What key is used to sign a message?

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

Answer: (A)

Digital signatures rely on asymmetric cryptography: the signer uses their private key to generate a signature on the message (typically on a hash of the message). This signature can be verified by anyone who has the corresponding public key, proving that the message came from the signer and hasn’t been altered. The private key must remain secret, which is what gives the signature its trust: only the holder of that key could have created it. A public key certificate is used to bind the public key to the signer’s identity and to establish trust in that key, but it isn’t the key used to sign messages itself. A session key is a symmetric key used to encrypt data for a limited period, not for creating signatures.