Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered.
There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different times in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice-versa.
Substitution over a single letter—simple substitution—can be demonstrated by writing out the alphabet in some order to represent the substitution. This is termed a substitution alphabet. The cipher alphabet may be shifted or reversed (creating the Caesar and Atbash ciphers, respectively) or scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged alphabet. Traditionally, mixed alphabets are created by first writing out a keyword, removing repeated letters in it, then writing all the remaining letters in the alphabet.
flee at once. we are discovered!
SIAA ZQ LKBA. VA ZOA RFPBLUAOAR!
Traditionally, the ciphertext is written out in blocks of fixed length, omitting punctuation and spaces; this is done to help avoid transmission errors and to disguise word boundaries from the plaintext. These blocks are called "groups", and sometimes a "group count" (i.e., the number of groups) is given as an additional check. Five letter groups are traditional, dating from when messages used to be transmitted by telegraph:
SIAAZ QLKBA VAZOA RFPBL UAOAR
If the length of the message happens not to be divisible by five, it may be padded at the end with "nulls". These can be any characters that decrypt to obvious nonsense, so the receiver can easily spot them and discard them.
The ciphertext alphabet is sometimes different from the plaintext alphabet; for example, in the pigpen cipher, the ciphertext consists of a set of symbols derived from a grid. For example:Such features make little difference to the security of a scheme, however — at the very least, any set of strange symbols can be transcribed back into an A-Z alphabet and dealt with as normal.
In lists and catalogues for sales people sometimes a very simple encryption is used to replace numeric digits by letters.
Although the number of possible keys is very large (26! ≈ 288.4, or about 88 bits), this cipher is not very strong, being easily broken. Provided the message is of reasonable length (see below), the cryptanalyst can deduce the probable meaning of the most common symbols by analyzing the frequency distribution of the ciphertext—frequency analysis. This allows formation of partial words, which can be tentatively filled in, progressively expanding the (partial) solution (see frequency analysis for a demonstration of this). In some cases, underlying words can also be determined from the pattern of their letters; for example, attract, osseous, and words with those two as the root are the only common English words with the pattern ABBCADB. Many people solve such ciphers for recreation, as with cryptogram puzzles in the newspaper.
According to the unicity distance of English, 27.6 letters of ciphertext are required to crack a mixed alphabet simple substitution. In practice, typically about 50 letters are needed, although some messages can be broken with fewer if unusual patterns are found. In other cases, the plaintext can be contrived to have a nearly flat frequency distribution, and much longer plaintexts will then be required by the user.
An early attempt to increase the difficulty of frequency analysis attacks on substitution ciphers was to disguise plaintext letter frequencies by homophony. In these ciphers, plaintext letters map to more than one ciphertext symbol. Usually, the highest-frequency plaintext symbols are given more equivalents than lower frequency letters. In this way, the frequency distribution is flattened, making analysis more difficult.
Since more than 26 characters will be required in the ciphertext alphabet, various solutions are employed to invent larger alphabets. Perhaps the simplest is to use a numeric substitution 'alphabet'. Another method consists of simple variations on the existing alphabet; uppercase, lowercase, upside down, etc. More artistically, though not necessarily more securely, some homophonic ciphers employed wholly invented alphabets of fanciful symbols. (See Poe's "The Gold-Bug" for a literary example; cf. the Voynich manuscript.)
An interesting variant is the nomenclator. Named after the public official who announced the titles of visiting dignitaries, this cipher combined a small codebook with large homophonic substitution tables. Originally the code was restricted to the names of important people, hence the name of the cipher; in later years it covered many common words and place names as well. The symbols for whole words (codewords in modern parlance) and letters (cipher in modern parlance) were not distinguished in the ciphertext. The Rossignols' Great Cipher used by Louis XIV of France was one; after it went out of use, messages in French archives were unbreakable for several hundred years.
Nomenclators were the standard fare of diplomatic correspondence, espionage, and advanced political conspiracy from the early fifteenth century to the late eighteenth century; most conspirators were and have remained less cryptographically sophisticated. Although government intelligence cryptanalysts were systematically breaking nomenclators by the mid-sixteenth century, and superior systems had been available since 1467, the usual response to cryptanalysis was simply to make the tables larger. By the late eighteenth century, when the system was beginning to die out, some nomenclators had 50,000 symbols.
Nevertheless, not all nomenclators were broken; today, cryptanalysis of archived ciphertexts remains a fruitful area of historical research.
In a polyalphabetic cipher, multiple cipher alphabets are used. To facilitate encryption, all the alphabets are usually written out in a large table, traditionally called a tableau. The tableau is usually 26×26, so that 26 full ciphertext alphabets are available. The method of filling the tableau, and of choosing which alphabet to use next, defines the particular polyalphabetic cipher. All such ciphers are easier to break than once believed, as substitution alphabets are repeated for sufficiently large plaintexts.
