The action of a Caesar cipher is to replace each plaintext letter with one a fixed number of places down the alphabet. This example is with a shift of three, so that a B in the plaintext becomes E in the ciphertext.

In cryptography, a Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a shift of 3, A would be replaced by D, B would become E, and so on. The method is named after Julius Caesar, who used it to communicate with his generals. Image File history File links Caesar3. ... Image File history File links Caesar3. ... The German Lorenz cipher machine, used in World War II for encryption of very high-level general staff messages Cryptography (or cryptology; derived from Greek κρυπτός kryptós hidden, and the verb γράφω gráfo write or λεγειν legein to speak) is the study of message secrecy. ... “Encrypt” redirects here. ... In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the units may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. ... In cryptography, plaintext is information used as input to an encryption algorithm; the output is termed ciphertext. ... ABCs redirects here, for the Alien Big Cats, see British big cats. ... For other uses, see Julius Caesar (disambiguation). ... This does not adequately cite its references or sources. ...

The encryption step performed by a Caesar cipher is often incorporated as part of more complex schemes, such as the Vigenère cipher, and still has modern application in the ROT13 system. As with all single alphabet substitution ciphers, the Caesar cipher is easily broken and in practice offers essentially no communication security. The Vigenère cipher is named for Blaise de Vigenère (pictured), although Giovan Batista Belaso had invented the cipher earlier. ... ROT13 replaces each letter by its partner 13 characters further along the alphabet. ...

Example

The transformation can be represented by aligning two alphabets; the cipher alphabet is the plain alphabet rotated left or right by some number of positions. For instance, here is a Caesar cipher using a right rotation of three places (the shift parameter, here 3, is used as the key): ABCs redirects here, for the Alien Big Cats, see British big cats. ... A key is a piece of information that controls the operation of a cryptography algorithm. ...

 Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC 

To encipher a message, simply look up each letter of the message in the "plain" line and write down the corresponding letter in the "cipher" line. To decipher, do the reverse. “Encrypt” redirects here. ...

 Plaintext: the quick brown fox jumps over the lazy dog Ciphertext: WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ 

The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,..., Z = 25. Encryption of a letter x by a shift n can be described mathematically as, Modular arithmetic (sometimes called modulo arithmetic, or clock arithmetic because of its use in the 24-hour clock system) is a system of arithmetic for integers, where numbers wrap around after they reach a certain value — the modulus. ...

Decryption is performed similarly,

(Note, there are different definitions for the modulo operation. In the above, the result is in the range 0...25. I.e., if x+n or x-n are not in the range 0...25, we have to subtract or add 26). In computing, the modulo operation finds the remainder of division of one number by another. ...

The replacement remains the same throughout the message, so the cipher is classed as a type of monoalphabetic substitution, as opposed to polyalphabetic substitution. In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the units may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. ... A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. ...

History and usage

The Caesar cipher is named for Julius Caesar, who used an alphabet with a left shift of three.

The Caesar cipher is named after Julius Caesar, who, according to Suetonius, used it with a left shift of three to protect messages of military significance: Wikipedia does not have an article with this exact name. ... Wikipedia does not have an article with this exact name. ... For other uses, see Julius Caesar (disambiguation). ... For other uses, see Julius Caesar (disambiguation). ... The Twelve Caesars is a set of twelve biographies of Julius Caesar and the first 11 emperors of the Roman Empire. ...

If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others. — Suetonius, Life of Julius Caesar 56 [1].

While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier. His nephew Augustus also used the cipher, but with a right shift of one, and it did not wrap around to the beginning of the alphabet: Augustus Caesar Caesar Augustus (Latin: IMP·CAESAR·DIVI·F·AVGVSTVS)¹ (23 September 63 BC – 19 August AD 14), known earlier in his life as Gaius Octavius or Gaius Julius Caesar Octavianus, was the first Roman Emperor and is traditionally considered the greatest. ...

Whenever he wrote in cipher, he wrote B for A, C for B, and the rest of the letters on the same principle, using AA for X. — Suetonius, Life of Augustus 88.

There is evidence that Julius Caesar used more complicated systems as well, and one writer, Aulus Gellius, refers to a (now lost) treatise on his ciphers: Aulus Gellius ( 125 - after 180), Latin author and grammarian, possibly of African origin, probably born and certainly brought up at Rome. ...

There is even a rather ingeniously written treatise by the grammarian Probus concerning the secret meaning of letters in the composition of Caesar's epistles. — Aulus Gellius, 17.9.1–5.

