Better to know some... than all 


Substitution ciphersIt consists the following types of classical ciphers: simple (or monoalphabetic) substitution, polygram substitution, and homophonic substitution. The difference between codes and ciphers is also noted. Monoalphabetic substitutionSuppose the ciphertext and plaintext character sets are the same. Let m = m1m2m3… be a plaintext message consisting of juxtaposed characters mi E A, where A is some fixed character alphabet such as A = {A,B…. Z}. A simple substitution cipher or monoalphabetic substitution cipher employs a permutation e over A, with encryption mapping Ee(m) = e(m1)e(m2)e(m3)… . Here juxtaposition indicates concatenation (rather than multiplication), and e(mi) is the character to which mi is mapped by e. Recognizing simple substitution. Monoalphabetic substitution alters the frequency of individual plaintext characters, but does not alter the frequency distribution of the overall character set. Thus, comparing ciphertext character frequencies to a table of expected letter frequencies (unigram statistics) in the plaintext language allows associations between ciphertext and plaintext characters. Polygram substitutionA simple substitution cipher substitutes for single plaintext letters. In contrast, polygram substitution ciphers involve groups of characters being substituted by other groups of characters. For example, sequences of two plaintext characters (digrams) may be replaced by other digrams. The same may be done with sequences of three plaintext characters (trigrams), or more generally using ngrams. Homophonic substitutionThe idea of homophonic substitution is for each fixed key k to associate with each plaintext unit (e.g., character) m a set S (k, m) of potential corresponding ciphertext units (generally all of common size). To encrypt m under k, randomly choose one element from this set as the ciphertext. To allow decryption, for each fixed key this onetomany encryption function must be injective on ciphertext space. Homophonic substitution results in ciphertext data expansion. In homophonic substitution, S (k, m) should be proportional to the frequency of m in the message space. The motivation is to smooth out obvious irregularities in the frequency distribution of ciphertext characters, which result from irregularities in the plaintext frequency distribution when simple substitution is used. While homophonic substitution complicates cryptanalysis based on simple frequency distribution statistics, sufficient ciphertext may nonetheless allow frequency analysis, in conjunction with additional statistical properties of plaintext manifested in the ciphertext. For example, in long ciphertexts each element of S (k, m) will occur roughly the same number of times. Codes vs. ciphersA technical distinction is made between ciphers and codes. Ciphers are encryption techniques which are applied to plaintext units (bits, characters, or blocks) independent of their semantic or linguistic meaning; the result is called ciphertext. In contrast, cryptographic codes operate on linguistic units such as words, groups of words, or phrases, and substitute (replace) these by designated words, letter groups, or number groups called codegroups. The key is a dictionarylike codebook listing plaintext units and their corresponding codegroups, indexed by the former; a corresponding codebook for decoding is reverseindexed. When there is potential ambiguity, codes in this context (vs. ciphers) may be qualified as cryptographic codebooks, to avoid confusion with errorcorrecting codes (ECcodes) used to detect and/or correct nonmalicious errors and authentication codes which provide data origin authentication. Several factors suggest that codes may be more difficult to break than ciphers: the key (codebook) is vastly larger than typical cipher keys; codes may result in data compression; and statistical analysis is complicated by the large plaintext unit block size. Opposing this are several major disadvantages: the coding operation not being easily automated (relative to an algorithmic mapping); and identical encryption of repeated occurrences of plaintext units implies susceptibility to knownplaintext attacks, and allows frequency analysis based on observed traffic. This implies a need for frequent rekeying (changing the codebook), which is both more costly and inconvenient. Consequently, codes are not commonly used to secure modern telecommunications. 