2.13. Generating Keying Material

2.13. Generating Keying Material

In the context of the IKE SA, four cryptographic algorithms are negotiated: an encryption algorithm, an integrity protection algorithm, a Diffie-Hellman group, and a pseudorandom function (PRF). The PRF is used for the construction of keying material for all of the cryptographic algorithms used in both the IKE SA and the Child SAs.

We assume that each encryption algorithm and integrity protection algorithm uses a fixed-size key and that any randomly chosen value of that fixed size can serve as an appropriate key. For algorithms that accept a variable-length key, a fixed key size MUST be specified as part of the cryptographic transform negotiated (see Section 3.3.5 for the definition of the Key Length transform attribute). For algorithms for which not all values are valid keys (such as DES or 3DES with key parity), the algorithm by which keys are derived from arbitrary values MUST be specified by the cryptographic transform. For integrity protection functions based on Hashed Message Authentication Code (HMAC), the fixed key size is the size of the output of the underlying hash function.

It is assumed that PRFs accept keys of any length, but have a preferred key size. The preferred key size MUST be used as the length of SK_d, SK_pi, and SK_pr (see Section 2.14). For PRFs based on the HMAC construction, the preferred key size is equal to the length of the output of the underlying hash function. Other types of PRFs MUST specify their preferred key size.

Keying material will always be derived as the output of the negotiated PRF algorithm. Since the amount of keying material needed may be greater than the size of the output of the PRF, the PRF is used iteratively. The term "prf+" describes a function that outputs a pseudorandom stream based on the inputs to a pseudorandom function called "prf".

In the following, | indicates concatenation. prf+ is defined as:

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 continues until all the material needed to compute all required keys has been output from prf+. The keys are taken from the output string without regard to boundaries (e.g., if the required keys are a 256-bit Advanced Encryption Standard (AES) key and a 160-bit HMAC key, and the prf function generates 160 bits, the AES key will come from T1 and the beginning of T2, while the HMAC key will come from the rest of T2 and the beginning of T3).

The constant concatenated to the end of each prf function is a single octet. The prf+ function is not defined beyond 255 times the size of the prf function output.