Final answer:
In zero-knowledge protocols, being 'sound' means a false statement cannot be convincingly proven true. It ensures the integrity and security of cryptographic systems against fraudulent proofs.
Step-by-step explanation:
To require a zero-knowledge protocol to be "sound" means that if the prover is wrong, he or she cannot convince the verifier with any significant probability. In other words, a sound zero-knowledge protocol ensures that if the statement being proved is false, the dishonest prover cannot trick the honest verifier into believing the statement is true. This is critical to the integrity of any cryptographic system, preventing the system from accepting false proofs as truths and thus providing security against fraudulent claims.
Semantically, 'soundness' joins 'completeness' and 'zero-knowledgeness' as cornerstone properties of such protocols. Soundness guarantees that bad actors cannot subvert the protocol, completeness ensures that honest provers can always convince honest verifiers of true statements, and zero-knowledgeness protects the prover's secret information during the verification process.