One document matched: draft-bmoeller-tls-downgrade-scsv-00.xml

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<rfc category="std"
     ipr="trust200902"
     docName="draft-bmoeller-tls-downgrade-scsv-00"
     updates="2246,4346,5246">

  <?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>

  <?rfc toc="yes" ?>
  <?rfc symrefs="yes" ?>
  <?rfc sortrefs="yes"?>
  <?rfc iprnotified="no" ?>
  <?rfc strict="yes" ?>

  <front>
    <title abbrev="TLS Downgrade SCSV">
      TLS Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks
    </title>

    <author initials="B." surname="Moeller" fullname="Bodo 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="2013" month="September" day="25"/>

    <abstract>
      <t>
        This document defines a Signaling Cipher Suite Value (SCSV) that can be used to prevent
        protocol downgrades for Transport Layer Security (TLS).
      </t>
    </abstract>
  </front>


  <middle>
    <section anchor="sec.intro" title="Introduction">

      <t>
        To work around interoperability problems with legacy servers, many TLS client
        implementations do not rely on the TLS protocol version negotiation mechanism alone, but
        will intentionally reconnect using a downgraded protocol if initial handshake attempts fail.
        Such clients may fall back to connections in which they announce a version as low as TLS 1.0
        (or even its predecessor, SSL 3.0) as the highest supported version, and make no attempt of
        using TLS extensions.
      </t>

      <t>
        While such protocol downgrades can be a useful last resort for connections to actual legacy
        servers, there's a risk that active attackers could exploit the downgrade strategy to weaken
        the cryptographic security of connections.  (For example, if a server requires the client's
        Supported Elliptic Curves Extension <xref target="RFC4492"/> to choose a forward-secure key
        exchange algorithm, the attacker could interfere with connections using this extension to
        get the client and server to instead use a key exchange algorithm that does not providing
        forward secrecy.)  Also, handshake errors due to network glitches could similary be
        misinterpreted as interaction with a legacy server and result in a protocol downgrade.
      </t>

      <t>
        All unnecessary protocol downgrades are undesirable (e.g., from TLS 1.2 to TLS 1.1 if both
        the client and the server actually do support TLS 1.1); they can be particularly critical if
        they mean losing the TLS extension feature (when downgrading to TLS 1.0 without extensions,
        or to SSL 3.0).  This document defines a Signaling Cipher Suite Value (SCSV) that can be
        employed to prevent such protocol downgrades between clients and servers that comply to this
        document.
      </t>

      <t>
        This specification applies to implementations of TLS 1.0 <xref target="RFC2246"/>, TLS 1.1
        <xref target="RFC4346"/>, and TLS 1.2 <xref target="RFC5246"/>.  (It is particularly relevant
        if such implementations also include support for predecessor protocol SSL 3.0 <xref
        target="RFC6101"/>.)  It can be applied similarly to later protocol versions.
      </t>

      <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"/>.
      </t>
    </section>


    <section anchor="sec.value" title="The TLS_DOWNGRADE_SCSV Signaling Cipher Suite Value">

      <t>
        This document defines one new cipher suite value:
      </t>

      <t>
        <![CDATA[
                 TLS_DOWNGRADE_SCSV         {0x##, 0x##}
        ]]>
      </t>

      <t>
        [[Code point TBD.]]
      </t>

      <t>
        This is a signaling cipher suite value, i.e., it does not actually correspond to a suite of
        cryptosystems, and it can never be selected by the server in the handshake; rather, its
        presence in the client hello message serves as a backwards-compatible signal from the client
        to the server.
      </t>
    </section>


    <section anchor="sec.server" title="Server behavior">

      <t>
        This section specifies server behavior when receiving the TLS_DOWNGRADE_SCSV cipher suite
        from a client in ClientHello.cipher_suites.

