<?xml version="1.0"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY RFC2119 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY RFC2616 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2616.xml">
<!ENTITY RFC4492 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4492.xml">
<!ENTITY RFC5246 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5246.xml">
<!ENTITY RFC5288 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5288.xml">
<!ENTITY RFC5289 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5289.xml">
<!ENTITY RFC7507 SYSTEM "http://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7507.xml">
]>
<?rfc strict="yes" ?>
<?rfc toc="yes" ?>
<?rfc compact="yes" ?>
<?rfc subcompact="no" ?>
<?rfc symrefs="yes" ?>
<rfc category="exp" docName="draft-ietf-tls-falsestart-01" ipr="trust200902">
  <front>
    <title abbrev="TLS False Start">
      Transport Layer Security (TLS) False Start
    </title>

    <author fullname="Adam Langley" initials="A." surname="Langley">
      <organization abbrev="Google">
        Google Inc.
      </organization>
      <address>
        <postal>
          <street>345 Spear St</street>
          <city>San Francisco</city>
          <region>CA</region>
          <code>94105</code>
          <country>USA</country>
        </postal>
        <email>agl@google.com</email>
      </address>
    </author>

    <author fullname="Nagendra Modadugu" initials="N." surname="Modadugu">
      <organization abbrev="Google">
        Google Inc.
      </organization>
      <address>
        <postal>
          <street>1600 Amphitheatre Parkway</street>
          <city>Mountain View</city>
          <region>CA</region>
          <code>94043</code>
          <country>USA</country>
        </postal>
        <email>nagendra@cs.stanford.edu</email>
      </address>
    </author>

    <author fullname="Bodo Moeller" initials="B." surname="Moeller">
      <organization abbrev="Google">
        Google Switzerland GmbH
      </organization>
      <address>
        <postal>
          <street>Brandschenkestrasse 110</street>
          <code>8002</code>
          <city>Zurich</city>
          <country>Switzerland</country>
        </postal>
        <email>bmoeller@acm.org</email>
      </address>
    </author>

    <date year="2015" month="November" day="2" />
    <area>Security</area>
    <workgroup>TLS Working Group</workgroup>

    <abstract>
      <t>
        This document specifies an optional behavior of TLS client implementations,
        dubbed False Start.
        It affects only protocol timing, not on-the-wire protocol data,
        and can be implemented unilaterally.
        A TLS False Start reduces handshake latency to one round trip.
      </t>
    </abstract>
  </front>

  <middle>
    <section title="Requirements Notation">
      <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
        <xref target="RFC2119">RFC 2119</xref>.
      </t>
    </section>

    <section title="Introduction">
      <t>
        A full handshake in TLS protocol versions up to TLS 1.2 <xref target="RFC5246" />
        requires two full protocol rounds (four flights)
        before the handshake is complete
        and the protocol parties may begin to send application data.
        Thus, using TLS can add a latency penalty of two network round-trip times for application
        protocols in which the client sends data first, such as <xref target="RFC2616">HTTP</xref>.
      </t>

      <figure>
        <artwork><![CDATA[
   Client                                               Server

   ClientHello                  -------->
                                                   ServerHello
                                                  Certificate*
                                            ServerKeyExchange*
                                           CertificateRequest*
                                <--------      ServerHelloDone
   Certificate*
   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   Finished                     -------->
                                            [ChangeCipherSpec]
                                <--------             Finished
   Application Data             <------->     Application Data

     Figure 1 [RFC5246].  Message flow for a full handshake
        ]]></artwork>
      </figure>

      <t>
        This document describes a technique that alleviates the latency burden imposed by TLS:
        the client-side TLS False Start.
        If certain conditions are met,
        the client can start to send application data when the full handshake
        is only partially complete, namely, when the client has sent its own
        <spanx style="verb">ChangeCipherSpec</spanx> and <spanx style="verb">Finished</spanx>
        messages (thus having updated its TLS Record Protocol write state as negotiated in the
        handshake), but has yet to receive the server's
        <spanx style="verb">ChangeCipherSpec</spanx> and <spanx style="verb">Finished</spanx>
        messages.
        (By section 7.4.9 of <xref target="RFC5246" />, after a full handshake,
        the client would have to delay sending application data
        until it has received and validated the server's
        <spanx style="verb">Finished</spanx> message.)
        Accordingly, the latency penalty for using TLS with HTTP can be kept at one round-trip time.
      </t>

