Final answer:
An HMAC uses an additional secret key to provide authentication, ensuring that the message is from a legitimate source and has not been altered. It also offers integrity verification but does not encrypt the message content.
Step-by-step explanation:
HMAC, which stands for Hash-based Message Authentication Code, uses an additional secret key as an input to the hash function. The main reason for this is to provide authentication. An HMAC is used to verify that the message has come from a legitimate source and has not been altered in transit, hence ensuring the authenticity of the message. The key ensures that the hash output is unique to the combination of the message and the key, making it difficult for an attacker to forge a valid HMAC without knowing the secret key.
While HMACs also provide integrity verification by ensuring that the message has not been tampered with, they do not provide encryption, as they do not conceal the content of the message. HMAC is not specifically designed to prevent Denial of Service (DoS) attacks, although ensuring the authentication of messages can help mitigate certain types of DoS attacks where message authentication is a factor.