@misc{cryptoeprint:2004:242,
author = {Vladimir Shpilrain and Gabriel Zapata},
title = {Combinatorial group theory and public key cryptography},
howpublished = {Cryptology ePrint Archive, Report 2004/242},
year = {2004},
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.