Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Experimental July 8, 2010
Expires: January 9, 2011
Additional Master Secret Inputs for TLS
draft-hoffman-tls-master-secret-input-03
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 Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on January 9, 2011.
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1. Introduction
Some TLS 1.2[RFC5246] and DTLS 1.2 [4347bis] 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. 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
(http://www.iana.org/assignments/tls-extensiontype-values/ tls-
extensiontype-values.xhtml) are extensions with master secret input.
5. Acknowledgements
Much of the text in this document is derived from text written by
Eric Rescorla, Margaret Salter, and Jerry Solinas.
6. Normative References
[4347bis] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
progress), October 2009.
[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.
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Author's Address
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
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