Digital signature

A digital signature is created by hashing a message and encrypting the digest with the sender’s private key. Anyone holding the sender’s public key can decrypt the signature, hash the received message themselves, and compare: a match proves the message is unaltered (integrity) and could only have been signed by the private-key holder (authenticity). Because only the sender possesses that private key, the sender cannot later deny signing: nonrepudiation, the property the exam cares about most.

The caveats decide questions. A signature provides no confidentiality: the message itself travels in the clear unless it is separately encrypted, using the machinery of asymmetric encryption in the other direction. The key directions are unforgiving: sign with the sender’s private key, verify with the sender’s public key. And nonrepudiation is only as strong as the binding between that public key and a real identity, which is what a PKI certificate exists to provide.

Exam relevance: nonrepudiation is the trigger word, and digital signatures are the only mechanism that provides it. The confusable is the MAC or HMAC: it also proves integrity and authenticity, but both parties share the same secret key, so either could have produced it and neither can be held to it. If a scenario demands proof of origin that stands up against a denying sender, the answer is a digital signature.