📝HMAC

tags

§ Cryptography

RFC

RFC 2104 - HMAC: Keyed-Hashing for Message Authentication

• Hash-based Message Authentication Code

• Can be used to verify both data integrity and authenticity of a message.

• Can be used with any cryptographic hash function.

• size of the HMAC output is the same as that of the hash function

• HMAC does not encode message. The message should be sent along with the HMAC. All other parties will hash the message and should receive the same HMAC value.

• HMAC is not vulnerable to Length extension attack

• inputs:

  • key

  • message

• output:

  • HMAC hash value of fixed length

Algorithm:

• two pads are produced from the key (outer and inner) by xoring key with byte-filled arrays of 0x5c and 0x36 respectively.

  • these constants are computed so that pads have large hamming distance

• $\mathrm{HMAC}(key, data) = \mathrm{Hash}(opad || \mathrm{Hash}(ipad || data))$ where $||$ is concatenation