Patent ID: 7684088

Claim:
A method for depositing a digital mark on a printed or engraved surface, the digital mark containing digital information in encoded form, the method comprising the steps of: generating the digital mark by selecting at least one set of pixels having a point size at least as small as 300 dots per inch; initializing a pseudo random number generator with a key and with the digital information; generating an output from the pseudo random number generator initialized by the key and the digital information; and modifying the set of pixels according to the output of a pseudo random number generator such that each pixel in the set of pixels that corresponds to a bit in the output of the pseudorandom number generator having a first value is modified, and each pixel in the set of pixels that corresponds to a bit in the output of the pseudorandom number generator having a second value is not modified; printing a visual element on the surface, the visual element not including the digital mark; and overprinting the digital mark on the surface after the visual element has been printed on the surface without taking into account any local color variations of the surface such that the luminosity of the surface is decreased, the digital mark being detectable on the printed or engraved surface, but the digital mark not being detectable in a copy of the printed or engraved surface when the printed or engraved surface is reproduced.