Transposition cipher

In cryptography, the transposition is one of the two basic encryption classes. The characters of a message ( the plaintext ) to re-order. Although each character remains unchanged, however, the point at which it is changed. This is in contrast to the class of ( mono- alphabetic or poly-alphabetic ) substitution, in which each character of the plaintext Although retains his place, but replaced by another character ( "substituted" ) is.

Method

A word with example only three letters ( "off") can be switched on only six different ways to be ( " transpose " ), namely: from, asu, uas, usa, sua, sow. The length of the string, however, the number of possible arrangements increases more than exponentially.

A random transposition longer strings so generates a relatively high level of security, but there is a receiver facing the same enigma as a possible eavesdropper, namely how to decipher the encrypted message (see also: anagram ). Therefore manageable systems of transposition must be agreed - these are called controlled transpositions.

Each transposition can be attributed to a polyalphabetic substitution. This is done using the Hill cipher (see en: Hill cipher) with a matrix that contains exactly in each row and each column is a one and usually only zeros. This produces an image which is identical to the original transposition.

Example

As a very simple and illustrative example of a regulated transposition is here the " fence " transposition serve: the letters of the text are written alternately on two lines, so that the first one on the top, the second on the bottom, the third letter again on the top line and so on. Finally, the string on the bottom line is appended to the upper line:

A really confusing garden fence, in capital letters A REALLY ENGENDERED CONFUSION GARDEN FENCE is encrypted:

E N I K I H E W R E D R A N T A N   I W R L C V R R I N E G R E T O ENIKIHEWREDRATNAN IWRLCVRIRNEGREZU - encryption by transposition has taken place. The recipient can decrypt the message by simply reverses the process, so the two character blocks reordered writes in two lines.

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