@misc{cryptoeprint:2003:082,
author = {Tetsu Iwata and Kaoru Kurosawa},
title = {Stronger Security Bounds for OMAC, TMAC and XCBC},
howpublished = {Cryptology ePrint Archive, Report 2003/082},
year = {2003},
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.