@misc{cryptoeprint:2014:653,
author = {Thorsten Kleinjung and Joppe W. Bos and Arjen K. Lenstra},
title = {Mersenne factorization factory},
howpublished = {Cryptology ePrint Archive, Report 2014/653},
year = {2014},
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.