It is unknown how effective the Caesar cipher was at the time, but it is likely to have been reasonably secure, not least because few of Caesar's enemies would have been literate, let alone able to consider cryptanalysis. Assuming that an attacker could read the message, there is no record at that time of any techniques for the solution of simple substitution ciphers. The earliest surviving records date to the 9th century in the Arab world with the discovery of frequency analysis. This article is about the ability to read and write. ... Cryptanalysis (from the Greek kryptós, hidden, and analýein, to loosen or to untie) is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. ... For other uses, see Arab (disambiguation). ... In mathematics, physics and signal processing, frequency analysis is a method to decompose a function, wave, or signal into its frequency components so that it is possible to have the frequency spectrum. ...

A Caesar cipher with a shift of one appears on the back of the Mezuzah.^[1] Mezuzah (IPA: ) (Heb. ...

In the 19th century, the personal advertisements section in newspapers would sometimes be used to exchange messages encrypted using simple cipher schemes. Kahn (1967) describes instances of lovers engaging in secret communications enciphered using the Caesar cipher in The Times. Even as late as 1915, the Caesar cipher was in use: the Russian army employed it as a replacement for more complicated ciphers which had proved to be too difficult for their troops to master; German and Austrian cryptanalysts had little difficulty in decrypting their messages. David Kahn is a US historian, journalist and writer. ... The Times is a national newspaper published daily in the United Kingdom since 1788. ...

Caesar ciphers can be found today in children's toys such as secret decoder rings. A Caesar shift of thirteen is also performed in the ROT13 algorithm, a simple method of obfuscating text used in some Internet forums to obscure text (such as joke punchlines and story spoilers), but not used as a method of encryption. A secret decoder was an inexpensive toy popular among young children during from 1930s through rest of the 20th century. ... ROT13 replaces each letter by its partner 13 characters further along the alphabet. ... In mathematics, computing, linguistics, and related disciplines, an algorithm is a finite list of well-defined instructions for accomplishing some task that, given an initial state, will terminate in a defined end-state. ... Look up spoiler in Wiktionary, the free dictionary. ...

The Vigenère cipher uses a Caesar cipher with a different shift at each position in the text; the value of the shift is defined using a repeating keyword. If a single-use keyword is as long as the message and chosen randomly then this is a one-time pad cipher, unbreakable if the users maintain the keyword's secrecy. Keywords shorter than the message (e.g., "Complete Victory"), used historically, introduce a cyclic pattern that might be detected with a statistically advanced version of frequency analysis. The Vigenère cipher is named for Blaise de Vigenère (pictured), although Giovan Batista Belaso had invented the cipher earlier. ... Excerpt from a one-time pad. ... The Vigenère cipher is named for Blaise de Vigenère (pictured), although Giovan Batista Belaso had invented the cipher earlier. ...

In April 2006, fugitive Mafia boss Bernardo Provenzano was captured in Sicily partly because of cryptanalysis of his messages written in a variation of the Caesar cipher. Provenzano's cipher used numbers, so that "A" would be written as "4", "B" as "5", and so on.^[2] This article is about the criminal society. ... Bernardo Provenzano in 1959, aged 26. ... Sicily ( in Italian and Sicilian) is an autonomous region of Italy and the largest island in the Mediterranean Sea, with an area of 25,708 km² (9,926 sq. ...

Breaking the cipher

The Caesar cipher can be easily broken even in a ciphertext-only scenario. Two situations can be considered: 1) an attacker knows (or guesses) that some sort of simple substitution cipher has been used, but not specifically that it is a Caesar scheme; and 2) an attacker knows that a Caesar cipher is in use, but does not know the shift value. In cryptography, a ciphertext-only attack is a form of cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. ...

In the first case, the cipher can be broken using the same techniques as for a general simple substitution cipher, such as frequency analysis or pattern words. While solving, it is likely that an attacker will quickly notice the regularity in the solution and deduce that a Caesar cipher is the specific algorithm employed. In mathematics, physics and signal processing, frequency analysis is a method to decompose a function, wave, or signal into its frequency components so that it is possible to have the frequency spectrum. ...

The distribution of letters in a typical sample of English language text has a distinctive and predictable shape. A Caesar shift "rotates" this distribution, and it is possible to determine the shift by examining the resultant frequency graph.

In the second instance, breaking the scheme is even more straightforward. Since there are only a limited number of possible shifts (26 in English), they can each be tested in turn in a brute force attack. One way to do this is to write out a snippet of the ciphertext in a table of all possible shifts — a technique sometimes known as "completing the plain component". The example given is for the ciphertext "EXXEGOEXSRGI"; the plaintext is instantly recognisable by eye at a shift of four. Another way of viewing this method is that, under each letter of the ciphertext, the entire alphabet is written out in reverse starting at that letter. This attack can be accelerated using a set of strips prepared with the alphabet written down them in reverse order. The strips are then aligned to form the ciphertext along one row, and the plaintext should appear in one of the other rows. English single letter frequencies. ... English single letter frequencies. ... The EFFs US$250,000 DES cracking machine contained over 1,800 custom chips and could brute force a DES key in a matter of days — the photograph shows a DES Cracker circuit board fitted with several Deep Crack chips. ...

