Question 1

What is HMAC?

Accepted Answer

HMAC (Hash-based Message Authentication Code, RFC 2104) combines a cryptographic hash function with a secret key to create message authentication codes. It provides both data integrity and authenticity verification. HMAC processes data through nested hash operations: HMAC(K, m) = H((K ⊕ opad) || H((K ⊕ ipad) || m)), where K is the key, m is the message, H is the hash function, and opad/ipad are padding constants. This construction ensures that even if the hash function has weaknesses, HMAC remains secure.

Question 2

Which hash function should I use for HMAC?

Accepted Answer

SHA-256 is recommended for most applications offering 128-bit security strength. Use SHA-512 for 256-bit security in high-value systems or SHA-384 for 192-bit security. SHA-3 (Keccak) provides the latest NIST standard with different internal structure. Avoid MD5-HMAC and SHA-1-HMAC for new security-critical applications due to hash collision vulnerabilities, though HMAC construction itself remains secure. For legacy system compatibility, SHA-1-HMAC may still be acceptable as HMAC provides additional security over plain hashing.

Question 3

What is the recommended HMAC key length?

Accepted Answer

The key length should match the hash output size for optimal security: 32 bytes (256 bits) for SHA-256, 64 bytes (512 bits) for SHA-512, 20 bytes (160 bits) for SHA-1. Keys shorter than the hash output reduce security strength. Keys longer than the hash block size (64 bytes for SHA-256, 128 bytes for SHA-512) are first hashed before use. Use cryptographically secure random number generators (CSPRNG) to generate HMAC keys, never use passwords or predictable values directly as HMAC keys.

Question 4

Where is HMAC commonly used?

Accepted Answer

HMAC is widely deployed in: API authentication (AWS Signature, OAuth 2.0, HMAC-based tokens), JWT (JSON Web Tokens) with HS256/HS384/HS512 algorithms, webhook signature verification (GitHub, Stripe, PayPal), TLS/SSL (HMAC for record integrity in cipher suites), IPsec (authentication headers and ESP), challenge-response protocols, secure file integrity verification, and blockchain/cryptocurrency transaction signing. HMAC is preferred over simple hashing for authentication because it requires knowledge of the secret key.

Question 5

Can I truncate HMAC output?

Accepted Answer

Yes, HMAC supports truncation while maintaining security. RFC 2104 recommends keeping at least half the hash output length: minimum 16 bytes for SHA-256-HMAC (128 bits of security), 20 bytes for SHA-512-HMAC (160 bits). Common truncations include HMAC-SHA-256-128 (16 bytes), HMAC-SHA-512-256 (32 bytes). Truncation reduces MAC size for bandwidth-constrained environments while preserving authentication strength against forgery attacks. Never truncate below 80 bits (10 bytes) for security applications.

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