        <list style="symbols">
          <t>
            If TLS_DOWNGRADE_SCSV appears in ClientHello.cipher_suites and the highest protocol
            version supported by the server is higher than the version indicated in
            ClientHello.client_version, the server MUST respond with a (fatal) protocol_version alert.
          </t>

          <t>
            If TLS_DOWNGRADE_SCSV appears in ClientHello.cipher_suites and the ClientHello message
            does not include field client_hello_extension_list at all (<xref target="RFC3546"/>, <xref
            target="RFC4366"/>, <xref target="RFC5246"/>), the server MUST respond with a (fatal)
            protocol_version alert.
          </t>
        </list>

        Otherwise (either TLS_DOWNGRADE_SCSV does not appear, or it appears *and* the client's
        protocol version is at least the highest protocol version supported by the server *and*
        there is a client_hello_extension_list - possibly empty), the server proceeds with the
        handshake as usual.
      </t>

      <t>
        (Note that the TLS specifications require server implementations that do not support TLS
        extensions to tolerate extra data at the end of the ClientHello message, at the location
        where the later specifications added the field client_hello_extension_list. The server
        behavior mandated here relies on that requirement; intolerance for such extra data is a bug
        that must be fixed before adding server-side support for TLS_DOWNGRADE_SCSV.)
      </t>
    </section>


    <section anchor="sec.client" title="Client behavior">

      <t>
        The TLS_DOWNGRADE_SCSV cipher suite value is meant for use by clients that repeat a
        connection attempt with a downgraded protocol in order to avoid interoperability problems
        with legacy servers.  This section specifies when to send it.

        <list style="symbols">
          <t>
            If a client sends a ClientHello.client_version containing a lower value than the latest
            (highest-valued) version supported by the client, it SHOULD include the
            TLS_DOWNGRADE_SCSV cipher suite value in ClientHello.cipher_suites.  This does not apply
            when the client intends to perform an abbreviated handshake to resume a previously
            negotiated session and sets ClientHello.client_version to the protocol version
            negotiated for that session.
          </t>

          <t>
            If a client that supports TLS extensions and has TLS extensions to send in the handshake
            entirely omits client_hello_extension_list from the ClientHello message, it SHOULD
            include the TLS_DOWNGRADE_SCSV cipher suite value in ClientHello.cipher_suites.
          </t>
        </list>
      </t>
      
      <t>
        Note that in the above, a protocol version is not considered supported by the client if it
        has been disabled by any applicable system or user settings: it is about the highest
        protocol version that the client would attempt using in a handshake, not about the highest
        protocol version implemented if its use is not actually enabled.  (For example, if the
        implementation supports TLS 1.2 but the user has disabled this protocol version, a TLS 1.1
        handshake is expected and does not warrant sending TLS_DOWNGRADE_SCSV.)
      </t>
    </section>


    <section anchor="sec.security" title="Security Considerations">

      <t>
        <xref target="sec.client"/> does not require client implementations to send
        TLS_DOWNGRADE_SCSV in any particular case, it merely recommends it; behavior can be adapted
        according to the client's security needs.  For example, during the initial deployment of a
        new protocol version (when some interoperability problems may have to be expected), smoothly
        falling back to the previous protocol version in case of problems may be preferrable to
        potentially not being able to connect at all: so TLS_DOWNGRADE_SCSV could be omitted for
        this particular protocol downgrade step.
      </t>

      <t>
        However, it is particularly strongly recommended to sent TLS_DOWNGRADE_SCSV when downgrading
        to SSL 3.0 as the CBC cipher suites in SSL 3.0 have weaknesses that cannot be addressed by
        implementation workarounds like the remaining weaknesses in later (TLS) protocol versions.
      </t>
    </section>
  </middle>


  <back>
    <references title="Normative References">
      &rfc2119;
      &rfc2246;
      &rfc4346;
      &rfc5246;
      &rfc3546;
      &rfc4366;
    </references>

    <references title="Informal References">
      &rfc4492;
      &rfc6101;
    </references>


    <section anchor="app.ack" title="Acknowledgements">

      <t>
        This specification was inspired by an earlier proposal by Eric Rescorla.
      </t>
    </section>
  </back>

</rfc>