Which cryptographic operation provides authentication in public-key cryptography?

• A. Encrypt with the recipient's public key
• B. Decrypt with the recipient's private key
• C. Sign with the private key
• D. Verify with the public key

Answer: (C)

Authentication in public-key cryptography is achieved through digital signatures. When someone signs a message with their private key, they create a signature that can be verified by others using the signer’s public key. This proves that the message was created by the holder of the private key and that it hasn’t been altered.

Encrypting with the recipient’s public key ensures confidentiality, not authentication. Decrypting with the recipient’s private key is the step to read the message, which doesn’t establish who sent it. Verifying with the public key is the verification step that confirms a signature was created with the corresponding private key; it validates authenticity, but the act that actually provides authentication is signing with the private key.