      <t>
        When an earlier TLS session is resumed,
        TLS uses an abbreviated handshake with only three protocol flights.
        For application protocols in which the client sends data first,
        this abbreviated handshake adds just one round-trip time to begin with,
        so there is no need for a client-side False Start.
        However, if the server sends application data first,
        the abbreviated handshake adds two round-trip times,
        and this could be reduced to just one added round-trip time
        by doing a server-side False Start.
        There is little need for this in practice,
        so this document does not consider server-side False Starts further.
      </t>

      <t>
        Note also that TLS versions 1.3 <xref target="tls13" /> and beyond
        are out of scope for this document.
        False Start will not be needed with these newer versions since
        protocol flows minimizing the number of round trips have become a first-order design goal.
      </t>

      <t>
        In a False Start, when the client sends application data before it has received and verified
        the server's <spanx style="verb">Finished</spanx> message, there are two possible
        outcomes:
        <list style="symbols">
          <t>
            The handshake completes successfully:
            <!-- Colon introduces multiple sentences, so continue with capital letter -->
            Once both <spanx style="verb">Finished</spanx> messages have been received and verified,
            this retroactively validates the handshake.  In this case, the transcript of protocol
            data carried over the transport underlying TLS will look as usual, apart from the
            different timing.
          </t>
          <t>
            The handshake fails:
            <!-- Colon introduces multiple sentences, so continue with capital letter -->
            If a party does not receive the other side's <spanx style="verb">Finished</spanx>
            message, or if the <spanx style="verb">Finished</spanx> message's contents are not
            correct, the handshake never gets validated.  This means that an attacker may have
            removed, changed, or injected handshake messages.  In this case, data has been sent over
            the underlying transport that would not have been sent without the False Start.
          </t>
        </list>
        The latter scenario makes it necessary to restrict when a False Start is allowed,
        as described in this document.
        <xref target="sec.compatibility" /> considers basic requirements for using False Start.
        <xref target="sec.client-requirements" />
        specifies the behavior for clients, referring to important
        security considerations in <xref target="sec.security" />.
      </t>
    </section>

    <section anchor="sec.compatibility" title="False Start Compatibility">
      <t>
        TLS False Start as described in detail in the subsequent sections,
        if implemented, is an optional feature.
      </t>

      <t>
        A TLS server implementation is defined to be "False Start compatible" if it tolerates
        receiving TLS records on the transport connection early, before the protocol
        has reached the state to process these.
        For successful use of client-side False Start in a TLS connection, the server has to be False Start
        compatible.  Out-of-band knowledge that the server is False Start compatible may be available,
        e.g. if this is mandated by specific application profile standards.
        As discussed in <xref target="app.implementation" />,
        the requirement for False Start compatibility does generally not pose a hindrance in practice.
      </t>
    </section>

    <section anchor="sec.client-requirements" title="Client-side False Start">
      <t>
        This section specifies a change to the behavior of TLS client implementations
        in full TLS handshakes.
      </t>

      <t>
        When the client has sent its
        <spanx style="verb">ChangeCipherSpec</spanx> and <spanx style="verb">Finished</spanx>
        messages, its default behavior following <xref target="RFC5246" />
        is to not send application data until it has received the server's
        <spanx style="verb">ChangeCipherSpec</spanx> and <spanx style="verb">Finished</spanx>
        messages, which completes the handshake.
        With the False Start protocol modification, the client MAY send application data earlier
        (under the new Cipher Spec) if each of the following conditions is satisfied:
        <list style="symbols">
          <t>
            The application layer has requested the TLS False Start option.
          </t>
          <t>
            The symmetric cipher defined by the cipher suite negotiated in this handshake has been
            whitelisted for use with False Start according to the Security Considerations in
            <xref target="sec.security.symmetric" />.
          </t>
          <t>
            The protocol version chosen by ServerHello.server_version has been
            whitelisted for use with False Start according to the Security Considerations in
            <xref target="sec.security.version" />.
          </t>
          <t>
            The key exchange method defined by the cipher suite negotiated in this handshake
            and, if applicable, its parameters
            have been whitelisted for use with False Start according to
            the Security Considerations in <xref target="sec.security.keyexchange" />.
          </t>
          <t>
            In the case of a handshake with client authentication,
            the client certificate type has been whitelisted for use with False Start according to
            the Security Considerations in <xref target="sec.security.keyexchange" />.
          </t>
        </list>
        The rules for receiving data from the server remain unchanged.
      </t>

      <t>
        Note that the TLS client cannot infer the presence of an authenticated
        server until all handshake messages have been received.
        With False Start, unlike with the default handshake behavior,
        applications are able to send data before this point has been reached:
        from an application point of view,
        being able to send data does not imply that an authenticated peer is present.
        Accordingly, it is
        recommended that TLS implementations allow the application layer to query whether the
        handshake has completed.
      </t>
    </section>

