A specific implementation of the Hash-based Message Authentication Code (HMAC) algorithm that uses the SHA-1 (Secure Hash Algorithm 1) cryptographic hash function. It is designed to verify both the data integrity and the authenticity of a message by using a shared secret key between communicating parties.
In HMAC-SHA1, the message is processed with SHA-1 in a two-pass structure: first by combining the key with an inner padding and hashing it with the message, and then by combining the result with an outer padding and hashing again. This process makes HMAC resistant to certain cryptographic attacks that can affect raw hash functions, and it ensures that only someone with the correct secret key can generate or validate the authentication code.
While SHA-1 itself is now considered cryptographically weak against collision attacks, HMAC-SHA1 remains widely used in protocols like SRTP, IPsec, and TLS for compatibility and performance reasons, though many systems are migrating toward stronger hash functions such as SHA-256.
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