Another brute force approach is to match up the frequency distribution of the letters. By graphing the frequencies of letters in the ciphertext, and by knowing the expected distribution of those letters in the original language of the plaintext, a human can easily spot the value of the shift by looking at the displacement of particular features of the graph. This is known as frequency analysis. For example in the English language the plaintext frequencies of the letters E, T, (usually most frequent), and Q, Z (typically least frequent) are particularly distinctive. Computers can also do this by measuring how well the actual frequency distribution matches up with the expected distribution; for example, the chi-square statistic can be used. In mathematics, physics and signal processing, frequency analysis is a method to decompose a function, wave, or signal into its frequency components so that it is possible to have the frequency spectrum. ... The English language is a West Germanic language that originates in England. ... Pearsons chi-square test (χ2) is one of a variety of chi-square tests – statistical procedures whose results are evaluated by reference to the chi-square distribution. ...

For natural language plaintext, there will, in all likelihood, be only one plausible decryption, although for extremely short plaintexts, multiple candidates are possible. For example, the ciphertext MPQY could, plausibly, decrypt to either "aden" or "know" (assuming the plaintext is in English); similarly, "ALIIP" to "dolls" or "wheel"; and "AFCCP" to "jolly" or "cheer" (see also unicity distance). Port of Aden (around 1910). ... Unicity distance is a term used in cryptography referring to the length of an original ciphertext needed to break the cipher by reducing the number of possible spurious keys to zero in a brute force attack. ...

Multiple encryptions and decryptions provide no additional security. This is because two encryptions of, say, shift A and shift B, will be equivalent to an encryption with shift A + B. In mathematical terms, the encryption under various keys forms a group. This picture illustrates how the hours on a clock form a group under modular addition. ...

References

• David Kahn, The Codebreakers — The Story of Secret Writing, 1967. ISBN 0-684-83130-9.
• F.L. Bauer, Decrypted Secrets, 2nd edition, 2000, Springer. ISBN 3-540-66871-3.
• Chris Savarese and Brian Hart, The Caesar Cipher, 1999 link

Current logo of The Register. ... David Kahn is a US historian, journalist and writer. ... Friedrich Ludwig Bauer (born June 10, 1924) in Regensburg) is a german computer scientist and professor emeritus at Munich_University_of_Technology. ...

External links

Listen to this article ( info/dl)

This audio file was created from an article revision dated 2005-04-14, and may not reflect subsequent edits to the article. (Audio help)

More spoken articles

   

• A Windows program to encode text featuring a "smart" dictionary attack to automatically decode cipher text
• A Caesar shift decoder in Excel format allowing you to see all 25 possibilities simultaneously
• An online Caesar cipher encoder
• A Caesar cipher solver: brute force attack
• Caesar cipher encryptor/decryptor: based on letter frequencies
• The Caesar Shift discussed on The Beginner's Guide to Cryptography
• Historical sources for Caesar's use of the cipher
• JavaScript Shift Cipher Encryptor/Decryptor
• Code Cracker Cracks many classic encryption codes including Caesar cipher, trying all permutations
• Online Caesar cipher encoder
• Generate your own secret codes
• Caesar Cipher Game An open-source Java game, with detailed explanation and comments
• Caesar Cipher. A Java client side implementation with sources. (GPL)

• Spy Tools Cryptography Online tool to automatically crack all kinds of basic codes and ciphers
• Online Caesar cipher encoder/decoder

