HMACSHA512

HMAC-SHA512 is a specific instance of the HMAC (Hash-based Message Authentication Code) construction that uses the SHA-512 hash function as its underlying primitive. It produces a 512-bit (64-byte) authentication tag, which can be used to verify both the integrity and the authenticity of a message when a shared secret key is known only to the communicating parties.

The HMAC construction works with a secret key and a message. If the key is longer than the hash function’s block size, it is first hashed to produce a shorter key; otherwise it is padded with zeros to the block size. For SHA-512, the block size is 128 bytes. The key is then split into two derived keys by XOR-ing with inner and outer padding constants (ipad and opad). The inner hash is computed as H((K' XOR ipad) || m), and the outer hash is H((K' XOR opad) || inner_hash). The final output is the 64-byte HMAC value.

Security and usage considerations: HMAC-SHA512 combines the secret key with the message in a way that provides strong message authentication and protects against certain cryptographic attacks, including length-extension. Its 512-bit tag offers high resistance to forgery in typical applications. It is widely used in protocols and systems such as TLS, API authentication, and JSON Web Tokens (as HS512). Practical use requires a strong, randomly generated key, proper key management, and use of established cryptographic libraries to avoid side-channel leaks or implementation errors. While SHA-512 can be faster on 64-bit platforms, choosing between HMAC variants should consider security requirements, interoperability, and performance.