One document matched: draft-ietf-tls-extractor-07.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY RFC2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY RFC5226 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5226.xml">
<!ENTITY RFC5216 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5216.xml">
<!ENTITY RFC3711 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3711.xml">
<!ENTITY RFC4346 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4346.xml">
<!ENTITY RFC5246 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5246.xml">
<!ENTITY RFC5281 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5281.xml">
<!ENTITY RFC4347 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4347.xml">
<!ENTITY I-D.ietf-avt-dtls-srtp SYSTEM "http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-avt-dtls-srtp">
]>
<!-- $Id -->
<?rfc inline="yes"?>
<?rfc toc="yes" ?>
<?rfc symrefs="yes" ?>
<?rfc iprnotified="no" ?>
<!--<?rfc strict="yes" ?>-->
<?rfc compact="yes" ?>
<?rfc sortrefs="no" ?>
<?rfc colonspace='yes' ?>
<rfc category="std" docName="draft-ietf-tls-extractor-07.txt"
ipr="pre5378Trust200902">
<front>
<title abbrev="TLS Exporters">Keying Material Exporters 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="07" month="September" year="2009" />
<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>
<list style="hanging">
<t hangText="Note:">The mechanism described in this document was previously
known as "TLS Extractors" but was changed to avoid a name conflict with
the use of the term "Extractor" in the cryptographic community.
</t>
</list>
</t>
<t>A number of protocols wish to leverage Transport Layer Security (TLS) <xref target="RFC5246"/> 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"/>, a key management scheme for 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 export keying material (later called Exported Keying Material
or EKM) from TLS/DTLS to an application or protocol residing at an
upper-layer, and securely agree on the upper-layer context
where the keying material will be used. The mechanism for exporting
the keying material has the following requirements:
</t>
<t>
<list style="symbols">
<t>Both client and server need to be able to export the same EKM value.</t>
<t>EKM values should be indistinguishable from random data
by attackers who don't know the master_secret.</t>
<t>It should be possible to export 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 fulfill these
requirements. This mechanism is compatible with all versions of TLS.
</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 using an exporter to obtain 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 a minimum, there should be some
mechanism for signalling that an exporter 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 exported 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 exporter 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 <xref target="I-D.ietf-avt-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 the 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 exported by 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="Exporter Definition" anchor="exporter.def">
<t>
The output of the exporter is intended to be used in a single
scope, which is associated with the TLS session, the label, and the context
value.
</t>
<t> The exporter takes three input values</t>
<t>
<list style="symbols">
<t>a disambiguating label string,</t>
<t>a per-association context value provided by the application using
the exporter, and</t>
<t>a length value.</t>
</list>
</t>
<t>
It then computes:
</t>
<t>
<figure>
<artwork><![CDATA[
PRF(SecurityParameters.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 5226 <xref target="RFC5226"/>.
Note that exporter labels have the potential to collide with
existing PRF labels. In order to prevent this, labels SHOULD
begin with "EXPORTER". This is not a MUST because there are
existing uses which have labels which do not begin with
this prefix.
</t>
<t>
The context value allows the application using the exporter
to mix its own data with the TLS PRF for the exporter
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.
Because the context value is mixed with the master_secret via
the PRF, it is safe to mix confidential information into the
extractor provided that the master_secret will not be known to
the attacker.
</t>
</section>
<section anchor="section.sec" title="Security Considerations">
<t>
The prime security requirement for exporter 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="exporter.def"/>, where the attacker can control the context value; such an attacker
MUST NOT be able to predict the output of the exporter.
Similarly, an attacker who does not know the master secret should
not be able to distinguish valid exporter 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 exporter 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 exporters 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>
<t>
With certain TLS cipher suites, the TLS master secret is not
necessarily unique to a single TLS session. In particular, with RSA
key exchange, a malicious party acting as TLS server in one session
and TLS client in another session can cause those two sessions to
have the same TLS master secret (though the sessions must be
established simultaneously to get adequate control of the Random
values). Applications using the EKM need
to consider this in how they use the EKM; in some cases, requiring
the use of other cipher suites (such as those using Diffie-Hellman key
exchange) may be advisable.
</t>
<t>
Designing a secure mechanism that uses extractors is not
necessarily straightforward. This document only provides
the extractor mechanism, but the problem of agreeing
on the surrounding context and the meaning of the
information passed to and from the extractor remains.
Any new uses of the extractor mechanism should be subject
to careful review.
</t>
</section>
<section anchor="section.iana" title="IANA Considerations">
<t>
IANA is requested to create (has created) a TLS Exporter Label
registry for this purpose. The initial contents of the registry
are given below:
</t>
<figure><artwork>
Value Reference Note
----------------------------- --------- ----
client finished [RFC5246] (1)
server finished [RFC5246] (1)
master secret [RFC5246] (1)
key expansion [RFC5246] (1)
client EAP encryption [RFC5216]
ttls keying material [RFC5281]
ttls challenge [RFC5281]
</artwork></figure>
<t>
Note(1): These entries are reserved and MUST NOT be used for
the purpose described in RFC XXXX, in order to avoid
confusion with similar, but distinct use in RFC 5246.
</t>
<t>
[ RFC Editor: Please replace 'XXXX' above by the RFC number assigned
to this document and delete this remark. ]
</t>
<t>
Future values are allocated via RFC 5226 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 and to Vijay Gurbani and Alfred Hoenes for
editorial comments.
</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2119;
&RFC5226;
&RFC5246;
</references>
<references title="Informative References">
&RFC5216;
&RFC3711;
&RFC4347;
&I-D.ietf-avt-dtls-srtp;
&RFC5281;
</references>
</back>
</rfc>
<!-- Keep this comment at the end of the file
Local variables:
mode: xml
sgml-omittag:nil
sgml-shorttag:nil
sgml-namecase-general:nil
sgml-general-insert-case:lower
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:2
sgml-indent-data:nil
sgml-parent-document:nil
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
-->
| PAFTECH AB 2003-2026 | 2026-04-23 08:25:25 |