In public key encryption, 'signing' is the act of ________.

• A. Adding the password to the challenge message and hashing the two
• B. Hashing the plain text message
• C. Encrypting the message digest with its own private key
• D. Encrypting the message digest with its own public key

Answer: (C)

Signing creates a digital signature by taking a hash of the message and encrypting that digest with the sender’s private key. This binds the message to the signer, because only the private key holder could produce that signature. Anyone with the signer’s public key can verify by decrypting the signature to recover the digest and then comparing it to a fresh hash of the message; a match confirms both integrity and authenticity.

That’s why encrypting the digest with the signer’s private key is the correct description. Simply hashing the plain text doesn’t prove who sent it, and adding a password or hashing without a private-key operation doesn’t provide non-repudiation. Encrypting with the public key wouldn’t enable the signer to create a verifiable signature at all, since the private key is what proves identity in signing.