Internet Engineering Task Force (IETF)                        P. Hoffman
Request for Comments: 6358                                VPN Consortium
Category: Experimental                                      January 2012
ISSN: 2070-1721


               Additional Master Secret Inputs for TLS

Abstract

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

Status of This Memo

  This document is not an Internet Standards Track specification; it is
  published for examination, experimental implementation, and
  evaluation.

  This document defines an Experimental Protocol for the Internet
  community.  This document is a product of the Internet Engineering
  Task Force (IETF).  It represents the consensus of the IETF
  community.  It has received public review and has been approved for
  publication by the Internet Engineering Steering Group (IESG).  Not
  all documents approved by the IESG are a candidate for any level of
  Internet Standard; see Section 2 of RFC 5741.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  http://www.rfc-editor.org/info/rfc6358.

Copyright Notice

  Copyright (c) 2012 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (http://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of
  the Trust Legal Provisions and are provided without warranty as
  described in the Simplified BSD License.





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  This document may contain material from IETF Documents or IETF
  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 [RFC6347] 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
  itself.

  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
  unchanged.

1.1.  Conventions Used in This Document

  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 [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 +
                         )[0..47];

  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
  master_secret.






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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.  Acknowledgments

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

5.  Normative References

  [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.

  [RFC6347]   Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security version 1.2", RFC 6347, January 2012.

Author's Address

  Paul Hoffman
  VPN Consortium

  EMail: [email protected]










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