@misc{cryptoeprint:2002:176,
author = {Masahiko Takenaka, Takeshi Shimoyama, Takeshi Koshiba},
title = {Theoretical Analysis of ``Correlations in RC6'' },
howpublished = {Cryptology ePrint Archive, Report 2002/176},
year = {2002},
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.