@misc{cryptoeprint:2011:245,
author = {Jean-Pierre Flori and Hugues Randriam},
title = {On the Number of Carries Occuring in an Addition $\mod 2^k-1$},
howpublished = {Cryptology ePrint Archive, Report 2011/245},
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.