Editing One-time pad (section)

{{short description|Encryption technique}}
{{Distinguish|One-time password}}
{{Redirect | One time code | the pre-arranged phrase
| Code (cryptography)#One-time code
}}
{{lead extra info|date=May 2023}}
[[File:NSA DIANA one time pad.tiff|thumb|A format of one-time pad used by the U.S. [[National Security Agency]], code named DIANA. The table on the right is an aid for converting between plaintext and ciphertext using the characters at left as the key.]]

The '''one-time pad''' ('''OTP''') is an [[encryption]] technique that cannot be [[Cryptanalysis|cracked]] in [[cryptography]]. It requires the use of a single-use [[pre-shared key]] that is larger than or equal to the size of the message being sent. In this technique, a [[plaintext]] is paired with a random secret [[Key (cryptography)|key]] (also referred to as a ''one-time pad''). Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using [[Modular arithmetic|modular addition]].<ref>{{Citation |last=Lugrin |first=Thomas |title=One-Time Pad |date=2023 |work=Trends in Data Protection and Encryption Technologies |pages=3–6 |editor-last=Mulder |editor-first=Valentin |place=Cham |publisher=Springer Nature Switzerland |language=en |doi=10.1007/978-3-031-33386-6_1 |isbn=978-3-031-33386-6 |editor2-last=Mermoud |editor2-first=Alain |editor3-last=Lenders |editor3-first=Vincent |editor4-last=Tellenbach |editor4-first=Bernhard|doi-access=free }}</ref>

The resulting [[ciphertext]] is impossible to decrypt or break if the following four conditions are met:<ref name="Numbers Stations">{{cite web |url=http://www.numbers-stations.com/intro |title=Intro to Numbers Stations |access-date=13 September 2014 |url-status = dead|archive-url=https://web.archive.org/web/20141018031055/http://www.numbers-stations.com/intro |archive-date=18 October 2014 |df=dmy-all }}</ref><ref>{{cite web |url=http://www.cryptomuseum.com/crypto/otp.htm |title=One-Time Pad (OTP) |publisher=Cryptomuseum.com |access-date=2014-03-17 |url-status = dead|archive-url=https://web.archive.org/web/20140314175211/http://www.cryptomuseum.com/crypto/otp.htm |archive-date=2014-03-14 }}</ref>

# The key must be at least as long as the plaintext.
# The key must be [[True random|truly random]]. 
# The key must never be reused in whole or in part.
# The key must be kept completely secret by the communicating parties.
These requirements make the OTP the only known encryption system that is mathematically proven to be unbreakable under the principles of information theory.<ref name="shannon">{{cite journal| last=Shannon| first=Claude| title=Communication Theory of Secrecy Systems| journal=Bell System Technical Journal| volume=28| issue=4| pages=656–715| year=1949| doi=10.1002/j.1538-7305.1949.tb00928.x| url=https://www.cs.virginia.edu/~evans/greatworks/shannon1949.pdf}}</ref>

Digital versions of one-time pad ciphers have been used by nations for critical [[diplomatic communications|diplomatic]] and [[military communication]], but the problems of secure [[key distribution]] make them impractical for many applications.

The concept was first described by [[Frank Miller (cryptographer)|Frank Miller]] in 1882,<ref>{{Cite wikisource|title=Telegraphic Code to Insure Privacy and Secrecy in the Transmission of Telegrams|author=Frank Miller|date=1882}}</ref><ref name="BELLOVIN1">{{cite journal |journal = Cryptologia |last = Bellovin |first = Steven M. |title = Frank Miller: Inventor of the One-Time Pad |volume = 35 |issue = 3 |issn = 0161-1194 |year = 2011 |doi = 10.1080/01611194.2011.583711 |pages = 203–222 |s2cid = 35541360 |url = https://academiccommons.columbia.edu/doi/10.7916/D80G3SZ2/download }}</ref> the one-time pad was re-invented in 1917. On July 22, 1919, U.S. Patent 1,310,719 was issued to [[Gilbert Vernam]] for the [[Exclusive or|XOR]] operation used for the encryption of a one-time pad.<ref name=Google>{{cite web|url=https://patents.google.com/patent/US1310719|website=google.com|access-date=3 February 2016|title='Secret signaling system patent' on Google.Com|url-status = live|archive-url=https://web.archive.org/web/20160311030400/http://www.google.com/patents/US1310719|archive-date=11 March 2016}}</ref> One-time use came later, when [[Joseph Mauborgne]] recognized that if the key tape were totally random, then [[cryptanalysis]] would be impossible.<ref>{{cite book| last=Kahn| first=David| title=The Codebreakers| publisher=[[Macmillan Publishers (United States)|Macmillan]]| year=1996| isbn=978-0-684-83130-5| pages=397–8 |author-link=David Kahn (writer)| title-link=The Codebreakers}}</ref>
To increase security, one-time pads were sometimes printed onto sheets of highly flammable [[nitrocellulose]], so that they could easily be burned after use.