@misc{cryptoeprint:2011:254,
author = {T. Kleinjung and A.K. Lenstra and D. Page and N.P. Smart},
title = {Using the Cloud to Determine Key Strengths},
howpublished = {Cryptology ePrint Archive, Report 2011/254},
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.