HMACSHA1

HmacSHA1 is a specific instance of the HMAC (Hash-based Message Authentication Code) construction that uses the SHA-1 cryptographic hash function as its underlying primitive. It provides data integrity and authenticity by combining a secret key with a message to produce a fixed-size authentication tag. HMACs are widely used to verify that a message has not been altered and that it originated from a party possessing the shared key.

The HMAC construction operates with a key of up to the block size of the hash function. For SHA-1, the block size is 64 bytes. If the key is longer than 64 bytes, it is hashed with SHA-1 to produce a 20-byte value, then padded with zeros to 64 bytes. If the key is shorter, it is padded with zeros to 64 bytes. The key is then XORed with two fixed pads: ipad (0x36) and opad (0x5c). The inner hash is computed as H((K ⊕ ipad) || m), and the final HMAC is H((K ⊕ opad) || inner_hash), where H denotes the SHA-1 hash function and m is the message.

HmacSHA1 is defined in the general HMAC standard as described in RFC 2104, and SHA-1 has a 160-bit output. While SHA-1 itself has known weaknesses in collision resistance, HMAC-SHA1 is still considered secure for many practical applications, because the security of HMAC relies on the compression function and the key rather than collision resistance alone. Nevertheless, many standards and security advisories increasingly recommend using stronger hash functions, such as HMAC-SHA256 or higher, for new designs.

Common uses include message authentication in protocols and APIs, such as legacy TLS cipher suites and OAuth 1.0a signatures, where a shared secret is used to generate and verify the HMAC-SHA1 tag.