2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/140406We present the optimal collective attack on a Quantum Key Distribution (QKD) protocol in the "device-independent" security scenario, where no assumptions are made about the way the QKD devices work or on what quantum system they operate. Our main result is a tight bound on the Holevo information between one of the authorized parties and the eavesdropper, as a function of the amount of violation of a Bell-type inequality.5 pages, 1 figure. v2: minor modificationsQuantum Physicstext