@misc{cryptoeprint:2012:588,
author = {Hans-Joachim Knobloch},
title = {Breaking Public Keys - How to Determine an Unknown RSA Public Modulus},
howpublished = {Cryptology ePrint Archive, Report 2012/588},
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.