@misc{cryptoeprint:2011:438,
author = {Philippe Camacho and Alejandro Hevia},
title = {Short Transitive Signatures for Directed Trees},
howpublished = {Cryptology ePrint Archive, Report 2011/438},
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.