We 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.
@article{arxiv.quant-ph/0702152,
title = {Device-independent security of quantum cryptography against collective attacks},
author = {Antonio Acin and Nicolas Brunner and Nicolas Gisin and Serge Massar and Stefano Pironio and Valerio Scarani},
journal= {arXiv preprint arXiv:quant-ph/0702152},
year = {2007}
}