@misc{cryptoeprint:2012:544,
author = {Florian Mendel and Vincent Rijmen and Deniz Toz and Kerem Varici},
title = {Differential Analysis of the LED Block Cipher},
howpublished = {Cryptology ePrint Archive, Report 2012/544},
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.