HMAC (RFC 2104)

1. Definition of HMAC

   H((K xor opad) || H((K xor ipad) || message)).

2. Keys

   Key length guidance and hashing long keys.

3. Implementation note

   Precomputing inner and outer pad states.

4. Truncated output

   Truncating HMAC tags safely.

5. Security

   Security properties and collision resistance requirements.

6. FIPS 198-1 HMAC standard

   NIST definition of keyed-hash message authentication.

7. HMAC-SHA test vectors

   Test vectors for HMAC-SHA-224/256/384/512.

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