@misc{cryptoeprint:2004:048,
author = {Bartosz Zoltak},
title = {Tail-MAC: A Message Authentication Scheme for Stream Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2004/048},
year = {2004},
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.