[noise] HKDF input sizes

[Stephen Touset]

The current documentation is unclear about the size of the inputs to HKDF. It’s clear that the outputs are either 32 or 64 bytes, but are the inputs always expected to be 32 bytes? I would argue that since all uses of HKDF use only the first 32 bytes of each output, the whole thing should simply be specified to take 32 byte inputs and always return 32 byte outputs.

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Stephen Touset
stephen at squareup.com