Now that there are dedicated struct for initial parameters, make the
HMAC params provide a lc_hash_ctx to be used internally. This makes HMAC
less special and allows it to work with any hash, instead of needing
dedicated inits and ctx_news. As an upside, now it's possible to have a
one-pass HMAC.
This unbreaks {,X}ChaCha20-Poly1305. Thanks to the use of params, the
{,X}ChaCha20-Poly1305 doesn't need to reach into the guts of ChaCha20
state. Move {en,de}cryption and key setup into its own functions.
This breaks the tests, which will be fixed in the following commit.
This allows for more flexibility in the future. While at it, do note
that the RFC and draft implementations are followed. In particular, in
XChaCha20, hardcode the high word of the counter to 0.
This commit breaks ChaCha20-Poly1305. It'll be fixed in a subsequent
commit.
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