5226], is as follows:
o 0-63 (decimal): Standards Action o 0-63 (decimal): Standards Action
o 64-223 (decimal): Specification Required o 64-223 (decimal): Specification Required
o 224-255 (decimal): reserved for Private Use o 224-255 (decimal): reserved for Private Use
8. Acknowledgments 8. Acknowledgments
skipping to change at page 11, line 32 skipping to change at page 10, line 32
Sean Turner and Joe Salowey, as well as the responsible security area Sean Turner and Joe Salowey, as well as the responsible security area
director, Stephen Farrell, for their support. director, Stephen Farrell, for their support.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224",
RFC 3874, September 2004.
[RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMAC-SHA)", RFC 4634, July 2006. (SHA and HMAC-SHA)", RFC 4634, July 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011. Extension Definitions", RFC 6066, January 2011.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013.
9.2. Informative References 9.2. Informative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC6574] Tschofenig, H. and J. Arkko, "Report from the Smart Object [RFC6574] Tschofenig, H. and J. Arkko, "Report from the Smart Object
Workshop", RFC 6574, April 2012. Workshop", RFC 6574, April 2012.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and [RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7250, June 2014. (DTLS)", RFC 7250, June 2014.
Appendix A. Example
The Wireshark trace of an example TLS exchange shown in Figure 2
illustrates the use of an ECC-based ciphersuite with a 256 bit key.
ECC allows for a small certificate size compared to RSA with
equivalent security strength. The Certificate message provided by
the server is with 557 bytes (including the record layer header) one
of the largest message even though it only contains a single
certificate (i.e., no intermediate CA certificates). The client-
provided Certificate message has a length of 570 bytes (also
including the record layer header).
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 121
Handshake Protocol: Client Hello
Handshake Type: Client Hello (1)
Length: 117
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: c61b966bba2781c50b07c3278c43f5892b3d...
Session ID Length: 0
Cipher Suites Length: 10
Cipher Suites (5 suites)
Compression Methods Length: 1
Compression Methods (1 method)
Extensions Length: 66
Extension: server_name
Extension: signature_algorithms
Extension: elliptic_curves
Extension: ec_point_formats
Client <-- Server:
TLSv1.2 Record Layer: Handshake Protocol: Server Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 87
Handshake Protocol: Server Hello
Handshake Type: Server Hello (2)
Length: 83
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: 82d3d09b44149d738b7002da4ff5a986fe11...
Session ID Length: 32
Session ID: d069a74661088676b98db8346070278a7475b617a0...
Cipher Suite: Unknown (0xc0ad)
Compression Method: null (0)
Extensions Length: 11
Extension: renegotiation_info
Extension: ec_point_formats
TLSv1.2 Record Layer: Handshake Protocol: Certificate
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 557
Handshake Protocol: Certificate
Handshake Type: Certificate (11)
Length: 553
Certificates Length: 550
Certificates (550 bytes)
Certificate Length: 547
Certificate (id-at-commonName=localhost,
id-at-organizationName=PolarSSL,id-at-countryName=NL)
signedCertificate
algorithmIdentifier (iso.2.840.10045.4.3.2)
Padding: 0
encrypted: 30650231009a2c5cd7a6dba2e5640df0b94ed...
TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 215
Handshake Protocol: Server Key Exchange
Handshake Type: Server Key Exchange (12)
Length: 211
TLSv1.2 Record Layer: Handshake Protocol: Certificate Request
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 78
Handshake Protocol: Certificate Request
Handshake Type: Certificate Request (13)
Length: 74
Certificate types count: 1
Certificate types (1 type)
Signature Hash Algorithms Length: 2
Signature Hash Algorithms (1 algorithm)
Distinguished Names Length: 66
Distinguished Names (66 bytes)
TLSv1.2 Record Layer: Handshake Protocol: Server Hello Done
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 4
Handshake Protocol: Server Hello Done
Handshake Type: Server Hello Done (14)
Length: 0
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Certificate
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 570
Handshake Protocol: Certificate
Handshake Type: Certificate (11)
Length: 566
Certificates Length: 563
Certificates (563 bytes)
Certificate Length: 560
Certificate (id-at-commonName=PolarSSL Test Client 2,
id-at-organizationName=PolarSSL,id-at-countryName=NL)
signedCertificate
algorithmIdentifier (iso.2.840.10045.4.3.2)
Padding: 0
encrypted: 306502304a650d7b2083a299b9a80ffc8dee8...
TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 138
Handshake Protocol: Client Key Exchange
Handshake Type: Client Key Exchange (16)
Length: 134
TLSv1.2 Record Layer: Handshake Protocol: Certificate Verify
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 80
Handshake Protocol: Certificate Verify
Handshake Type: Certificate Verify (15)
Length: 76
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol:
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Client <-- Server:
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Figure 2: Example TLS Exchange (without Cached Info Extension).
The total size of the TLS exchange shown in Figure 2 is 1932 bytes
whereas the exchange shown in Figure 3 reduces the size to 1323 bytes
by omitting the Certificate and the CertificateRequest messages. As
it can be seen, the use of the cached info extension leads to an on-
the-wire improvement of more than 600 bytes.
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 121 + 21
Handshake Protocol: Client Hello
Handshake Type: Client Hello (1)
Length: 117 + 21
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: c61b966bba2781c50b07c3278c43f5892b3d...
Session ID Length: 0
Cipher Suites Length: 10
Cipher Suites (5 suites)
Compression Methods Length: 1
Compression Methods (1 method)
Extensions Length: 66
Extension: server_name
Extension: signature_algorithms
Extension: elliptic_curves
Extension: ec_point_formats
Extension: cached_info
Client <-- Server:
TLSv1.2 Record Layer: Handshake Protocol: Server Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 87 + 5
Handshake Protocol: Server Hello
Handshake Type: Server Hello (2)
Length: 83 + 5
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: 82d3d09b44149d738b7002da4ff5a986fe11...
Session ID Length: 32
Session ID: d069a74661088676b98db8346070278a7475b617a0...
Cipher Suite: Unknown (0xc0ad)
Compression Method: null (0)
Extensions Length: 11 + 5
Extension: renegotiation_info
Extension: ec_point_formats
Extension: cached_info
TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 215
Handshake Protocol: Server Key Exchange
Handshake Type: Server Key Exchange (12)
Length: 211
TLSv1.2 Record Layer: Handshake Protocol: Server Hello Done
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 4
Handshake Protocol: Server Hello Done
Handshake Type: Server Hello Done (14)
Length: 0
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Certificate
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 570
Handshake Protocol: Certificate
Handshake Type: Certificate (11)
Length: 566
Certificates Length: 563
Certificates (563 bytes)
Certificate Length: 560
Certificate (id-at-commonName=PolarSSL Test Client 2,
id-at-organizationName=PolarSSL,id-at-countryName=NL)
signedCertificate
algorithmIdentifier (iso.2.840.10045.4.3.2)
Padding: 0
encrypted: 306502304a650d7b2083a299b9a80ffc8dee8...
TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 138
Handshake Protocol: Client Key Exchange
Handshake Type: Client Key Exchange (16)
Length: 134
TLSv1.2 Record Layer: Handshake Protocol: Certificate Verify
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 80
Handshake Protocol: Certificate Verify
Handshake Type: Certificate Verify (15)
Length: 76
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol:
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Client <-- Server:
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Figure 3: Example TLS Exchange (with Cached Info Extension).
Note: To accomplish further on-the-wire handshake size message
reductions the Certificate message sent by the client can be reduced
in size by using the Client Certificate URL extension.
Authors' Addresses Authors' Addresses
Stefan Santesson Stefan Santesson
3xA Security AB 3xA Security AB
Scheelev. 17 Scheelev. 17
Lund 223 70 Lund 223 70
Sweden Sweden
Email: sts@aaa-sec.com Email: sts@aaa-sec.com
Hannes Tschofenig Hannes Tschofenig
ARM Ltd. ARM Ltd.
Hall in Tirol 6060 Hall in Tirol 6060
Austria Austria
Email: Hannes.tschofenig@gmx.net Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at URI: http://www.tschofenig.priv.at
 End of changes. 28 change blocks. 
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