[gnutls-devel] GnuTLS | Provide high-level KDF API (#813)

[Development of GNU's TLS library]

Daiki Ueno commented on a discussion: https://gitlab.com/gnutls/gnutls/issues/813#note_281041008

> Seeing how this is implemented in openssl and PKCS#11, such an API can get very messy and hard to use if everything needs to be handled.

While they have a good reason (i.e. new API addition is not easy) to provide a single API function that handles all KDF variants, I'm not sure GnuTLS needs to impose such restriction.

> Not sure if it helps, but checking what model could fit for such a demanding/extensible API, the closest I see is some function similar to gnutls_session_set_verify_cert2 with gnutls_vdata_types_t, but most likely on steroids.

I'm not a big fan of this idea, because that would move the error checking to the run time.

Given that GnuTLS (and nettle) currently only supports HKDF and PBKDF2, I propose to add 3 distinct functions for HKDF-Extract, HKDF-Expand, and PBKDF2 derivation. That way, most of the necessary parameters could be checked at compile time. Of course, it would be a problem if we support Argon2 or similar, but I don't think the number of supported KDFs explode in near future.

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