Cipher: Difference between revisions

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{{short description|Algorithm for encrypting and decrypting information}}

~~{{More footnotes|date=March 2009}}~~

In [[cryptography]], a '''cipher''' (or '''cypher''') is an [[algorithm]] for performing [[encryption]] or [[decryption]]—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "[[code (cryptography)|code]]", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially [[classical cryptography]].

In [[cryptography]], a '''cipher''' (or '''cypher''') is an [[algorithm]] for performing [[encryption]] or [[decryption]]—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "[[code (cryptography)|code]]", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially [[Classical cipher|classical cryptography]].

Codes generally substitute different length strings of characters in the output, while ciphers generally substitute the same number of characters as are input. A code maps one meaning with another. Words and phrases can be coded as letters or numbers. Codes typically have direct meaning from input to key. Codes primarily function to save time. Ciphers are algorithmic. The given input must follow the cipher's process to be solved. Ciphers are commonly used to encrypt written information.

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In casual contexts, "code" and "cipher" can typically be used interchangeably; however, the technical usages of the words refer to different concepts. Codes contain meaning; words and phrases are assigned to numbers or symbols, creating a shorter message.

An example of this is the [[commercial code (communications)|commercial telegraph code]] which was used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of [[~~telegram~~]]s.

An example of this is the [[commercial code (communications)|commercial telegraph code]] which was used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of [[telegrams]].

Another example is given by whole word ciphers, which allow the user to replace an entire word with a symbol or character, much like the way written Japanese utilizes [[Kanji]] (meaning Chinese characters in Japanese) characters to supplement the native Japanese characters representing syllables. An example using English language with Kanji could be to replace "The quick brown fox jumps over the lazy dog" by "The quick brown 狐 jumps 上 the lazy 犬". [[Shorthand|Stenographers]] sometimes use specific symbols to abbreviate whole words.

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===Historical===

[[File:Caesar cipher left shift of 3.svg|thumb|Visual representation of how Caesar's Cipher works.]]

The [[Caesar cipher|Caesar Cipher]] is one of the earliest known cryptographic systems. Julius Caesar used a cipher that shifts the letters in the alphabet in place by three and wrapping the remaining letters to the front to write to Marcus Tullius [[Cicero]] in approximately 50 BC.{{cn|date=~~March 2023~~}}

The [[Caesar cipher|Caesar Cipher]] is one of the earliest known cryptographic systems. Julius Caesar used a cipher that shifts the letters in the alphabet in place by three and wrapping the remaining letters to the front to write to Marcus Tullius [[Cicero]] in approximately 50 BC.<ref>{{Cite web |title=Suetonius • Life of Julius Caesar |url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#56 |access-date=2026-05-29 |website=penelope.uchicago.edu}}</ref>

Historical pen and paper ciphers used in the past are sometimes known as [[classical cipher]]s. They include simple [[substitution cipher]]s (such as [[ROT13]]) and [[transposition cipher]]s (such as a [[Rail Fence Cipher]]). For example, "GOOD DOG" can be encrypted as "PLLX XLP" where "L" substitutes for "O", "P" for "G", and "X" for "D" in the message. Transposition of the letters "GOOD DOG" can result in "DGOGDOO". These simple ciphers and examples are easy to crack, even without plaintext-ciphertext pairs.<ref>{{Cite journal|last=Saltzman|first=Benjamin A.|title=Vt hkskdkxt: Early Medieval Cryptography, Textual Errors, and Scribal Agency (Speculum, forthcoming)|url=https://www.academia.edu/35034685|journal=Speculum|year=2018 |volume=93|issue=4|page=975|doi=10.1086/698861 |jstor=26584834 |s2cid=165362817|language=en}}</ref><ref>{{Cite book |last=Janeczko |first=Paul B |title=Top Secret |year=2004}}</ref>

