What is a method to prove that a user authored a document?

A digital signature serves as a method to prove that a user authored a document because it employs cryptographic techniques to verify the identity of the signer. When a user digitally signs a document, it creates a unique hash of the document's contents. This hash is then encrypted with the user's private key, forming the digital signature. Upon receiving the signed document, anyone with access to the user's public key can decrypt the signature, verifying that it matches the hash of the document. This ensures both the authenticity and the integrity of the document, confirming that it was indeed authored by the legitimate user who holds the associated private key.

In contrast, a video surveillance recording may provide evidence of a user's presence but doesn't authenticate authorship of the document itself. Biometric authentication, while confirming identity at the time of access, does not create a verifiable link to the specific actions taken afterward, such as authoring a document. A receipt from a service might indicate a transaction but lacks the necessary cryptographic framework to substantiate authorship on its own. Thus, a digital signature is specifically designed for this purpose, making it the correct choice.