@misc{cryptoeprint:2006:451,
author = {Jongsung Kim},
title = {Combined Differential, Linear and Related-Key Attacks on Block Ciphers and MAC Algorithms},
howpublished = {Cryptology ePrint Archive, Report 2006/451},
year = {2006},
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.