What digital document identifies a valid user or system?

• A. Certificate
• B. Token
• C. Key
• D. Password

Answer: (A)

Digital certificates serve as identity documents in cryptographic systems. They bind a public key to real-world identity information and are issued and signed by a trusted Certificate Authority. The certificate includes the subject’s identity, the public key, the issuer, a validity period, and the CA’s digital signature. Because the signature can be verified through the CA’s trusted root, anyone can confirm that the public key indeed belongs to the claimed entity, enabling reliable authentication of a valid user or system in protocols like TLS or mutual SSL.

A token is something you possess to prove authentication, not a formal identity document. A key is a cryptographic value used for encryption or signing, but by itself it doesn’t prove who you are. A password is something you know, used to prove knowledge of a secret, rather than a verifiable identity document.