2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90711In quantum cryptography, the level of security attainable by a protocol which implements a particular task $N$ times bears no simple relation to the level of security attainable by a protocol implementing the task once. Useful partial security, and even near-perfect security in an appropriate sense, can be obtained for $N$ copies of a task which itself cannot be securely implemented. We illustrate this with protocols for quantum bit string commitment and quantum random number generation between mistrustful parties.6 pages. To appear in Proceedings of QCMC02Quantum PhysicsCryptography and Securitytext