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Internet Engineering Task Force (IETF)                        P. HoffmanRequest for Comments: 6358                                VPN ConsortiumCategory: Experimental                                      January 2012ISSN: 2070-1721Additional Master Secret Inputs for TLSAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Hoffman                      Experimental                       [Page 1]

RFC 6358                  Additional TLS Inputs             January 2012   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 notHoffman                      Experimental                       [Page 2]

RFC 6358                  Additional TLS Inputs             January 2012   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.Hoffman                      Experimental                       [Page 3]

RFC 6358                  Additional TLS Inputs             January 20123.  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: paul.hoffman@vpnc.orgHoffman                      Experimental                       [Page 4]