In the 1640s, the Parliamentarian commander, [[Edward Montagu, 2nd Earl of Manchester]], developed ciphers to send coded messages to his allies during the [[English Civil War]].<ref>{{cite web |title=English Civil War cipher belonging to Cromwell ally goes on display |url=https://the-past.com/news/english-civil-war-cipher-belonging-to-cromwell-ally-goes-on-display/ |website=The Past |date=12 July 2023 |access-date=4 August 2023}}</ref> The English theologian John Wilkins published a book in 1641 titled "Mercury, or The Secret and Swift Messenger" and described a musical cipher wherein letters of the alphabet were substituted for music notes.<ref>{{cite web |title=Types of Ciphers |url=https://www.audiocipher.com/post/types-of-ciphers |website=AudioCipher |date=26 February 2025 |access-date=14 March 2025}}</ref><ref>{{cite web |title=Mercury: Or the Secret and Swift Messenger |url=https://www.lybrary.com/mercury-or-the-secret-and-swift-messenger-p-5897.html |website=Lybrary |access-date=14 March 2025}}</ref> This species of melodic cipher was depicted in greater detail by author Abraham Rees in his book Cyclopædia (1778). <ref>{{cite web |title=Cyclopaedia |url=https://archive.org/details/cyclopaediaoruni08rees |website=Internet Archive |access-date=14 March 2025}}</ref>

In the 1640s, the Parliamentarian commander, [[Edward Montagu, 2nd Earl of Manchester]], developed ciphers to send coded messages to his allies during the [[English Civil War]].<ref>{{cite web |title=English Civil War cipher belonging to Cromwell ally goes on display |url=https://the-past.com/news/english-civil-war-cipher-belonging-to-cromwell-ally-goes-on-display/ |website=The Past |date=12 July 2023 |access-date=4 August 2023}}</ref> The English theologian John Wilkins published a book in 1641 titled "Mercury, or The Secret and Swift Messenger" and described a musical cipher wherein letters of the alphabet were substituted for music notes.<ref>{{cite web |title=Types of Ciphers |url=https://www.audiocipher.com/post/types-of-ciphers |website=AudioCipher |date=26 February 2025 |access-date=14 March 2025}}</ref><ref>{{cite web |title=Mercury: Or the Secret and Swift Messenger |url=https://www.lybrary.com/mercury-or-the-secret-and-swift-messenger-p-5897.html |website=Lybrary |access-date=14 March 2025}}</ref> This species of melodic cipher was depicted in greater detail by author Abraham Rees in his book Cyclopædia (1778).<ref>{{cite web |title=Cyclopaedia |url=https://archive.org/details/cyclopaediaoruni08rees |website=Internet Archive |access-date=14 March 2025}}</ref>

Simple ciphers were replaced by [[polyalphabetic substitution]] ciphers (such as the [[Vigenère cipher|Vigenère]]) which changed the substitution alphabet for every letter. For example, "GOOD DOG" can be encrypted as "PLSX TWF" where "L", "S", and "W" substitute for "O". With even a small amount of known or estimated plaintext, simple polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.<ref>{{citation|first=Douglas R.|last=Stinson|author-link = Doug Stinson|title=Cryptogtaphy / Theory and Practice|year=1995|publisher=CRC Press|isbn=0-8493-8521-0|page=45}}</ref> It is possible to create a secure pen and paper cipher based on a [[one-time pad]], but these have other disadvantages.

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In a pure mathematical attack, (i.e., lacking any other information to help break a cipher) two factors above all count:

* Computational power available, i.e., the computing power which can be brought to bear on the problem. ~~It is important to note that average~~ performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of [[exhaustive search]] for a key (i.e., "brute force" attack) substantially.

* Computational power available, i.e., the computing power which can be brought to bear on the problem. Average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of [[exhaustive search]] for a key (i.e., "brute force" attack) substantially.

* [[Key size]], i.e., the size of key used to encrypt a message. As the key size increases, so does the complexity of [[exhaustive search]] to the point where it becomes impractical to crack encryption directly.

Since the desired effect is computational difficulty, in theory one would choose an [[algorithm]] and desired difficulty level, thus decide the key length accordingly.

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* {{cite journal | last=Caldwell | first=William Casey | title=Shakespeare's ''Henry V'' and the Ciphers of History | journal=[[SEL: Studies in English Literature 1500–1900]] | volume=61 | issue=2 | date=2022 | issn=1522-9270 | doi=10.1353/sel.2022.0003 | pages=241–268}}

* {{Cite book |last=Gaines |first=Helen Fouché |title=Cryptanalysis |year=1956 |isbn=0-486-20097-3 |edition=Corrected |publisher=[[Dover Publications]] |publication-place=New York |url=https://archive.org/details/cryptanalysis00hele |ol=7634764M |author-link=Helen Fouché Gaines |orig-date=1939}}

