Method for authenticating an illustration

A method for authenticating an illustration, said method comprising:

• using an optical sensor, taking a picture of the illustration and a two-dimensional barcode,
• wherein encoding a message in the form of a two-dimensional barcode, said barcode comprising a set of blocks, each of which codes a fragment of said message, wherein each block comprises rows, columns, and a set of coding subblocks, wherein each subblock comprises bits comprised:
• defining or identifying a set of distinctive points in the illustration, wherein each of said distinctive points is selected from the group consisting a pixel and a set of two-by-two adjacent pixels, wherein each distinctive point has a contrast gradient that, in one direction and at a predefined distance from said distinctive point, exceeds a predefined threshold value,
• calculating a set of attributes according to at least some of the distinctive points of said set,
• selecting, from among the attributes thus calculated, an attribute that makes it possible to define a digital fingerprint, and
• recording, in the message, one from among:
• the set of attributes and the digital fingerprint, wherein the digital fingerprint is one of unsigned and uncompressed, unsigned and compressed, signed and uncompressed, and signed and compressed,
• decoding the message of said two-dimensional barcode,
• in the picture taken by the optical sensor:
• defining or identifying a set of distinctive points in the illustration,
• recalculating a set of attributes according to at least some of the distinctive points of said set, and
• selecting, from among the set of recalculated attributes, a set of at least one attribute making it possible to define a digital fingerprint, and
• comparing the value of the difference between the decoded attributes of said two-dimensional barcode and the recalculated attributes to a predetermined threshold value recorded in a memory.