How are digital signatures used in cryptographic protocols?

• A. To provide confidentiality
• B. To verify the authenticity and integrity of messages
• C. To randomize key material
• D. To compress messages

Answer: (B)

Verifying authenticity and integrity is the main purpose of digital signatures in cryptographic protocols. When someone signs a message, they create a signature from a hash of the message using their private key. Anyone who has the signer’s public key can verify that signature against the message. If verification succeeds, you can be confident that the message was created by the claimed sender (authenticity) and that it has not been altered since it was signed (integrity). This also supports non-repudiation: the signer cannot deny authorship because only their private key could have produced that signature.

It’s important to distinguish this from confidentiality, which is about keeping content secret. Confidentiality is achieved through encryption, not signatures. A signature does not hide the message; it proves who wrote it and that the content remains unchanged. Similarly, signatures don’t serve to randomize key material or compress messages; those are separate functions tied to key generation and data processing, not to the signing operation. In practice, signing is often combined with encryption in various ways (for example, sign-then-encrypt) to provide both authenticity and confidentiality as needed.