@misc{cryptoeprint:2004:059,
author = {Michael Backes and Birgit Pfitzmann},
title = {Symmetric Encryption in a Simulatable Dolev-Yao Style Cryptographic Library},
howpublished = {Cryptology ePrint Archive, Report 2004/059},
year = {2004},
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.