* {{Cite conference |last1=Ho |first1=Yean Li |last2=Samsudin |last3=Belaton |first3=Bahari |first2=Azman |title=Heuristic Cryptanalysis of Classical and Modern Ciphers |conference=2005 13th IEEE International Conference on Networks, Jointly held with the 2005 IEEE 7th Malaysia International Conference on Communication ~~|url=https://ieeexplore.ieee.org/document/1635595~~ |publisher=[[IEEE]] |date=2005 |volume=2 |pages=710–715 |doi=10.1109/ICON.2005.1635595 |publication-place=Kuala Lumpur ~~|access-date=2024-06-30 |url-access=subscription~~}}

* {{Cite conference |last1=Ho |first1=Yean Li |last2=Samsudin |last3=Belaton |first3=Bahari |first2=Azman |title=Heuristic Cryptanalysis of Classical and Modern Ciphers |conference=2005 13th IEEE International Conference on Networks, Jointly held with the 2005 IEEE 7th Malaysia International Conference on Communication |publisher=[[IEEE]] |date=2005 |volume=2 |pages=710–715 |doi=10.1109/ICON.2005.1635595 |publication-place=Kuala Lumpur }}

* {{Cite book |last=Kahn |first=David |title=The Codebreakers – The Story of Secret Writing |year=1996 |isbn=0-684-83130-9 |edition=Revised |location=New York |oclc=35159231 |author-link=David Kahn (writer) |orig-date=1967 |title-link=The Codebreakers}}

* {{Cite web |url=https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/ciphers-vs-codes |title=Ciphers vs. codes |access-date=2024-06-30 |website=[[Khan Academy]] |archive-url=https://web.archive.org/web/20240117234055/https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/ciphers-vs-codes |archive-date=2024-01-17 |url-status=live}}

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==External links==

* [https://ciphermuseum.com Cipher Museum] – Educational site with interactive cipher demonstrations and cryptanalysis tools

* [https://caesarcipher.org/ciphers/identifier Cipher Identifier] – Free tool to automatically identify which classical cipher was used to encrypt a given text

* [http://www.scholarpedia.org/article/Secure_communications_using_the_KLJN_scheme Kish cypher]

