@misc{cryptoeprint:2003:236,
author = {Elena Trichina},
title = {Combinational Logic Design for AES SubByte Transformation on Masked Data},
howpublished = {Cryptology ePrint Archive, Report 2003/236},
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.