@misc{cryptoeprint:2011:202,
author = {Markku-Juhani O. Saarinen},
title = {Cycling Attacks on GCM, GHASH and Other Polynomial MACs and Hashes},
howpublished = {Cryptology ePrint Archive, Report 2011/202},
year = {2011},
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.