On Mon, Jan 04, 2021 at 10:49:13PM +0100, Stephan Müller wrote: > RFC5869 specifies an extract and expand two-step key derivation > function. The HKDF implementation is provided as a service function that > operates on a caller-provided HMAC cipher handle. HMAC isn't a "cipher". > The extract function is invoked via the crypto_hkdf_setkey call. Any reason not to call this crypto_hkdf_extract(), to match the specification? > RFC5869 > allows two optional parameters to be provided to the extract operation: > the salt and additional information. Both are to be provided with the > seed parameter where the salt is the first entry of the seed parameter > and all subsequent entries are handled as additional information. If > the caller intends to invoke the HKDF without salt, it has to provide a > NULL/0 entry as first entry in seed. Where does "additional information" for extract come from? RFC 5869 has: HKDF-Extract(salt, IKM) -> PRK Inputs: salt optional salt value (a non-secret random value); if not provided, it is set to a string of HashLen zeros. IKM input keying material There's no "additional information". > > The expand function is invoked via the crypto_hkdf_generate and can be > invoked multiple times. This function allows the caller to provide a > context for the key derivation operation. As specified in RFC5869, it is > optional. In case such context is not provided, the caller must provide > NULL / 0 for the info / info_nvec parameters. Any reason not to call this crypto_hkdf_expand() to match the specification? - Eric
