Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Updates: 4347, 5246                                     January 29, 2010
(if approved)
Intended status: Standards Track
Expires: August 2, 2010

                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

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1.  Introduction

   Some TLS [RFC5246] and DTLS [RFC4347] 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.  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.

   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
   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 RFC
   5246 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 +

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

   Some of the ideas in this document came from earlier documents
   authored by Eric Rescorla, Margaret Salter, and Jerry Solinas.

6.  Normative References

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

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

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

Author's Address

   Paul Hoffman
   VPN Consortium