One document matched: draft-ietf-tls-extractor-03.xml

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<rfc category="std" docName="draft-ietf-tls-extractor-03.txt" ipr="full3978">
  <front>
    <title abbrev="TLS Extractors">Keying Material Extractors for Transport Layer Security (TLS)</title>

    <author fullname="Eric Rescorla" initials="E." surname="Rescorla">
      <organization>Network Resonance</organization>

      <address>
        <postal>
          <street>2064 Edgewood Drive</street>

          <city>Palo Alto</city>

          <region>CA</region>

          <code>94303</code>

          <country>USA</country>
        </postal>

        <email>ekr@networkresonance.com</email>
      </address>
    </author>

    <date day="01" month="November" year="2008" />

    <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.
      </t>
    </abstract>
  </front>

  <middle>
    <section title="Introduction">
    <t>A number of protocols wish to leverage Transport Layer Security (TLS) <xref target="RFC4346"/> or Datagram TLS (DTLS) <xref target="RFC4347"/>
      to perform key establishment but then use some of the keying material
      for their own purposes. A typical example is DTLS-SRTP <xref target="I-D.ietf-avt-dtls-srtp"/>, which uses DTLS to perform a key exchange and 
      negotiate the SRTP <xref target="RFC3711"/> protection suite and
      then uses the DTLS master_secret to generate the SRTP keys.
    </t>
    <t>
      These applications imply a need to be
      able to extract keying material (later called Exported Keying Material
      or EKM) from TLS/DTLS, and securely agree on the upper-layer context
      where the keying material will be used. The mechanism for extracting
      the keying material has the following requirements:
      </t>
      <t>
      <list style="symbols">
      <t>Both client and server need to be able to extract the same EKM value.</t>
      <t>EKM values should be indistinguishable from random 
         by attackers who don't know the master_secret.</t>
      <t>It should be possible to extract multiple EKM values
	 from the same TLS/DTLS association.</t>
      <t>Knowing one EKM value should not reveal any 
         information about the master_secret or about other EKM values.</t>
      </list>
      </t>
    <t>
    The mechanism described in this document is intended to fill these
    requirements.
    </t>
    </section>
    <section title="Conventions Used In This Document">
      <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"></xref>.</t>
    </section>
    <section title="Binding to Application Contexts" anchor="sec.binding">
     <t>
     In addition to extracting keying material, an application using the
     keying material has to securely establish the upper-layer 
     context where the keying material will be used.  The details of this
     context depend on the application, but it could include things such as
     algorithms and parameters that will be used with the keys,
     identifier(s) for the endpoint(s) who will use the keys, identifier(s)
     for the session(s) where the keys will be used, and the lifetime(s)
     for the context and/or keys. At minimum, there should be some
     mechanism for signalling that an extractor will be used.
     </t>
     <t>
     This specification does not mandate a single mechanism for agreeing
     on such context; instead, there are several possibilities that
     can be used (and can complement each other). For example:
     </t>
     <t>
     <list style="symbols">
     <t>One important part of the context -- which application will
        use the extracted keys -- is given by the disambiguating label 
	string (see Section 4).
     </t>
     <t>Information about the upper-layer context can be included in
     the optional data after the extractor label (see Section 4).
     </t>
     <t>Information about the upper-layer context can be exchanged in TLS
     extensions included in the ClientHello and ServerHello messages.  
     This approach is used in [DTLS-SRTP].
     The handshake messages are protected by the Finished messages, so
     once the handshake completes, the peers will have the same view of the
     information. Extensions also allow a limited form of negotiation:
     for example, the TLS client could propose several alternatives for
     some context parameters, and TLS server could select one of them.
     </t>
     <t>The upper-layer protocol can include its own handshake which can
      be protected using the keys extracted from TLS.
     </t>
     </list>
     </t>
     <t>
     It is important to note that just embedding TLS messages in the
     upper-layer protocol may not automatically secure all the important
     context information, since the upper-layer messages are not covered by
     TLS Finished messages.
    </t>
    </section>
    <section title="Extractor Definition" anchor="extractor.def">
    <t>
      The output of the extractor is intended to be used in a single
      scope, which is associated with the TLS session, the label, and the context
      value. 
    </t>
    <t>
    An extractor takes as input three values:
