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Versions: 00 01                                                         
Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                        January 29, 2010
Expires: August 2, 2010

                   Additional Random Extension to TLS


   This document specifies a TLS/DTLS extension that uses the additional
   master secret inputs to achieve useful security properties.

Status of this Memo

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

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1.  Introduction

   This document defines a TLS [RFC5246] and DTLS [RFC4347] extension to
   provide additional random values for the derivation of the
   master_secret.  This extension is a "extensions with master secret
   input" as defined in [MASTERSECRETINPUT].

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.  The additional_random Extension

   TLS and DTLS use a 32-byte "Random" value consisting of a 32-bit time
   value and 28 randomly generated bytes.  The client and server each
   contribute a Random value which is then mixed with secret keying
   material to produce the final per-association keying material.  In
   some application environments, it is desirable to have the client
   and/or the server be able to input more random material in the master
   key calculation than is allowed by the fixed-length Random value.

   The additional_random extension to TLS and DTLS adds a variable
   amount of client-specified and/or server-specified opaque randomness
   to the master key calculation.

   The extension data field of the additional_random extention contains
   a Additional_Random_Data structure:

      struct {
         opaque additional_random_value<0..2^16>
      } Additional_Random_Data;

   The additional_random_value is a single opaque random octet string.
   The recipient of an additional_random extension MUST NOT try to parse
   the additional_random_value.

   Negotiating the additional_random extension has the same semantics as
   negotiating any other TLS/DTLS extension.  In addition, the size of
   the additional_random_value provided by the client does not indicate
   anything about the expected size of the additional_random_value from
   the server.

   The extension type for the additional_random extension is {TBD}.

   The additional_random extension is a extension with master secret
   input as defined in [MASTERSECRETINPUT].  The entire extension
   (including the extension type {TBD}, the length value, and the
   Additional_Random_Data) is used as the additional_ms_input value for
   calculating the master secret.

3.  Security Considerations

   Adding greater entropy to the master secret calculation does not have
   any negative security impacts on the master secret.

4.  IANA Considerations

   This document defines an extension to TLS, in accordance with RFC
   4366.  The following is to be added to the TLS Extensions registry
   (http://www.iana.org/assignments/tls-extensiontype-values/ tls-

     enum { additional_random(TBD) }

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

              Hoffman, P., "Additional Master Secret Inputs for TLS",
              draft-hoffman-tls-master-secret-input (work in progress),
              January 2010.

   [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

   Email: paul.hoffman@vpnc.org