A digital signature is a cryptographic mechanism used to verify the authenticity and integrity of a digital message or document. It involves encrypting a hash of the document with the sender's private key, creating a unique signature. Statement 1 describes a certificate of authority, not the signature itself. Statement 2 describes authentication, often achieved through methods like passwords or tokens, rather than the primary function of a digital signature. Statement 3 accurately reflects the core purpose of a digital signature: to provide a verifiable electronic mark on a document while ensuring its content has not been tampered with.