@misc{cryptoeprint:2007:474,
author = {Martin Cochran},
title = {Notes on the Wang et al. $2^{63}$ SHA-1 Differential Path},
howpublished = {Cryptology ePrint Archive, Report 2007/474},
year = {2007},
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.