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 c ...
, various methods such as
void pantograph 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 ( , short for Joint Photographic Experts Group and sometimes retroactively referred to as JPEG 1) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degr ...
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 concealed information would not be evident to an unsuspecting person's examination. In computing/ ...
,
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.H.T. Sencar, M. Ramkumar and A.N. Akansu: ''Data Hiding Fundamentals and Applications: Content Security in Digital Multimedia'' ...
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
3D, 3-D, 3d, or Three D may refer to:
Science, technology, and mathematics
* A three-dimensional space in mathematics Relating to three-dimensionality
* 3D computer graphics, computer graphics that use a three-dimensional representation of geome ...
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 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 s ...
.
References
{{reflist
Forgery
Authentication methods
Security