@@ Line 1: / Line 1: @@
 {{short description|Algorithm for encrypting and decrypting information}}
 {{Other uses}}
-{{More footnotes|date=March 2009}}
 [[File:Edward Larsson 1885 I.jpg|thumb|[[Edward Larsson]]'s [[Cipher runes|rune cipher]] resembling that found on the [[:en:Kensington Runestone|Kensington Runestone]]. Also includes runically unrelated [[:en:blackletter|blackletter]] writing style and [[:en:pigpen cipher|pigpen cipher]].]]
-In [[cryptography]], a '''cipher''' (or '''cypher''') is an [[algorithm]] for performing [[encryption]] or [[decryption]]&mdash;a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "[[code (cryptography)|code]]", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially [[classical cryptography]].
+In [[cryptography]], a '''cipher''' (or '''cypher''') is an [[algorithm]] for performing [[encryption]] or [[decryption]]&mdash;a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "[[code (cryptography)|code]]", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially [[Classical cipher|classical cryptography]].
 Codes generally substitute different length strings of characters in the output, while ciphers generally substitute the same number of characters as are input. A code maps one meaning with another. Words and phrases can be coded as letters or numbers. Codes typically have direct meaning from input to key. Codes primarily function to save time. Ciphers are algorithmic. The given input must follow the cipher's process to be solved. Ciphers are commonly used to encrypt written information.
@@ Line 25: / Line 24: @@
 In casual contexts, "code" and "cipher" can typically be used interchangeably; however, the technical usages of the words refer to different concepts. Codes contain meaning; words and phrases are assigned to numbers or symbols, creating a shorter message.
-An example of this is the [[commercial code (communications)|commercial telegraph code]] which was used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of [[telegram]]s.
+An example of this is the [[commercial code (communications)|commercial telegraph code]] which was used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of [[telegrams]].
 Another example is given by whole word ciphers, which allow the user to replace an entire word with a symbol or character, much like the way written Japanese utilizes [[Kanji]] (meaning Chinese characters in Japanese) characters to supplement the native Japanese characters representing syllables. An example using English language with Kanji could be to replace "The quick brown fox jumps over the lazy dog" by "The quick brown 狐 jumps 上 the lazy 犬". [[Shorthand|Stenographers]] sometimes use specific symbols to abbreviate whole words.
@@ Line 40: / Line 39: @@
 ===Historical===
 [[File:Caesar cipher left shift of 3.svg|thumb|Visual representation of how Caesar's Cipher works.]]
-The [[Caesar cipher|Caesar Cipher]] is one of the earliest known cryptographic systems. Julius Caesar used a cipher that shifts the letters in the alphabet in place by three and wrapping the remaining letters to the front to write to Marcus Tullius [[Cicero]] in approximately 50 BC.{{cn|date=March 2023}}
+The [[Caesar cipher|Caesar Cipher]] is one of the earliest known cryptographic systems. Julius Caesar used a cipher that shifts the letters in the alphabet in place by three and wrapping the remaining letters to the front to write to Marcus Tullius [[Cicero]] in approximately 50 BC.<ref>{{Cite web |title=Suetonius • Life of Julius Caesar |url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#56 |access-date=2026-05-29 |website=penelope.uchicago.edu}}</ref>
 Historical pen and paper ciphers used in the past are sometimes known as [[classical cipher]]s. They include simple [[substitution cipher]]s (such as [[ROT13]]) and [[transposition cipher]]s (such as a [[Rail Fence Cipher]]). For example, "GOOD DOG" can be encrypted as "PLLX XLP" where "L" substitutes for "O", "P" for "G", and "X" for "D" in the message.  Transposition of the letters "GOOD DOG" can result in "DGOGDOO". These simple ciphers and examples are easy to crack, even without plaintext-ciphertext pairs.<ref>{{Cite journal|last=Saltzman|first=Benjamin A.|title=Vt hkskdkxt: Early Medieval Cryptography, Textual Errors, and Scribal Agency (Speculum, forthcoming)|url=https://www.academia.edu/35034685|journal=Speculum|year=2018 |volume=93|issue=4|page=975|doi=10.1086/698861 |jstor=26584834 |s2cid=165362817|language=en}}</ref><ref>{{Cite book |last=Janeczko |first=Paul B |title=Top Secret |year=2004}}</ref>
-In the 1640s, the Parliamentarian commander, [[Edward Montagu, 2nd Earl of Manchester]], developed ciphers to send coded messages to his allies during the [[English Civil War]].<ref>{{cite web |title=English Civil War cipher belonging to Cromwell ally goes on display |url=https://the-past.com/news/english-civil-war-cipher-belonging-to-cromwell-ally-goes-on-display/ |website=The Past |date=12 July 2023 |access-date=4 August 2023}}</ref> The English theologian John Wilkins published a book in 1641 titled "Mercury, or The Secret and Swift Messenger" and described a musical cipher wherein letters of the alphabet were substituted for music notes.<ref>{{cite web |title=Types of Ciphers |url=https://www.audiocipher.com/post/types-of-ciphers |website=AudioCipher |date=26 February 2025 |access-date=14 March 2025}}</ref><ref>{{cite web |title=Mercury: Or the Secret and Swift Messenger |url=https://www.lybrary.com/mercury-or-the-secret-and-swift-messenger-p-5897.html |website=Lybrary |access-date=14 March 2025}}</ref> This species of melodic cipher was depicted in greater detail by author Abraham Rees in his book Cyclopædia (1778). <ref>{{cite web |title=Cyclopaedia |url=https://archive.org/details/cyclopaediaoruni08rees |website=Internet Archive |access-date=14 March 2025}}</ref>
