What mechanism allows the system to verify both the source and content of a message without using any other means?

The correct choice is HMAC (hash-based message authentication code) because it combines a cryptographic hash function with a secret cryptographic key to verify both the integrity and authenticity of a message. This dual verification is crucial in ensuring that the message has not been altered in transit (integrity) and that it comes from an authenticated source (authenticity).

When a sender generates an HMAC for a message, they process the message data along with a secret key through a hash function. This results in a unique HMAC value that is sent along with the message. The receiver, who also possesses the same secret key, can compute the HMAC on their end using the received message. If the computed HMAC matches the one sent with the message, it confirms that the message is intact and originates from a legitimate source that knows the secret key.

Other options, while relevant to cryptographic processes, operate differently. Poly1305 is a message authentication code but is not as widely utilized as HMAC and does not provide a combination of strength and versatility as HMAC does. MD5 and SHA-256 are hashing algorithms that provide integrity checks but do not implement the additional layer of authentication through secret keys, making them insufficient alone for verifying both source and content