Copy-evident documents have features that make it detectable that a copy is not the original. In

security printing Security printing is the field of the printing industry that deals with the printing of items such as banknotes, cheques, passports, tamper-evident labels, security tapes, product authentication, stock certificates, postage stamps and identity ca ...

, various methods such as

void pantograph In security printing, void pantograph refers to a method of making copy-evident and tamper-resistant patterns in the background of a document. Normally these are invisible to the eye, but become obvious when the document is photocopied. Typically t ...

are used to create patterns that are hard to copy exactly and when copied inexactly produces an easily detectable image. This can include halftone screens that are affected by the copying process, fine line patterns in different directions that are differently affected by copying and hence produce a mark, metallic inks causing diffraction or reflection. Copy-evident computer image files are also possible, for example by embedding a high-frequency pattern in a

JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...

image that is imperceptible, but produces an obvious message when re-compressed with a different quality factor. Some forms of

steganography Steganography ( ) is the practice of representing information within another message or physical object, in such a manner that the presence of the information is not evident to human inspection. In computing/electronic contexts, a computer file, ...

, paper watermarking, and

digital watermarking A digital watermark is a kind of marker covertly embedded in a noise-tolerant signal such as audio, video or image data. It is typically used to identify ownership of the copyright of such signal. "Watermarking" is the process of hiding digital inf ...

can be regarded as copy-evidence, although detecting that a document or file has been copied typically requires special inspection, tools or software. For example, 3D printed objects can be equipped with marks of genuinity that can be identified using a measurement device but are hard to deduce from sample objects; copies will lack these marks.Daniel G. Aliaga, Mikhail J. Atallah. Genuinity Signatures: Designing Signatures for Verifying 3D Object Genuinity. Computer Graphics Forum Volume 28, Issue 2, pages 437–446, April 2009 http://www.cs.purdue.edu/cgvlab/papers/aliaga/eg2009.pdf As another example, a

copy detection pattern A copy detection pattern (CDP) or graphical code is a small Randomness, random or Pseudorandomness, pseudo-random digital image which is printed on documents, labels or products for counterfeit detection. Authentication is made by scanning th ...

References

{{reflist Forgery Authentication methods Security