    <section anchor="sec.security" title="Security Considerations">
      <t>
        In a TLS handshake, the <spanx style="verb">Finished</spanx> messages serve to validate
        the entire handshake.  These messages are based on a hash of the handshake so far
        processed by a PRF keyed with the new master secret (serving as a MAC),
        and are also sent under the new Cipher Spec with its keyed MAC,
        where the MAC key again is derived from the master secret.
        The protocol design relies on the assumption that
        any server and/or client authentication done during the
        handshake carries over to this.  While an attacker could, for example, have changed the
        cipher suite list sent by the client to the server and thus influenced cipher suite
        selection (presumably towards a less secure choice) or could have made other modifications
        to handshake messages in transmission, the attacker would not be able to round off the
        modified handshake with a valid <spanx style="verb">Finished</spanx> message:
        every TLS cipher suite is presumed to key the PRF appropriately
        to ensure unforgeability.  Once the handshake has been validated by verifying
        the <spanx style="verb">Finished</spanx> messages, this confirms that the
        handshake has not been tampered with, thus bootstrapping secure encryption
        (using algorithms as negotiated) from secure authentication.
      </t>

      <t>
        Using False Start interferes with this approach of bootstrapping secure encryption from
        secure authentication, as application data may have already been sent before
        <spanx style="verb">Finished</spanx> validation confirms that the handshake has not been
        tampered with -- so there is generally no hope to be sure that communication with the
        expected peer is indeed taking place during the False Start.
        Instead, the security goal is to ensure that if anyone at all can decrypt the application
        data sent in a False Start, this must be the legitimate peer: while an attacker could be
        influencing the handshake (restricting cipher suite selection, modifying key exchange
        messages, etc.), the attacker should not be able to benefit from this.
        The TLS protocol already relies on such a security property for authentication --
        with False Start, the same is needed for encryption.
        This motivates the rules put forth in the following subsections.
      </t>

      <t>
        It is prudent for applications to be even more restrictive.
        If heuristically a small list of cipher suites and a single protocol version
        is found to be sufficient for the majority of TLS handshakes in practice,
        it could make sense to forego False Start for any handshake
        that does not match this expected pattern,
        even if there is no concrete reason to assume a cryptographic weakness.
        Similarly, if handshakes almost always use ephemeral ECDH over one of a few named curves,
        it could make sense to disallow False Start with any other supported curve.
      </t>

      <section anchor="sec.security.symmetric" title="Symmetric Cipher">
        <t>
          Clients MUST NOT use the False Start protocol modification in a handshake
          unless the cipher suite uses a symmetric cipher that is considered cryptographically strong.
        </t>

        <t>
          Implementations may have their own classification of ciphers (and may additionally allow
          the application layer to provide a classification), but generally only symmetric ciphers
          with an effective key length of 128 bits or more can be considered strong.
          Also, various ciphers specified for use with TLS are known to have cryptographic
          weaknesses regardless of key length
          (none of the ciphers specified in <xref target="RFC4492" /> and <xref target="RFC5246" />
          can be recommended for use with False Start).
          The AES_128_GCM_SHA256 or AES_256_GCM_SHA384 ciphers specified in
          <xref target="RFC5288" /> and <xref target="RFC5289" /> can be considered
          sufficiently strong for most uses.
          Implementations that support additional cipher
          suites have to be careful to whitelist only suitable symmetric ciphers; if in doubt, False
          Start should not be used with a given symmetric cipher.
        </t>

        <t>
          While an attacker can change handshake messages to force a downgrade to a less secure
          symmetric cipher than otherwise would have been chosen, this rule ensures that in such
          a downgrade attack no application data will be sent under an insecure symmetric cipher.
        </t>
      </section>

      <section anchor="sec.security.version" title="Protocol Version">
        <t>
          Clients MUST NOT use the False Start protocol modification in a handshake unless the
          protocol version chosen by ServerHello.server_version has been whitelisted for this
          use.
        </t>

        <t>
          Generally, to avoid potential protocol downgrade attacks,
          implementations should whitelist only
          their latest (highest-valued) supported TLS protocol version
          (and, if applicable, any earlier protocol versions that
          they would use in fallback retries without TLS_FALLBACK_SCSV <xref target="RFC7507" />).
        </t>

        <t>
          The details of nominally identical cipher suites can differ between protocol versions,
          so this reinforces <xref target="sec.security.symmetric" />.
        </t>
      </section>

