What is the function of a digital signature?

• A. To encrypt the entire message
• B. To verify the authenticity and integrity of a message or document
• C. To provide user authentication for a login
• D. To sign a certificate for a website

Answer: (B)

A digital signature serves to prove who sent something and that it hasn’t been changed since it was signed. The signer creates a signature by taking a hash of the data and encrypting that hash with their private key. Anyone with the signer’s public key can then decrypt the signature and compare it to a fresh hash of the received data. If the hashes match, you have confidence that the data came from that signer and has not been altered. This also supports non-repudiation—the signer can’t deny having signed it later.

It’s not meant to encrypt the entire message for confidentiality, so it doesn’t protect the content from others who might intercept it. It’s also not specifically about authenticating a user for a login. Signing a certificate is a related concept but involves a trusted authority signing a certificate to bind an identity to a public key, whereas the signature on ordinary data is about verifying the data’s origin and integrity.