Caesar cipher. ... Image File history File links Sound-icon. ... Year 2005 (MMV) was a common year starting on Saturday (link displays full calendar) of the Gregorian calendar. ... April 14 is the 104th day of the year (105th in leap years) in the Gregorian calendar, with 261 days remaining. ... Image File history File links Sound-icon. ... In cryptography, the ADFGVX cipher was a field cipher used by the German Army during World War I. ADFGVX was in fact an extension of an earlier cipher called ADFGX. Invented by Colonel Fritz Nebel and introduced in March 1918, the cipher was a fractionating transposition cipher which combined a... The Affine cipher is a special case of the more general substitution cipher. ... The Alberti Cipher was the first ever Polyalphabetic Cipher. ... Atbash is a simple substitution cipher for the Hebrew alphabet. ... A tabula recta for use with an autokey cipher An autokey cipher is a cipher which incorporates the message (the plaintext) into the key. ... In classical cryptography, the bifid cipher is a cipher which combines the Polybius square with transposition, and uses fractionation to achieve diffusion. ... A book cipher is a cipher in which the key is the identity of a book. ... The Four-square cipher is a manual symmetric encryption technique. ... Hills cipher machine, from figure 4 of the patent In classical cryptography, the Hill cipher is a polygraphic substitution cipher based on linear algebra. ... A keyword is a monoalphabetic substitution. ... -1... In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation. ... The pigpen cipher uses graphical symbols assigned according to a key similar to the above diagram. ... The Playfair system was invented by Charles Wheatstone, who first described it in 1854. ... A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. ... In cryptography, the Polybius square, also known as the Polybius checkerboard, is a device invented by the Ancient Greek historian and scholar Polybius, described in Hist. ... The Rail Fence Cipher is a form of transposition cipher that gets its name from the way in which it is encoded. ... The Reihenschieber was a hand cipher system used by Germany. ... Reservehandverfahren (RHV) (German: Reserve Hand Procedure) was a German Naval World War II hand-cipher system used as a backup method when no working Enigma machine was available[1]. The cipher had two stages: a transposition followed by bigram substitution. ... ROT13 replaces each letter by its partner 13 characters further along the alphabet. ... In classical cryptography, the runnning key cipher is a type of polyalphabetic substitution cipher in which a text, typically from a book, is used to provide a very long key stream. ... This article is about the encryption device; for the Dune character, see Scytale (Dune). ... This article refers to a cipher devised in 2006 by Mr Justice Peter Smith. ... The Solitaire cryptographic algorithm was designed by Bruce Schneier for use in Neal Stephensons Cryptonomicon (where it was initially called Pontifex). ... In cryptography, a straddling checkerboard is a device for converting an alphabetic plaintext into digits whilst simultaneously achieving fractionation (a simple form of information diffusion) and homophony (a simple method for suppressing peaks of the frequency distribution). ... In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the units may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. ... The Tap Code is a code, commonly used by prisoners in jail to communicate with one another. ... In classical cryptography, a transposition cipher changes one character from the plaintext to another (to decrypt the reverse is done). ... In classical cryptography, the trifid cipher is a cipher invented around 1901 by Felix Delastelle, which extends the concept of the bifid cipher to a third dimension, allowing each symbol to be fractionated into 3 elements instead of two. ... The Two-square cipher is a manual symmetric encryption technique. ... The VIC cipher was a pencil and paper cipher used by the Soviet spy Reino Hayhanen, codenamed VICTOR. It was arguably the most complex hand-operated cipher ever seen. ... The Vigenère cipher is named for Blaise de Vigenère (pictured), although Giovan Batista Belaso had invented the cipher earlier. ... A typical distribution of letters in English language text. ... In cryptography, coincidence counting is the technique (invented by William F. Friedman) of putting two texts side-by-side and counting the number of times that a letter appears next to itself in both copies. ... For the plants which reproduce by using spores, see Cryptogam. ... Bacons cipher or the Baconian cipher is a method of steganography (a method of hiding a secret message as opposed to a true cipher) devised by Francis Bacon. ... In cryptography, the Polybius square, also known as the Polybius checkerboard, is a device invented by the Ancient Greek historian and scholar Polybius, described in Hist. ... This article is about the encryption device; for the Dune character, see Scytale (Dune). ... In cryptography, a straddling checkerboard is a device for converting an alphabetic plaintext into digits whilst simultaneously achieving fractionation (a simple form of information diffusion) and homophony (a simple method for suppressing peaks of the frequency distribution). ... Tabula recta In cryptography, the tabula recta is a square table of alphabets, each one made by shifting the previous one to the left. ... The German Lorenz cipher machine, used in World War II for encryption of very high-level general staff messages Cryptography (or cryptology; derived from Greek κρυπτός kryptós hidden, and the verb γράφω gráfo write or λεγειν legein to speak) is the study of message secrecy. ... The history of cryptography dates back thousands of years. ... Cryptanalysis (from the Greek kryptós, hidden, and analýein, to loosen or to untie) is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. ... This article is intended to be an analytic glossary, or alternatively, an organized collection of annotated pointers. ... This article does not cite any references or sources. ... Encryption Decryption In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation. ... The operation of the keystream generator in A5/1, a LFSR-based stream cipher used to encrypt mobile phone conversations. ... A big random number is used to make a public-key pair. ... In cryptography, a cryptographic hash function is a hash function with certain additional security properties to make it suitable for use as a primitive in various information security applications, such as authentication and message integrity. ... A cryptographic message authentication code (MAC) is a short piece of information used to authenticate a message. ... A cryptographically secure pseudo-random number generator (CSPRNG) is a pseudo-random number generator (PRNG) with properties that make it suitable for use in cryptography. ...