      <section anchor="sec.security.keyexchange" title="Key Exchange and Client Certificate Type">
        <t>
          Clients MUST NOT use the False Start protocol modification in a handshake unless the
          cipher suite uses a key exchange method that has been whitelisted for this use.
          Also, clients MUST NOT use the False Start protocol modification unless
          any parameters to the key exchange methods (such as ServerDHParams, ServerECDHParams)
          have been whitelisted for this use.
          Furthermore, when using client authentication, clients MUST NOT use the False Start
          protocol modification unless the client certificate type has been whitelisted for this
          use.
        </t>

        <t>
          Implementations may have their own whitelists of key exchange methods, parameters,
          and client certificate types (and may additionally allow the application layer to specify
          whitelists).  Generally, out of the options from <xref target="RFC5246" />
          and <xref target="RFC4492" />, the following whitelists are recommended:
          <list style="symbols">
            <t>
              Key exchange methods: DHE_RSA, ECDHE_RSA, DHE_DSS, ECDHE_ECDSA
            </t>
            <t>
              Parameters: well-known DH groups (at least 3,072 bits),
              named curves (at least 256 bits)
            </t>
            <t>
              Client certificate types:
              none
            </t>
          </list>
          However, if an implementation that supports only key exchange methods from
          <xref target="RFC5246" /> and <xref target="RFC4492" />
          does not support any of the above key exchange methods,
          all of its supported key exchange methods can be whitelisted for False Start use.
          Care is required with any additional key exchange methods,
          as these may not have similar properties.
        </t>

        <t>
          The recommended whitelists are such that
          if cryptographic algorithms suitable for forward secrecy would
          possibly be negotiated, no False Start will take place if the current handshake
          fails to provide forward secrecy.
          (Forward secrecy can be achieved using ephemeral Diffie-Hellman or ephemeral
          Elliptic-Curve Diffie-Hellman;
          there is no forward secrecy when a using key exchange method of
          RSA, RSA_PSK, DH_DSS, DH_RSA, ECDH_ECDSA, or ECDH_RSA,
          or a client certificate type of
          rsa_fixed_dh, dss_fixed_dh, rsa_fixed_ecdh, or ecdsa_fixed_ecdh.)
          As usual, the benefits of forward secrecy may need to be balanced against efficiency,
          and accordingly even implementations that support the above key exchange methods
          might whitelist further key exchange methods and client certificate types.
        </t>

        <t>
          Client certificate types rsa_sign, dss_sign, and ecdsa_sign do allow forward security,
          but using False Start with any of these means
          sending application data tied to the client's signature
          before the server's authenticity (and, thus, the CertificateRequest message)
          has been completely verified,
          so these too are not generally suitable for the client certificate type whitelist.
        </t>
      </section>
    </section>

    <section title="Acknowledgments">
      <t>
        The authors wish to thank
        Wan-Teh Chang, Ben Laurie, Martin Thomson, Eric Rescorla, and Brian Smith
        for their input.
      </t>
    </section>

    <section title="IANA Considerations">
      <t>
        None.
      </t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      &RFC2119;
      &RFC4492;
      &RFC5246;
      &RFC5288;
      &RFC5289;
    </references>

    <references title="Informative References">
      &RFC2616;
      &RFC7507;

      <reference anchor="tls13">
        <front>
          <title>The Transport Layer Security (TLS) Protocol Version 1.3</title>
          <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
            <organization />
          </author>
          <date month="October" year="2015" />
        </front>
        <seriesInfo name="Work in Progress," value="draft-ietf-tls-tls13-10" />
      </reference>

    </references>

    <section anchor="app.implementation" title="Implementation Notes">
      <t>
        TLS False Start is a modification to the TLS protocol, and some implementations that
        conform to <xref target="RFC5246" /> may have problems interacting with
        implementations that use the False Start modification.
        If the peer uses a False Start, application data records may be received
        directly following the peer's <spanx style="verb">Finished</spanx> message,
        before the TLS implementation has sent its own <spanx style="verb">Finished</spanx> message.
        False Start compatibility as defined in <xref target="sec.compatibility" />
        ensures that these records with application data will simply
        remain buffered for later processing.
      </t>

      <t>
        A False Start compatible TLS implementation does not
        have to be aware of the False Start concept, and is certainly not expected to detect whether
        a False Start handshake is currently taking place: thanks to transport layer buffering,
        typical implementations will be False Start compatible without having been
        designed for it.
      </t>
    </section>
  </back>
</rfc>
