@misc{cryptoeprint:2012:208,
author = {Jay Shah and Ayan Mahalanobis},
title = {A NEW GUESS-AND-DETERMINE ATTACK ON THE A5/1 STREAM CIPHER},
howpublished = {Cryptology ePrint Archive, Report 2012/208},
year = {2012},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.