@misc{cryptoeprint:2012:401,
author = {Martin Albrecht and Gregor Leander},
title = {An All-In-One Approach to Differential Cryptanalysis for Small Block Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2012/401},
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.