The correct answer is An encryption key. This choice is not a fundamental component of a digital signature.

Digital signatures are built from the original message, a fixed length digest produced by a secure hash function, and a signing operation that uses a private key and a corresponding public key for verification. A certificate is often used to bind the public key to an identity. The core idea is integrity and nonrepudiation rather than encrypting the message, so a generic encryption key is not listed as a fundamental element.

A message digest produced by a secure hash function is incorrect because the digest is what is actually signed in most schemes. Hashing provides a compact representation of the original data and ensures that any change to the document yields a different digest, which makes it essential to the signature process.

The original document or message being signed is incorrect because the document is the item whose integrity and authenticity are being proven. You need the original or its representation to produce and later verify the digest and signature.

A public key certificate is incorrect because certificates are commonly used to associate the public verification key with an identity and to establish trust. Verification requires the public key and a means to trust that key, and certificates provide that assurance.

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