Visual Cryptography

1
Visual Cryptography

• OR
• Reading Between the Lines

2
Sharing a Visual Secret

• Divide Image into Two Parts
• Key a transparency
• Cipher a printed page
• Separately, they are random noise
• Combination reveals an image
• Human vision system decodes message
• No computation required at receiver

3
Basic Scheme, Two Levels

• Two-level plaintext image (black/white)
• Each pixel encoded as
• a 2x2 cell
• in two shares (key and cipher)
• Each share has 2 black, 2 transparent subpixels
• When stacked, shares combine to
• Solid black
• Half black (seen as gray)

4
Basic Scheme, Two Levels

• Choose random cell in key (6 choices)
• Choose complementary (for black) or matching (for
  white) cell in cipher
• Key conveys no information by itself
• Neither does cipher
• To view, stack and align
• www.cse.psu.edu/rsharris/visualcryptography/two
  level.html

5
Extensions

• K of N secret sharing
• N shares
• Any K combine to reveal message
• Any less cant
• Gray level
• Could encode three levels in 2x2
• Can encode four levels in 3x3
• Color

6
Four Gray Levels

• Each pixel encoded as
• A 3x3 cell
• 3 black, 6 transparent
• Combine to 3, 4, 5, or 6 black
• Example
• http//www.cse.psu.edu/rsharris/visualcryptograp
  hy/deh.html

7
Continuous Gray level

• Each pixel encoded as 33 black circle
• Combine for any gray from 33 to 67 black

8
Possible Applications

• Bank customer identification
• Bank sends customer a set of transparencies (key)
  in advance
• Bank web site displays cipher
• Customer applies overlay, reads transaction key
• Customer enters transaction key
• Verifiable Receipts in Electronic Voting
• Anti spam-bot measure

9
References

• Naor and Shamir, Visual Cryptography, in Advances
  in Cryptology - Eurocrypt 94
• The Visual Cryptography Project
• www.esat.kuleuven.ac.be/rijmen/vc/