+In the 1640s, the Parliamentarian commander, [[Edward Montagu, 2nd Earl of Manchester]], developed ciphers to send coded messages to his allies during the [[English Civil War]].<ref>{{cite web |title=English Civil War cipher belonging to Cromwell ally goes on display |url=https://the-past.com/news/english-civil-war-cipher-belonging-to-cromwell-ally-goes-on-display/ |website=The Past |date=12 July 2023 |access-date=4 August 2023}}</ref> The English theologian John Wilkins published a book in 1641 titled "Mercury, or The Secret and Swift Messenger" and described a musical cipher wherein letters of the alphabet were substituted for music notes.<ref>{{cite web |title=Types of Ciphers |url=https://www.audiocipher.com/post/types-of-ciphers |website=AudioCipher |date=26 February 2025 |access-date=14 March 2025}}</ref><ref>{{cite web |title=Mercury: Or the Secret and Swift Messenger |url=https://www.lybrary.com/mercury-or-the-secret-and-swift-messenger-p-5897.html |website=Lybrary |access-date=14 March 2025}}</ref> This species of melodic cipher was depicted in greater detail by author Abraham Rees in his book Cyclopædia (1778).<ref>{{cite web |title=Cyclopaedia |url=https://archive.org/details/cyclopaediaoruni08rees |website=Internet Archive |access-date=14 March 2025}}</ref>
 Simple ciphers were replaced by [[polyalphabetic substitution]] ciphers (such as the [[Vigenère cipher|Vigenère]]) which changed the substitution alphabet for every letter.  For example, "GOOD DOG" can be encrypted as "PLSX TWF" where "L", "S", and "W" substitute for "O".  With even a small amount of known or estimated plaintext, simple polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.<ref>{{citation|first=Douglas R.|last=Stinson|author-link = Doug Stinson|title=Cryptogtaphy / Theory and Practice|year=1995|publisher=CRC Press|isbn=0-8493-8521-0|page=45}}</ref> It is possible to create a secure pen and paper cipher based on a [[one-time pad]], but these have other disadvantages.
@@ Line 66: / Line 65: @@
 In a pure mathematical attack, (i.e., lacking any other information to help break a cipher) two factors above all count:
-* Computational power available, i.e., the computing power which can be brought to bear on the problem. It is important to note that average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of [[exhaustive search]] for a key (i.e., "brute force" attack) substantially.
+* Computational power available, i.e., the computing power which can be brought to bear on the problem. Average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of [[exhaustive search]] for a key (i.e., "brute force" attack) substantially.
 * [[Key size]], i.e., the size of key used to encrypt a message. As the key size increases, so does the complexity of [[exhaustive search]] to the point where it becomes impractical to crack encryption directly.
 Since the desired effect is computational difficulty, in theory one would choose an [[algorithm]] and desired difficulty level, thus decide the key length accordingly.
@@ Line 89: / Line 88: @@
 * {{cite journal | last=Caldwell | first=William Casey | title=Shakespeare's ''Henry V'' and the Ciphers of History | journal=[[SEL: Studies in English Literature 1500–1900]] | volume=61 | issue=2 | date=2022 | issn=1522-9270 | doi=10.1353/sel.2022.0003 | pages=241–268}}
 * {{Cite book |last=Gaines |first=Helen Fouché |title=Cryptanalysis |year=1956 |isbn=0-486-20097-3 |edition=Corrected |publisher=[[Dover Publications]] |publication-place=New York |url=https://archive.org/details/cryptanalysis00hele |ol=7634764M |author-link=Helen Fouché Gaines |orig-date=1939}}
-* {{Cite conference |last1=Ho |first1=Yean Li |last2=Samsudin |last3=Belaton |first3=Bahari |first2=Azman |title=Heuristic Cryptanalysis of Classical and Modern Ciphers |conference=2005 13th IEEE International Conference on Networks, Jointly held with the 2005 IEEE 7th Malaysia International Conference on Communication |url=https://ieeexplore.ieee.org/document/1635595  |publisher=[[IEEE]] |date=2005 |volume=2 |pages=710–715 |doi=10.1109/ICON.2005.1635595 |publication-place=Kuala Lumpur  |access-date=2024-06-30 |url-access=subscription}}
+* {{Cite conference |last1=Ho |first1=Yean Li |last2=Samsudin |last3=Belaton |first3=Bahari |first2=Azman |title=Heuristic Cryptanalysis of Classical and Modern Ciphers |conference=2005 13th IEEE International Conference on Networks, Jointly held with the 2005 IEEE 7th Malaysia International Conference on Communication |publisher=[[IEEE]] |date=2005 |volume=2 |pages=710–715 |doi=10.1109/ICON.2005.1635595 |publication-place=Kuala Lumpur  }}
 * {{Cite book |last=Kahn |first=David |title=The Codebreakers – The Story of Secret Writing |year=1996 |isbn=0-684-83130-9 |edition=Revised |location=New York |oclc=35159231 |author-link=David Kahn (writer) |orig-date=1967 |title-link=The Codebreakers}}
 * {{Cite web |url=https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/ciphers-vs-codes |title=Ciphers vs. codes |access-date=2024-06-30 |website=[[Khan Academy]] |archive-url=https://web.archive.org/web/20240117234055/https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/ciphers-vs-codes |archive-date=2024-01-17 |url-status=live}}
@@ Line 101: / Line 100: @@
 ==External links==
 {{Wiktionary|cipher}}
+* [https://ciphermuseum.com Cipher Museum] – Educational site with interactive cipher demonstrations and cryptanalysis tools
+* [https://caesarcipher.org/ciphers/identifier Cipher Identifier] – Free tool to automatically identify which classical cipher was used to encrypt a given text
 * [http://www.scholarpedia.org/article/Secure_communications_using_the_KLJN_scheme Kish cypher]

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