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Additional Master Secret Inputs for TLS

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6358.
Author Paul E. Hoffman
Last updated 2015-10-14 (Latest revision 2010-07-08)
RFC stream Internet Engineering Task Force (IETF)
Stream WG state (None)
Document shepherd (None)
IESG IESG state RFC 6358 (Experimental)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Sean Turner
IESG note
Send notices to (None)
Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Experimental                               July 8, 2010
Expires: January 9, 2011

                Additional Master Secret Inputs for TLS


   This document describes a mechanism for using additional master
   secret inputs with Transport Layer Security (TLS) and Datagram TLS

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 2011.

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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF

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   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Introduction

   Some TLS 1.2[RFC5246] and DTLS 1.2 [4347bis] extensions want to mix
   particular data into the calculation of the master_secret.  This
   mixing creates a cryptographic binding of the added material directly
   into the secret that is used to protect the TLS session.  For
   example, some systems want to be sure that there is sufficient
   randomness in the TLS master_secret, and this can be accomplished by
   adding it directly to the master_secret calculations.

   This document describes a framework for TLS and DTLS extensions to
   meet these requirements.  In an extension that uses this framework, a
   client and server provide data in the handshake using normal TLS
   extensions, and then this data is combined with the ClientHello and
   ServerHello random values during the derivation of the master_secret.

   Extensions that specify data to be added to the master secret are
   called "extensions with master secret input".  An extension with
   master secret input must specify the additional input that comes from
   the client and/or the server.  Note that the term "and/or" is used
   here because the definition of the extension might cause input to the
   master secret to come from only one of the participants.

   Note that extensions that do not specify that they are extensions
   with master secret input cannot be extensions with master secret
   input.  That is, every extension that does not call itself an
   extension with master secret input is treated just like a normal
   extension.  Also note that this document only describes a framework;
   if an extension uses this framework, and a client and server both
   implement the extension, no signaling about the use of master secret
   input is needed: that comes as part of the extension definition

   Use of one or more of these extensions changes the way that the
   master secret is calculated in TLS and DTLS.  That is, if the
   handshake has no extensions, or only extensions that are not

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   extensions with master secret input, the master secret calculation is

1.1.  Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Master Secret Calculation Modifications for TLS and DTLS

   When an extension with master secret input is present in the
   handshake, the additional master secret input values MUST be mixed
   into the pseudorandom function (PRF) calculation along with the
   client and server random values during the computation of the
   master_secret.  For the calculation of the master secret, the
   extensions MUST be sorted by extension type order.  Note that TLS 1.2
   specifies that there can only be one extension per type, and the
   extensions can appear in mixed order.

   Each extension with master secret input adds its own specified input,
   called "additional_ms_input_1" for the extension with master secret
   input that has the lowest type number, "additional_ms_input_2" for
   the extension with master secret input with the second lowest type
   number, and so on.

   The calculation of the master_secret becomes:

      master_secret = PRF(pre_master_secret, "master secret",
                          ClientHello.random +
                          ClientHello.additional_ms_input_1 +
                          ClientHello.additional_ms_input_2 +
                          . . .
                          ClientHello.additional_ms_input_N +
                          ServerHello.random +
                          ServerHello.additional_ms_input_1 +
                          ServerHello.additional_ms_input_2 +
                          . . .
                          ServerHello.additional_ms_input_N +

   Using the specified order of the additional_ms_input_n fields in the
   master_secret is required for interoperability.  Otherwise, a server
   and a client would not know how to unambiguously calculate the same

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3.  Security Considerations

   This modification to TLS and DTLS increases the amount of data that
   an attacker can inject into the master secret calculation.  This
   potentially would allow an attacker who had partially compromised the
   inputs to the master secret calculation greater scope for influencing
   the output.  Hash-based PRFs like the one used in TLS master secret
   calculations are designed to be fairly indifferent to the input size.

   The additional master secret input may have no entropy; in fact, it
   might be completely predictable to an attacker.  TLS is designed to
   function correctly even when the PRF used in the master secret
   calculation has a great deal of predictable material because the PRF
   is used to generate distinct keying material for each connection.
   Thus, even in the face of completely predictable additional master
   secret input values, no harm is done to the resulting PRF output.
   When there is entropy in these values, that entropy is reflected in
   the PRF output.

4.  IANA Considerations

   [[ This section should be removed at the time of RFC publication. ]]
   No IANA registries are changed or created by this document.  At the
   time that this document was written, none of the extensions in the
   IANA TLS registry
   ( tls-
   extensiontype-values.xhtml) are extensions with master secret input.

5.  Acknowledgements

   Much of the text in this document is derived from text written by
   Eric Rescorla, Margaret Salter, and Jerry Solinas.

6.  Normative References

   [4347bis]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
              progress), October 2009.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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Author's Address

   Paul Hoffman
   VPN Consortium


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