Editing Visual cryptography (section)

{{Short description|Cryptographic technique}}
[[File:visual_cryptography_development.svg|thumb|Development of masks to let overlaying ''n'' transparencies A, B,... printed with black rectangles reveal a secret image &mdash; ''n'' = 4 requires 16 (2<sup>4</sup>) sets of codes each with 8 (2<sup>4-1</sup>) subpixels, which can be laid out as 3×3 with the extra bit always black]]
'''Visual cryptography''' is a [[cryptography|cryptographic]] technique  which allows visual information (pictures, text, etc.) to be encrypted in such a way that the decrypted information appears as a visual image.

One of the best-known techniques has been credited to [[Moni Naor]] and [[Adi Shamir]], who developed it in 1994.<ref>{{Cite book |doi = 10.1007/BFb0053419|chapter = Visual cryptography|title = Advances in Cryptology – EUROCRYPT'94|volume = 950|pages = 1–12|series = Lecture Notes in Computer Science|year = 1995|last1 = Naor|first1 = Moni|last2 = Shamir|first2 = Adi|isbn = 978-3-540-60176-0}}</ref> They demonstrated a visual [[secret sharing]] scheme, where a [[binary image]] was broken up into ''n'' shares so that only someone with all ''n'' shares could decrypt the image, while any {{nowrap|''n'' − 1}} shares revealed no information about the original image. Each share was printed on a separate transparency, and decryption was performed by overlaying the shares. When all ''n'' shares were overlaid, the original image would appear. There are several generalizations of the basic scheme including ''k''-out-of-''n'' visual cryptography,<ref>{{cite journal |doi=10.1023/A:1008280705142|year=1997|last1=Verheul|first1=Eric R.|title=Constructions and Properties of k out of n Visual Secret Sharing Schemes|journal=Designs, Codes and Cryptography|volume=11|issue=2|pages=179–196|last2=Van Tilborg|first2=Henk C. A.|s2cid=479227}}</ref><ref>{{Cite journal |doi = 10.1016/S0304-3975(99)00127-9|title = Extended capabilities for visual cryptography|journal = Theoretical Computer Science|volume = 250|issue = 1–2|pages = 143–161|year = 2001|last1 = Ateniese|first1 = Giuseppe|last2 = Blundo|first2 = Carlo|last3 = Santis|first3 = Alfredo De|last4 = Stinson|first4 = Douglas R.|doi-access = }}</ref> and using opaque sheets but illuminating them by multiple sets of identical illumination patterns under the recording of only one single-pixel detector.<ref>{{cite journal|last1=Jiao|first1=Shuming|last2=Feng|first2=Jun|last3=Gao|first3=Yang|last4=Lei|first4=Ting|last5=Yuan|first5=Xiaocong|title=Visual cryptography in single-pixel imaging|journal=Optics Express|year=2020|volume=28|issue=5|pages=7301–7313|doi=10.1364/OE.383240|pmid=32225961|arxiv=1911.05033|bibcode=2020OExpr..28.7301J |s2cid=207863416}}</ref>

Using a similar idea, transparencies can be used to implement a [[one-time pad]] encryption, where one transparency is a shared random pad, and another transparency acts as the ciphertext. Normally, there is an expansion of space requirement in visual cryptography. But if one of the two shares is structured recursively, the efficiency of visual cryptography can be increased to 100%.<ref>{{Cite journal |doi = 10.1080/0161-110291890768|title = Recursive Hiding of Secrets in Visual Cryptography|journal = Cryptologia|volume = 26|pages = 68–76|year = 2002|last1 = Gnanaguruparan|first1 = Meenakshi|last2 = Kak|first2 = Subhash|s2cid = 7995141}}</ref>

Some antecedents of visual cryptography are in patents from the 1960s.<ref>Cook, Richard C. (1960) [https://patents.google.com/patent/US4682954 Cryptographic process and enciphered product], United States patent 4,682,954.</ref><ref>Carlson, Carl O. (1961) [https://patents.google.com/patent/US3279095 Information encoding and decoding method], United States patent 3,279,095.</ref> Other antecedents are in the work on perception and secure communication.<ref>{{Cite journal |doi = 10.1364/OL.12.000377|pmid = 19741737|title = Encryption of pictures and shapes by random grids|journal = Optics Letters|volume = 12|issue = 6|pages = 377–9|year = 1987|last1 = Kafri|first1 = O.|last2 = Keren|first2 = E.|bibcode = 1987OptL...12..377K}}</ref><ref>{{Cite journal |doi = 10.1109/21.44016|title = Intuition, perception, and secure communication|journal = IEEE Transactions on Systems, Man, and Cybernetics|volume = 19|issue = 5|pages = 1016–1020|year = 1989|last1 = Arazi|first1 = B.|last2 = Dinstein|first2 = I.|last3 = Kafri|first3 = O.}}</ref>

Visual cryptography can be used to protect biometric templates in which decryption does not require any complex computations.<ref>{{cite conference |last1=Askari|first1=Nazanin|last2=Moloney|first2=Cecilia|last3=Heys|first3=Howard M.|title=Application of Visual Cryptography to Biometric Authentication|conference=NECEC 2011|date=November 2011|url=http://necec.engr.mun.ca/ocs2011/viewpaper.php?id=60&print=1|access-date=12 February 2015}}</ref>