In public-key cryptography, which key signs a message?

• A. Sender's public key
• B. Session key
• C. Sender's private key
• D. Public key certificate

Answer: (C)

Signing a message in public-key cryptography is done with the sender’s private key. The sender creates a digital signature by hashing the message and encrypting that hash with their private key. Anyone who has the corresponding public key can verify the signature, confirming that the message came from the holder of the private key and hasn’t been altered. The session key, on the other hand, is used for symmetric encryption to protect confidentiality, not for signing. The public key is used to verify signatures, not to sign, and a public key certificate is a trusted binding of a public key to an identity, not the act of signing the message itself.