@misc{cryptoeprint:2011:239,
author = {Shay Gueron},
title = {Efficient Software Implementations of Modular Exponentiation},
howpublished = {Cryptology ePrint Archive, Report 2011/239},
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.