Network Working Group E. Rescorla
Internet-Draft Network Resonance
Expires: June 16, 2007 M. Salter
National Security Agency
December 13, 2006
Opaque PRF Inputs for TLS
draft-rescorla-tls-opaque-prf-input-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
This document describes a mechanism for using opaque PRF inputs with
Transport Layer Security (TLS) and Datagram TLS (DTLS).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . . 3
3. The OpaquePRFInput Extension . . . . . . . . . . . . . . . . . 3
3.1. Negotiating the OpaquePRFInput Extension . . . . . . . . . 4
3.2. PRF Modifications . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
4.1. Threats to TLS . . . . . . . . . . . . . . . . . . . . . . 5
4.2. New Security Issues . . . . . . . . . . . . . . . . . . . . 5
4.3. Scope of Randomness . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
TLS [RFC4346] and DTLS [RFC4347] use a 32-byte "Random" value
consisting of a 32-bit time value time and 28 randomly generated
bytes:
struct {
uint32 gmt_unix_time;
opaque random_bytes[28];
} Random;
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 a number of United States Government applications, it is desirable
to have some material with the following properties:
1. It is contributed both by client and server.
2. It is arbitrary-length.
3. It is mixed into the eventual keying material.
4. It is structured and decodable by the receiving party.
These requirements are incompatible with the current Random
mechanism, which supports a short, fixed-length value. This document
describes a mechanism called "Opaque PRF Inputs for TLS" that meets
these requirements.
2. 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].
3. The OpaquePRFInput Extension
The OpaquePRFInput is carried in a new TLS extension called
"OpaquePRFInput".
struct {
opaque opaque_prf_input_value<0..2^16-1>;
} OpaquePRFInput;
The opaque_prf_input_value is an opaque byte-string which is
generated in an implementation-dependent fashion. It MAY be
generated by and/or made available to the TLS/DTLS-using application.
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3.1. Negotiating the OpaquePRFInput Extension
The client requests support for the opaque PRF input feature by
sending an "opaque_prf_input" extension in its ClientHello. The
"extension_data" field contains an OpaquePRFInput value.
When a server which does not recognize the "opaque_prf_input"
extension receives one, it will ignore it as required by [RFC4366].
A server which recognizes the extension MAY choose to ignore it, in
which case it SHOULD continue with the exchange as if it had not
received the extension.
If the server wishes to use the opaque PRF input feature, it MUST
send its own "opaque_prf_input" extension with an
opaque_prf_input_value equal in length to the client's
opaque_prf_input_value. Clients SHOULD check the length of the
server's opaque_prf_input_value and generate a fatal
"illegal_parameter" error if it is present but does does not match
the length that was transmitted in the ClientHello.
Because RFC 4366 does not permit servers to request extensions which
the client did not offer, the client may not offer the
"opaque_prf_input" extension even if the server requires it. In this
case, the server should generate a fatal "handshake_failure" alert.
Because there is no way to mark extensions as critical, the server
may ignore the "opaque_prf_input" extension even though the client
requires it. If a client requires the opaque PRF input feature but
the server does not negotiate it, the client SHOULD generate a fatal
"handshake_failure" alert.
3.2. PRF Modifications
When the opaque PRF input feature is in use, the opaque PRF input
values MUST be mixed into the PRF along with the client and server
random values during the PMS->MS conversion. Thus, the PRF becomes:
master_secret = PRF(pre_master_secret, "master secret",
ClientHello.random +
ClientHello.opaque_prf_input_value +
ServerHello.random +
ServerHello.opaque_prf_input_value)[0..47];
Because new extensions may not be introduced in resumed handshakes,
mixing in the opaque PRF inputs during the MS->keying material
conversion would simply involve mixing in the same material twice.
Therefore, the opaque PRF inputs are only used when the PMS is
converted into the MS.
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4. Security Considerations
4.1. Threats to TLS
When this extension is in use it increases the amount of data that an
attacker can inject into the PRF. This potentially would allow an
attacker who had partially compromised the PRF greater scope for
influencing the output. Hash-based PRFs like the one in TLS are
designed to be fairly indifferent to the input size (the input is
already greater than the block size of most hash functions), however
there is currently no proof that a larger input space would not make
attacks easier.
Another concern is that bad implementations might generate low
entropy opaque PRF input values. TLS is designed to function
correctly even when fed low-entropy random values because they are
primarily used to generate distinct keying material for each
connection.
4.2. New Security Issues
As noted in Section 3 it is anticipated that applications may want to
have access to the opaque PRF input values and that they may contain
data that is meaningful at a higher layer. Because the values are
covered by the TLS Finished message, they are integrity-protected by
TLS. However, the application must independently provide any
confidentiality necessary for those values.
4.3. Scope of Randomness
RFC 4366 specifies that when a session is resumed the extensions from
the original connection are used:
If, on the other hand, the older session is resumed, then the
server MUST ignore the extensions and send a server hello
containing none of the extension types. In this case, the
functionality of these extensions negotiated during the original
session initiation is applied to the resumed session.
This motivates why the the opaque PRF input does not get mixed into
the PRF when generating the keying material from the master secret.
Because the same opaque PRF inputs would be used for every connection
in a session, they would not provide any differentiation in the
keying material between the connections.
5. IANA Considerations
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This document defines an extension to TLS, in accordance with
[RFC4366]:
enum { opaque_prf_input (??) } ExtensionType;
[[ NOTE: These values need to be assigned by IANA ]]
6. Acknowledgements
This work was supported by the US Department of Defense.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[I-D.ietf-tls-rfc4346-bis]
Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.2", draft-ietf-tls-rfc4346-bis-02 (work in progress),
October 2006.
7.2. Informative References
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Authors' Addresses
Eric Rescorla
Network Resonance
2483 E. Bayshore #212
Palo Alto, CA 94303
USA
Email: ekr@networkresonance.com
Margaret Salter
National Security Agency
9800 Savage Rd.
Fort Meade 20755-6709
USA
Email: msalter@restarea.ncsc.mil
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