Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Updates: 5723, 6617, 6631, 7296, 8784, 9370 (if 20 February 2025
approved)
Intended status: Standards Track
Expires: 24 August 2025
Use of Variable-Length Output Preudo-Random Functions (PRFs) in the
Internet Key Exchange Protocol Version 2 (IKEv2)
draft-smyslov-ipsecme-ikev2-prf-plus-00
Abstract
This document specifies the use of variable-length output Preudo-
Random Functions (PRFs) in the Internet Key Exchange Protocol Version
2 (IKEv2). Current IKEv2 specification relies on traditional PRFs
with fixed output length for key derivation and uses iterative
application of a PRF (called "prf+") in cases when longer output is
required. Appearance of PRFs that can output as much bits as
requested allows to streamline the key derivation functions of IKEv2.
This document updates RFCs 5723, 6617, 6631, 7296, 8784, 9370 for the
cases when variable-length output Preudo-Random Functions are used in
IKEv2 and its extensions.
Status of This Memo
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This Internet-Draft will expire on 24 August 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Use of Variable-Length Output PRFs in IKEv2 . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
The Internet Key Exchange protocol version 2 (IKEv2) [RFC7296] is
used in the IP Security (IPsec) architecture for the Security
Association (SA) parameters negotiation and for establishing an
authenticated shared secret. As part of the SA parameters
negotiation a Pseudo-Random Function (PRF) is agreed upon. Keying
materials for established SAs are then derived from the shared secret
using the negotiated PRF.
For the purpose of deriving variable-size keying material to
accomodate the use of cryptographic algorithms with different
properties, a construction called prf+ is used in IKEv2. This
construction is defined in Section 2.13 of [RFC7296] and is provided
here for convenience:
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
This construction allows to get more output bits than the output
length of the negotiated PRF by applying it iteratively.
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Modern achievements in cryptography bore new class of cryptographic
primitives - PRFs with variable-length output. These functions are
able to produce as many pseudorandom bits as requested in one call.
One example of such PRFs is KMAC [SP-800-185].
When variable-length output PRFs are considered for the use in IKEv2
for the key derivation purposes, it is possible either to keep the
prf+ construction always performung exactly one iteration with the
PRF producing the needed number of output bits at once, or to remove
the prf+ construction completely and replace it with a single call to
the PRF, that would also produce the needed number of output bits.
This is discussed in Section 8.3.2 of [I-D.salter-ipsecme-sha3] for
the case of KMAC.
Since it is envisioned that more variable-length output PRFs could
appear in future, this specification aims establish a uniform
approach for using them in IKEv2. This document updates a number of
RFCs specified IKEv2 and its extensions for the cases when variable-
length output Preudo-Random Functions are used: [RFC7296], [RFC5723],
[RFC6617], [RFC6631], [RFC8784], and [RFC9370].
2. Terminology and Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Use of Variable-Length Output PRFs in IKEv2
When a new preudo-random function is introduced for use in IKEv2, the
corresponding document must define whether the preudo-random function
has fixed or variable output length. This doesn't affect the process
of code point allocation for this prf, but affects the way it is used
in IKEv2 for the purposes of key derivation.
In particular, when a variable-length output pseudo-random function
is negotiated in IKEv2 in the Pseudo-random Function (PRF) transform
type, then the following rules have affect.
1. When such a PRF is used in the context of "prf", then its output
length MUST be set to the preferred key size of this PRF.
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2. When such a PRF is used in the context of "prf+", then the prf+
construction MUST NOT be used. Instead, the PRF output length
MUST be set to the required number of pseudorandom bits in the
current context and the use of prf+ is replaced with a single
call to this PRF.
3. If an API to such a PRF includes additional inputs, like
"customization string" in KMAC, then these inputs MUST be null
(e.g. zero-length string).
4. Security Considerations
It is assumed that variable-output length PRFs are cryptographically
strong and doesn't decrease the entropy of the input key if the
output length is at least the size of the key.
5. IANA Considerations
This specification makes no request to IANA.
6. Acknowledgements
This document was inspired by draft-salter-ipsecme-sha3 (Use of SHA-3
in the Internet Key Exchange Protocol Version 2 (IKEv2) and IPsec).
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
7.2. Informative References
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[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
DOI 10.17487/RFC5723, January 2010,
<https://www.rfc-editor.org/info/rfc5723>.
[RFC6617] Harkins, D., "Secure Pre-Shared Key (PSK) Authentication
for the Internet Key Exchange Protocol (IKE)", RFC 6617,
DOI 10.17487/RFC6617, June 2012,
<https://www.rfc-editor.org/info/rfc6617>.
[RFC6631] Kuegler, D. and Y. Sheffer, "Password Authenticated
Connection Establishment with the Internet Key Exchange
Protocol version 2 (IKEv2)", RFC 6631,
DOI 10.17487/RFC6631, June 2012,
<https://www.rfc-editor.org/info/rfc6631>.
[RFC8784] Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov,
"Mixing Preshared Keys in the Internet Key Exchange
Protocol Version 2 (IKEv2) for Post-quantum Security",
RFC 8784, DOI 10.17487/RFC8784, June 2020,
<https://www.rfc-editor.org/info/rfc8784>.
[RFC9370] Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
Key Exchanges in the Internet Key Exchange Protocol
Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
2023, <https://www.rfc-editor.org/info/rfc9370>.
[SP-800-185]
Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived
functions: cSHAKE, KMAC, TupleHash and ParallelHash",
National Institute of Standards and Technology,
DOI 10.6028/nist.sp.800-185, December 2016,
<https://doi.org/10.6028/nist.sp.800-185>.
[I-D.salter-ipsecme-sha3]
S, B., R, A., and J. C, "Use of SHA-3 in the Internet Key
Exchange Protocol Version 2 (IKEv2) and IPsec", Work in
Progress, Internet-Draft, draft-salter-ipsecme-sha3-00, 4
October 2024, <https://datatracker.ietf.org/doc/html/
draft-salter-ipsecme-sha3-00>.
Author's Address
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Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd)
124460
Russian Federation
Phone: +7 495 276 0211
Email: svan@elvis.ru
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