@misc{cryptoeprint:2011:400,
author = {Gérard Cohen and Jean-Pierre Flori},
title = {On a generalized combinatorial conjecture involving addition $\mod 2^k - 1$},
howpublished = {Cryptology ePrint Archive, Report 2011/400},
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.