@misc{cryptoeprint:2011:431,
author = {Enrico Thomae and Christopher Wolf},
title = {Roots of Square: Cryptanalysis of Double-Layer Square and Square+},
howpublished = {Cryptology ePrint Archive, Report 2011/431},
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.