@misc{cryptoeprint:2002:154,
author = {Roberto M. Avanzi},
title = {On multi-exponentiation in cryptography},
howpublished = {Cryptology ePrint Archive, Report 2002/154},
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.