@misc{cryptoeprint:2008:078,
author = {Donghoon Chang and Mridul Nandi},
title = {A Short Proof of the PRP/PRF Switching Lemma},
howpublished = {Cryptology ePrint Archive, Report 2008/078},
year = {2008},
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.