@misc{cryptoeprint:2000:059,
author = {Gideon Samid},
title = {Essential Shannon Security with Keys Smaller Than the Encrypted Message},
howpublished = {Cryptology ePrint Archive, Report 2000/059},
year = {2000},
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.