The technique involves two black and white images, one a graphic one time pad page and the other a graphic prepared by a simple imaging transformation combining the one time pad page and a graphic containing the message.
After preparation, neither image has a preferred status in restoring the message.
When the two graphics are overlayed in exact registration and a *logical or* is used to show each pixel, the result is an image which restores a version of the original message

The technique is useful since the logical-or overlay operation can be readily accomplished using two transparencies or electronically by superimposing the two images and adding.
Further, the encryption and decryption processes are completely transparent (in the other sense of the word) so that the user need have no concerns about whether surreptitious, malicious extraction or insertion of information might occur somewhere inside the process. Equally important, the technique relies on direct physical steps, eliminating concerns about system obsolescence or operating system and application revisions rendering recovery difficult or impossible.

Prior art:
Moni Naor and Adi Shamir, Visual Cryptography, Eurocrypt 94
http://www.wisdom.weizmann.ac.il/%7Enaor/PAPERS/vis.ps

Their paper describes a similar technique with the same advantages as listed above but uses a different and less readily generated basic element. The technique explained here uses individual pixels, black or white as its basic element. The emphasis in the Naor, Naor and Shamir paper is on secret splitting between multiple parties rather than secure point to point information transfer or archival encryption for much later recovery.
ref. http://www.wisdom.weizmann.ac.il/%7Enaor/PUZZLES/visual.html

The technique.

A graphic consisting of random dots of black and white must be created to be the one time pad page. This image must have an equal amount of black and white, such that a negative image has the same average intensity as the positive. A copy of this graphic must be retained for later use in recovering the message.

A negative image of this graphic is also created.

Using well-known textual writing overlay, a text message to be encrypted is written on this negative.

The command "invert selection" is used to select the whole of this negative one time pad page but without the overwritten text.

This selection is copied and moved to the original one time pad page graphic. The result is a graphic with a random field of black and white dots which superficially is the same as the original one time pad page graphic. Call this graphic the encrypted image
The encrypted image, when overlayed using a logical-or with a copy of the original one time pad page graphic will result in a version of the original image.

Explaination

The encrypted image consists of two kinds of areas. Areas in which the random dots are exactly in phase with those of the one time pad and the other areas where the random dots are exactly out of phase with the dots of the one time pad. When a superimposed logical-or operation is applied to the encrypted image with the original one time pad image, the areas which are exactly in phase are unchanged, while the areas which are exactly out of phase are rendered as completely white (white corresponding to a logical one). If a XOR process were used instead of a logical-or, a completely recovered original image would result. Using the logical-or, a version of the image is recovered, generally readible with a correctly chosen resolution.

Theory

The technique can be dismissed as a trivial expansion of the usual one time pad technique. This is correct.

The technique can be called a simple application of direct sequence spread spectrum technology. This also is correct.

So what's new? Nothing. It's simple and useful.



John Bailey
May 20, 2003