HMAC

HMAC, or Hash Message Authentication Code is often used to verify the integrity of data and the authenticity of messages. It’s overall strength relies on several factors: the cryptographic ability of the underlying hash function, the length and quality of the secret key being used, and the length of the hash output. Iterative hash functions such as SHA-1 and MD5 can be used in HMAC calculations. In their respective instances, the resulting algorithms are then termed HMAC-SHA-1 and HMAC-MD5. These type of hash functions break messages down into blocks of a fixed size and iterates over them by way of compression. In this example, the output length of HMAC would be the same as the underlying hash function, which is 160 bits for SHA-1 and 128 bits for MD5. While it’s length can be truncated, such modification makes it more vulnerable to intrusion such as the infamous Birthday attack.