@misc{cryptoeprint:2008:510,
author = {Nadia Heninger and Hovav Shacham},
title = {Reconstructing RSA Private Keys from Random Key Bits},
howpublished = {Cryptology ePrint Archive, Report 2008/510},
year = {2008},
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.