@misc{cryptoeprint:2010:303,
author = {Ran Canetti and Sebastian Gajek},
title = {Universally Composable Symbolic Analysis of Diffie-Hellman based Key Exchange},
howpublished = {Cryptology ePrint Archive, Report 2010/303},
year = {2010},
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.