Hash-based Message Authentication Code

cryptography

Definition

Hash-based Message Authentication Code

A hash-based message authentication code (HMAC) is a MAC constructed from a hash function $h$ (with block size $B$) and a secret key $K$.

Construction

Let $k^{\prime }$ be the key padded or hashed depending on byte sequence length $B$:

$$k^{\prime }=\{\begin{array}{ll}\mathsf{pad}(h(k))& \text{if }k\text{ larger than }B\text{ bytes}\\ \mathsf{pad}(k)& \text{otherwise}\end{array}$$

Define two fixed pads:

• $\text{ipad}$: byte sequence of length $B$ with all bytes equal to $\text{0x36}$
• $\text{opad}$: byte sequence of length $B$ with all bytes equal to $\text{0x5c}$

Then

$$\mathrm{HMAC}(k,m)=h(k^{\prime }\oplus \text{opad}\Vert h(k^{\prime }\oplus \text{ipad}\Vert m))$$

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Footnotes

1. 192.019 Introduction to Security ↩