Which of the following describes the primary purpose of a digital signature?

• A. To verify authenticity and integrity of a message or document
• B. To encrypt the message for confidentiality
• C. To compress data for transmission
• D. To authenticate a user to login

Answer: (A)

Verifying authenticity and integrity of a message or document. A digital signature is created with the signer’s private key and attached to the data. The recipient uses the signer’s public key to verify the signature against the data; if it checks out, you know the signer is who they claim to be (authenticity) and that the content hasn’t been altered since it was signed (integrity). It also provides non-repudiation, since the signer cannot easily deny having signed it.

This isn’t about keeping content secret (encryption for confidentiality), nor about compressing data, nor about authenticating a user to log in.