Which key does the signer use to encrypt the message digest to create the signature?

• A. Signer's Public Key
• B. Signer's Private Key
• C. Recipient's Public Key
• D. Recipient's Private Key

Answer: (B)

When creating a digital signature, the signer hashes the message to produce a digest and then encrypts that digest with their private key. This use of the private key provides authentication and non-repudiation: only the signer, who holds the private key, could have produced that signature. Anyone can verify it later by decrypting with the signer’s public key and comparing the recovered digest to the hash of the received message. The recipient’s keys have no role in generating the signature, and using the recipient’s private key wouldn’t produce a valid signature from the signer.