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doc,lib,test: rename HKDF 'key' argument

Rodrigo Muino Tomonari requested to merge github/fork/tniessen/doc-rename-hkdf-key-arg into master

HKDF is a key derivation function, which, as the name implies, can be used to produce a key. However, the input is usually not a cryptographic key. (Internally, HKDF passes the "key" to HMAC, but not as a key, but as data to be hashed. The salt argument is actually used as the key internally.)

To avoid confusion and the ambiguity of having two (or three) "keys" (input keying material and output keying material, and internally the pseudorandom key), this renames the argument in documentation and error messages.

I'd be happy about alternatives to the name ikm, which is not exactly self-explanatory. Is inputKeyingMaterial (or inputKeyMaterial) too verbose?

Refs: #39471 (closed)

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