@misc{cryptoeprint:1996:013,
author = {Carlo Blundo, Alfredo De Santis, and Douglas R. Stinson},
title = {On the Contrast in Visual Cryptography Schemes},
howpublished = {Cryptology ePrint Archive, Report 1996/013},
year = {1996},
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.