@misc{cryptoeprint:2008:221,
author = {Ivan Damgard and Rune Thorbek},
title = {Efficient Conversion of Secret-shared Values Between Different Fields},
howpublished = {Cryptology ePrint Archive, Report 2008/221},
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.