Editing Substitution cipher (section)

====Security====
Although the traditional keyword method for creating a mixed substitution alphabet is simple, a serious disadvantage is that the last letters of the alphabet (which are mostly low frequency) tend to stay at the end. A stronger way of constructing a mixed alphabet is to generate the substitution alphabet completely randomly.

Although the number of possible substitution alphabets is very large (26! ≈ 2<sup>88.4</sup>, or about [[key size|88 bits]]), this cipher is not very strong, and is easily broken. Provided the message is of reasonable length (see below), the [[cryptanalysis|cryptanalyst]] can deduce the probable meaning of the most common symbols by analyzing the [[frequency distribution]] of the ciphertext. This allows formation of partial words, which can be tentatively filled in, progressively expanding the (partial) solution (see [[frequency analysis#An example|frequency analysis]] for a demonstration of this). In some cases, underlying words can also be determined from the pattern of their letters; for example, the [[English language|English]] words ''tater'', ''ninth'', and ''paper'' all have the pattern ''ABACD''. Many people solve such ciphers for recreation, as with [[cryptogram]] puzzles in the newspaper.

According to the [[unicity distance]] of [[English language|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 cryptanalyst.