@misc{cryptoeprint:2006:041,
author = {Birgit Pfitzmann and Matthias Schunter and Michael Waidner},
title = {Reactively Simulatable Certified Mail},
howpublished = {Cryptology ePrint Archive, Report 2006/041},
year = {2006},
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.