@misc{cryptoeprint:2006:170,
author = {Carlo Blundo and Stelvio Cimato and Alfredo De Santis},
title = {Visual Cryptography Schemes with Optimal Pixel Expansion},
howpublished = {Cryptology ePrint Archive, Report 2006/170},
year = {2006},
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.