[ Changed 28th August 1997 ]
Visual cryptography was introduced by Naor and Shamir as a way to allow fast visual decryption of graphic objects. No decryption device is required; instead decryption is done by fitting slides together. Several schemes were suggested which allow users to share secret pictures (and text) in an information theoretically secure way, so that deciphering is easy if all the shares are given, but it is impossible if one of them is missing. The drawbacks of all the existing methods are the exponentially small contrast of the deciphered picture as the number of shares increases, and the reduction in quality due to pixels' being represented by many smaller (black and white) pixels.
In this talk we suggest new visual cryptographic schemes based on light polarization which are better than the optimal existing schemes. Then we present an ultimate scheme which does not subdivide pixels, and in which the contrast is independent of the number of shares.
Joint work with Ayal Itzkovitz
NOTE: the time is nonstandard for a security seminar.