  Visual cryptography schemes have been introduced in 1994 by Naor and Shamir.
Their idea was to encode a secret image into $n$ shadow images and to give
exactly one such shadow image to each member of a group $P$ of $n$ persons.
Whereas most work in recent years has been done concerning the problem of
qualified and forbidden subsets of $P$ or the question of contrast optimizing,
in this paper we study extended visual cryptography schemes, i.e. shared secret
systems where any subset of $P$ shares its own secret.
