What is a digital signature, and what does it verify?

Question

Accepted Answer

A cryptographic mechanism that verifies the authenticity and integrity of a digital message or document, confirming it was created by a known sender and has not been altered. A digital signature is a cryptographic mechanism that provides authentication (confirming the identity of the signer), integrity (proving the content has not been altered since signing), and non-repudiation (preventing the signer from denying having signed). Digital signatures work by having the signer create a hash of the message and then encrypt that hash with their private key. Recipients can verify the signature by decrypting the hash with the signer's public key and comparing it to their own hash of the received message. If the hashes match, the message is authentic and unaltered.

A scanned image of a handwritten signature is simply a picture file that provides no cryptographic assurance - it can be easily copied, pasted onto any document, and does not verify integrity or prevent tampering. A unique username assigned for login purposes describes a user identifier, which is part of the identification process but does not provide the cryptographic proof of authenticity and integrity that digital signatures offer. An automated email signature block is a text template appended to emails for professional communication, containing contact details but providing no cryptographic verification of the sender's identity or message integrity.

Digital signatures are a core cryptographic concept on the Fundamentals of Cybersecurity exam. Students should understand how digital signatures use asymmetric cryptography and hashing to provide authentication, integrity, and non-repudiation. They should also understand the relationship between digital signatures and Public Key Infrastructure (PKI), and practical applications including code signing, document signing, and secure email.

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