Hash-based Message Authentication Code

A type of Hashing algorithm designed to produce MAC. Allows Separation of message from Secret Key so that the order of secret key + message doesn’t matter.

Algorithm

$$\text{HMAC}(K,m)=\text{Hash}((K^{\prime }\oplus \text{opad})||\text{Hash}(K^{\prime }\oplus \text{ipad}||m))$$

Where:

• $\text{Hash}$ is the Hash function
• $m$ is the message to be authenticated
• $K$ is the secret key
• $K^{\prime }$ is the block-size secret key defined as:

$$K^{\prime }=\{\begin{array}{ll}H(K)& \text{if }K\text{ is larger than block size}\\ K& \text{otherwise}\end{array}$$

• $||$ is concatenation
• $\oplus$ is bitwise XOR
• $\text{opad}$ is padding consisting of repeated $\{\text{0x5c},\text{0x5c},\dots \}$
• $\text{ipad}$ is padding consisting of repeated $\{\text{0x36},\text{0x36},\dots \}$

Implementations

• HMAC-DRBG
• HS256