One of the most popular was that of Blaise de Vigenère. First published in 1585, it was considered unbreakable until 1863, and indeed was commonly called le chiffre indéchiffrable (French for "indecipherable cipher").
In the Vigenère cipher, the first row of the tableau is filled out with a copy of the plaintext alphabet, and successive rows are simply shifted one place to the left. (Such a simple tableau is called a tabula recta, and mathematically corresponds to adding the plaintext and key letters, modulo 26.) A keyword is then used to choose which ciphertext alphabet to use. Each letter of the keyword is used in turn, and then they are repeated again from the beginning. So if the keyword is 'CAT', the first letter of plaintext is enciphered under alphabet 'C', the second under 'A', the third under 'T', the fourth under 'C' again, and so on. In practice, Vigenère keys were often phrases several words long.
In 1863, Friedrich Kasiski published a method (probably discovered secretly and independently before the Crimean War by Charles Babbage) which enabled the calculation of the length of the keyword in a Vigenère ciphered message. Once this was done, ciphertext letters that had been enciphered under the same alphabet could be picked out and attacked separately as a number of semi-independent simple substitutions - complicated by the fact that within one alphabet letters were separated and did not form complete words, but simplified by the fact that usually a tabula recta had been employed.
As such, even today a Vigenère type cipher should theoretically be difficult to break if mixed alphabets are used in the tableau, if the keyword is random, and if the total length of ciphertext is less than 27.6 times the length of the keyword. These requirements are rarely understood in practice, and so Vigenère enciphered message security is usually less than might have been.
Other notable polyalphabetics include:
Modern stream ciphers can also be seen, from a sufficiently abstract perspective, to be a form of polyalphabetic cipher in which all the effort has gone into making the keystream as long and unpredictable as possible.
To substitute pairs of letters would take a substitution alphabet 676 symbols long (). In the same De Furtivis Literarum Notis mentioned above, della Porta actually proposed such a system, with a 20 x 20 tableau (for the 20 letters of the Italian/Latin alphabet he was using) filled with 400 unique glyphs. However the system was impractical and probably never actually used.
The earliest practical digraphic cipher (pairwise substitution), was the so-called Playfair cipher, invented by Sir Charles Wheatstone in 1854. In this cipher, a 5 x 5 grid is filled with the letters of a mixed alphabet (two letters, usually I and J, are combined). A digraphic substitution is then simulated by taking pairs of letters as two corners of a rectangle, and using the other two corners as the ciphertext (see the Playfair cipher main article for a diagram). Special rules handle double letters and pairs falling in the same row or column. Playfair was in military use from the Boer War through World War II.
Several other practical polygraphics were introduced in 1901 by Felix Delastelle, including the bifid and four-square ciphers (both digraphic) and the trifid cipher (probably the first practical trigraphic).
The Hill cipher, invented in 1929 by Lester S. Hill, is a polygraphic substitution which can combine much larger groups of letters simultaneously using linear algebra. Each letter is treated as a digit in base 26: A = 0, B =1, and so on. (In a variation, 3 extra symbols are added to make the basis prime.) A block of n letters is then considered as a vector of n dimensions, and multiplied by a n x n matrix, modulo 26. The components of the matrix are the key, and should be random provided that the matrix is invertible in (to ensure decryption is possible). Astonishingly, a Hill cipher of dimension 6 was once implemented mechanically!
Unfortunately, the Hill cipher is vulnerable to a known-plaintext attack because it is completely linear, so it must be combined with some non-linear step to defeat this attack. The combination of wider and wider weak, linear diffusive steps like a Hill cipher, with non-linear substitution steps, ultimately leads to a substitution-permutation network (e.g., a Feistel cipher), so it is possible—from this extreme perspective—to consider modern block ciphers as a type of polygraphic substitution.
All of these were similar in that the substituted letter was chosen electrically from amongst the huge number of possible combinations resulting from the rotation of several letter disks. Since one or more of the disk rotated mechanically with each plaintext letter enciphered, the number of alphabets used was substantially more than astronomical. Early versions of these machine were, nevertheless, breakable. William F. Friedman of the US Army's SIS early found vulnerabilities in Hebern's rotor machine, and GC&CS's Dillwyn Knox solved versions of the Enigma machine (those without the "plugboard") well before WWII began. Traffic protected by essentially all of the German military Enigmas was broken by Allied cryptanalysts, most notably those at Bletchley Park, beginning with the German Army variant used in the early 1930s. This version was broken by inspired mathematical insight by Marian Rejewski in Poland.
The one-time pad is, in most cases, impractical as it requires that the key material be as long as the plaintext, actually random, used once and only once, and kept entirely secret from all except the sender and intended receiver. When these conditions are violated, even marginally, the one-time pad is no longer unbreakable. Soviet one-time pad messages sent from the US for a brief time during WWII used non-random key material. US cryptanalysts, beginning in the late 40s, were able to, entirely or partially, break a few thousand messages out of several hundred thousand. (See VENONA)