@misc{cryptoeprint:2008:133,
author = {P. Gaudry and D. Lubicz},
title = {The arithmetic of characteristic 2 Kummer surfaces},
howpublished = {Cryptology ePrint Archive, Report 2008/133},
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.