Copy detection pattern

A copy detection pattern (CDP), secure graphic or graphical code is a small random or pseudo-random digital image which is printed on documents, labels or products for counterfeit detection. Authentication is made by scanning the printed CDP using an image scanner or mobile phone camera. It is possible to store additional product-specific data into the CDP that will be decoded during the scanning process. A CDP can also be inserted into a 2D barcode to facilitate smartphone authentication and to connect with traceability data.
Principle.
The detection of counterfeits using a CDP relies on an "information loss principle", which states that every time a digital image is printed or scanned, some information is lost about the original digital image. A CDP is a maximum entropy image that attempts to take advantage of this information loss. Since producing a counterfeit CDP requires an additional scanning and printing processes, it will have less information than an original CDP. By measuring the information in the scanned CDP, the detector can determine whether the CDP is an original print or a copy.
CDPs aim to address limitations of optical security features such as security holograms. They are motivated by the need for security features that can be originated, managed and transferred digitally, and that are machine readable. Contrarily to many traditional security printing techniques, CDPs do not rely on Security by Obscurity, as the algorithm for generating CDPs can be public as long as the key used to generate it or the digital CDP is not revealed.
CDPs have also been described as a type of optical physical unclonable function. While they have been cited as a "powerful tool to detect copies", it is noted however that CDPs "require an extensive knowledge of printing technologies".
Security assessment.
The theoretical and practical assessment of the security level of CDPs, in other words the detector's ability to detect counterfeit attempts, is an ongoing area of research:
Applications.
CDPs are used for different physical item authentication applications:
Related techniques.
The EURion constellation and digital watermarks are inserted into banknotes to be detected by scanners, photocopiers and image processing software. However the objective of these techniques is not to detect whether a given banknote is a counterfeit, but to deter amateur counterfeiters from reproducing banknotes by blocking the device or software used to make the counterfeit.
Digital watermarks can may be used as well to differentiate original prints from counterfeits. A digital watermark may also be inserted into a 2D barcode. The fundamental difference between digital watermarks and CDPs is that a digital watermark must be embedded into an existing image while respecting a fidelity constraint, while the CDP does not have such constraint.