HMAC

HMAC (Hash-based Message Authentication Code) is a message authentication code combining a hash function with a secret key, providing integrity and authenticity.

Purpose

Constructs a secure MAC from an existing hash function, requiring no dedicated design.

Mechanism

HMAC(K, m) = H((K ⊕ opad) || H((K ⊕ ipad) || m)), where ipad/opad are fixed pads. Keys longer than the hash block are hashed first. Must be used with a secure hash (SHA-256); even though MD5/SHA-1 are broken as hashes, HMAC-MD5/SHA1 remain comparatively secure.

Use cases

API request signing (e.g. AWS SigV4)
JWT HS256 signing
TLS record MAC
Cookie tamper-proofing

Examples

Input
```
key + message
```
Output
```
HMAC-SHA256 tag
```
32-byte authentication tag

Common errors

Using the bare hash H(key||msg) as a MAC (vulnerable to length-extension attacks): Always use the HMAC construction

Related terms

SHA-256 is the 256-bit-output member of the SHA-2 family, widely used in TLS, blockchains, and cryptographic applications, with no known practical attacks.

SHA-512 is the 512-bit-output member of the SHA-2 family; faster than SHA-256 on 64-bit platforms, providing a higher security margin.

MD5 is a 128-bit cryptographic hash function designed by Ronald Rivest; proven insecure by collision attacks, it is now used only for non-security purposes such as checksums.

JWT (JSON Web Token) is a compact, self-contained token that securely conveys claims as a JSON object, widely used for stateless authentication and information exchange.

Related tools

Generate HMAC (Keyed-Hash Message Authentication Code) with SHA-1/256/512. Free, private, and 100% client-side.

Hash Calculator

Compute MD5, SHA-1, SHA-256, or SHA-512 hashes with hex or Base64 output. Free, private, and 100% client-side.

References