HMAC Based Hybrid Key Combiners for Multiple Keys

As a fundamental building block in security protocols, a key combiner is used to combine two or more input cryptographic keys, some potentially compromised, into a single pseudorandom output key. Most of the existing key combiners are for two input keys. This draft specifies two provably secure constructions of key combiners for multiple keys [WWW25], which is particularly useful in post-quantum migration where multiple input keys may be produced by different algorithms or even different approaches [RFC9370], [ETSI25]. Namely, here the input keys can be two or more, and the combined output key remains pseudorandom if at least one of the input keys is secure, i.e., uniformly random and unknown to an attacker. The two constructions, called HKCv1 and HKCv2, are based on the extract-then- expand paradigm of HMAC (Keyed-Hashing for Message Authentication) [RFC2104]. HKCv1 is designed for simultaneously available input keys using concatenation, while HKCv2 is tailored for scenarios where input keys arrive incrementally, using an iterative HMAC structure. [EDNOTE: .... ]