    </t>
    <t>
    <list style="symbols">
    <t>A disambiguating label string</t>
    <t>A per-association context value provided by the
       extractor using application</t>
    <t>A length value</t>
    </list>
    </t>
    <t>
    It then computes:
    </t>
    <t>
        <figure>
          <artwork><![CDATA[	
       PRF(master_secret, label, 
           SecurityParameters.client_random +
           SecurityParameters.server_random +
           context_value_length + context_value
           )[length]
]]></artwork>
    </figure>
     </t>
    <t>
      Where PRF is the TLS PRF in use for the session.
    The output is a pseudorandom bit string of length bytes
    generated from the master_secret. 
    </t>
    <t>
    Labels here have the same definition as in TLS, i.e.,
    an ASCII string with no terminating NULL.
    Label values beginning with "EXPERIMENTAL" MAY be used
    for private use without registration. All other
    label values MUST be registered via Specification Required as
    described by RFC 2434 <xref target="RFC2434"/>.
    Note that extractor labels have the potential to collide with 
    existing PRF labels. In order to prevent this, labels SHOULD
    begin with "EXTRACTOR". This is not a MUST because there are
    existing uses which have labels which do not begin with
    this prefix.
    </t>
    <figure>
	<artwork><![CDATA[
	opaque context<0..2^16-1>;
	]]>
	</artwork>
    </figure>
    <t>
    The context value allows the application using the extractor
    to mix its own data with the TLS PRF for the extractor
    output. One example of where this might be useful is 
    an authentication setting where the client credentials
    are valid for more than one identity; the context value
    could then be used to mix the expected identity into
    the keying material, thus preventing substitution attacks.
    The context value length is encoded as an unsigned
    16-bit quantity (uint16) representing the length of the
    context value. The context MAY be zero length.
    </t>
    </section>
    <section anchor="section.sec" title="Security Considerations">
    <t>
      The prime security requirement for extractor outputs is that
      they be independent. More formally, 
      after a particular TLS session, if an adversary is allowed to choose
      multiple (label, context value) pairs and is given the output of the PRF for
      those values, the attacker is still unable to distinguish between the output
      of the PRF for a (label, context value) pair (different from the ones that
      it submitted) and a random value of the same length. In particular,
      there may be settings, such as the one described in <xref target="extractor.def"/>, where the attacker can control the context value; such an attacker
      MUST not be able to predict the output of the extractor.
      Similarly, an attacker who does not know the master secret should
      not be able to distinguish valid extractor outputs from random values.
      The current set of TLS PRFs is believed to meet this objective, provided the
      master secret is randomly generated.
    </t>
    <t>
      Because an extractor produces the same value if applied twice
      with the same label to the same master_secret, it is critical
      that two EKM values generated with the same label not be used
      for two different purposes--hence the requirement for IANA
      registration. 
      However, because extractors depend on the TLS PRF, it is not a
      threat to the use of an EKM value generated from one label to reveal an
      EKM value generated from another label.
    </t>
    </section>
    <section anchor="section.iana" title="IANA Considerations">
    <t>
   IANA is requested to create (has created) a TLS Extractor Label 
   registry for this purpose. The initial contents of the registry 
   are given below:
    </t>
<figure><artwork>
   Value                          Reference
   -----                          ------------
   client finished                [RFC4346]
   server finished                [RFC4346]
   master secret                  [RFC4346]
   key expansion                  [RFC4346]
   client EAP encryption          [RFC2716]
   ttls keying material           [draft-funk-eap-ttls-v0-01]
</artwork></figure>
<t>
Future values are allocated via RFC2434 Specification Required
policy. The label is a string consisting of printable ASCII
characters.  IANA MUST also verify that one label is not a prefix
of any other label. For example, labels "key" or "master secretary"
are forbidden.
</t>
    </section>
    <section title="Acknowledgments">
    <t>
    Thanks to Pasi Eronen for valuable comments and the contents of the IANA section
    and <xref target="sec.binding"/>. Thanks to David McGrew for helpful
    discussion of the security considerations.
    </t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      &RFC2119;
      &RFC2434;
      &RFC4346;
    </references>
    <references title="Informational References">
      &RFC4347;
      &RFC3711;
      &I-D.ietf-avt-dtls-srtp;
    </references> 
  </back>
</rfc>
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