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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: paul.hoffman@vpnc.org










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