Designs, Codes and Cryptography

, Volume 35, Issue 3, pp 311–335 | Cite as

Optimal Colored Threshold Visual Cryptography Schemes

  • Stelvio Cimato
  • Roberto De PriscoEmail author
  • Alfredo De Santis


Visual cryptography schemes allow the encoding of a secret image into n shares which are distributed to the participants. The shares are such that only qualified subsets of participants can “visually” recover the secret image. Usually the secret image consist of black and white pixels. In colored threshold visual cryptography schemes the secret image is composed of pixels taken from a given set of c colors. The pixels expansion and the contrast of a scheme are two measures of the goodness of the scheme.

In this paper, we study c-color (k,n)-threshold visual cryptography schemes and provide a characterization of contrast-optimal schemes. More specifically we prove that there exists a contrast-optimal scheme that is a member of a special set of schemes, which we call canonical schemes, and that satisfy strong symmetry properties.

Then we use canonical schemes to provide a constructive proof of optimality, with respect to the pixel expansion, of c-color (n,n)-threshold visual cryptography schemes.

Finally, we provide constructions of c-color (2,n)-threshold schemes whose pixels expansion improves on previously proposed schemes.


visual crytography secret sharing schemes 


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Copyright information

© Springer Science+Business Media, Inc. 2005

Authors and Affiliations

  • Stelvio Cimato
    • 1
  • Roberto De Prisco
    • 1
    Email author
  • Alfredo De Santis
    • 1
  1. 1.Dipartimento di Informatica ed ApplicazioniUniversitá di SalernoBaronissi (SA)Italy

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