HMAC (Hash-based Message Authentication Code)

HMAC (Hash-based Message Authentication Code) is a mechanism for simultaneously verifying integrity (has the message been altered?) and authenticity (does the message come from the expected sender?). It combines a cryptographic hash function with a secret key.

How HMAC Works

HMAC(K, M) = H(K XOR opad || H(K XOR ipad || M))

Where:
  K    = secret key
  M    = message
  H    = hash function (e.g., SHA-256)
  ipad = inner pad (repeated 0x36)
  opad = outer pad (0x5c repeated)

In practice: HMAC calculates a deterministic "fingerprint" from the key and the message. Only those who know the same key can:

1. Calculate a valid HMAC
2. Verify a received HMAC

HMAC vs. simple hash function

Problem with simple hash: SHA-256(message) - anyone can compute the hash and create a new valid hash for a manipulated message.

HMAC solution: HMAC-SHA256(key, message) - without the secret key, no one can create or verify a valid MAC.

Length Extension Attack: Certain hash functions (MD5, SHA-1, SHA-256) are vulnerable to length extension attacks. HMAC protects against this with its double hash structure.

Practical Use

API Signatures: Almost all modern APIs (AWS, Stripe, GitHub Webhooks, Twilio) use HMAC-SHA256 for request authentication:

Signature = HMAC-SHA256(API_SECRET, timestamp + method + path + body)

JWT (JSON Web Tokens): HMAC-SHA256 (HS256) is one of the most common JWT signing algorithms—although asymmetric signatures (RS256, ES256) should be preferred for critical applications.

DKIM: Email signing uses HMAC-like signatures with private keys.

TLS: HMAC is used in the TLS record layer for message integrity (in TLS 1.2; TLS 1.3 uses AEAD modes that incorporate MACs).

Password Hashing Algorithms: PBKDF2 (Password-Based Key Derivation Function 2) uses HMAC internally for its iteration.

Security Recommendations

• Use HMAC-SHA-256 or HMAC-SHA-512 (MD5 and SHA-1 are obsolete)
• Keys should be at least as long as the hash output (256 bits for SHA-256)
• Rotate keys regularly
• Use constant-time comparison during verification (prevents timing attacks)
• Do not use HMAC for password hashing—use bcrypt/Argon2 instead

HMAC vs. Digital Signature

	HMAC	Digital Signature
Key type	Symmetric (same key)	Asymmetric (private/public)
Speed	Very fast	Slower
Non-repudiation	No (both parties know the key)	Yes (only the sender has the private key)
Typical use	API authentication, integrity between two trusting parties	Digital documents, certificates, code signing