Use of Variable-Length Output Pseudo-Random Functions (PRFs) in the Internet Key Exchange Protocol Version 2 (IKEv2)

The Internet Key Exchange protocol version 2 (IKEv2) 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 servers the role of a Key Deriation Function (KDF) in IKEv2 and is defined in Section 2.13 of and is provided here for convenience:

This construction allows to get more output bits than the output length of the negotiated PRF by applying it iteratively. 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 . 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 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 Pseudo-Random Functions are used: , , , , , , and .

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 when, and only when, they appear in all capitals, as shown here.

When a new pseudo-random function is introduced for use in IKEv2, the corresponding document must define whether the pseudo-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.

When such a PRF is defined for the use in IKEv2, a preferred key size for this function MUST be specified.
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.
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.
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).

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.

This specification makes no request to IANA.

This document was inspired by draft-salter-ipsecme-sha3 (Use of SHA-3 in the Internet Key Exchange Protocol Version 2 (IKEv2) and IPsec).