What is the role of a private key in a digital signature?

• A. Signs the message using a private key
• B. Verifies the message using a public key
• C. Encrypts the message for confidentiality
• D. Is stored in a certificate

Answer: (A)

In a digital signature, the private key is used to create the signature itself. The signer hashes the message and then encrypts that hash with their private key, producing a signature. Anyone with the corresponding public key can decrypt that signature to recover the hash and compare it to a fresh hash of the message. If they match, it proves who signed the message (authenticity) and that the message hasn’t been altered (integrity). It’s important that the private key stays secret, or else signatures could be forged. The other ideas aren’t the primary role: verification is done with the public key, signatures aren’t meant for confidentiality, and a certificate stores the public key and identity, not the private key.