permutation cipher

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permutation cipher

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permutation cipher

In classical cryptography, a permutation cipher is a transposition cipher in which the key is a
permutation.
To apply a cipher, a random permutation of size e is generated (the larger the value of
e the more secure the cipher). The plaintext is then broken into segments of size
e and the letters within that segment are permuted according to this key.
In theory, any transposition cipher can be viewed as a permutation cipher where e is equal to the length of the
plaintext; this is too cumbersome a generalisation to use in actual practice, however.

Identifying the cipher
Because the cipher doesn't change any of the characters, the ciphertext will have exactly
the same letter frequencies as the underlying plaintext. This means that the cipher can in many cases be identified as a
transposition by the close similarity of its letter statistics with the letter frequencies of the underlying language.

Breaking the cipher
Because the cipher operates on blocks of size e, the plaintext and the ciphertext have to have a length which is some
multiple of e. This causes two weakness in the system: firstly, that the plaintext may have to be padded (if the padding
is identifiable then part of the key is revealed) and secondly, information relating to the length of the key is revealed by the
length of the ciphertext. To see this, note that if the ciphertext is of length i then e must be one of the
divisors of i. With the different possible key sizes different possible permutations are
tried to find the permutation which results in the highest number of frequent bigrams and trigrams as found in the underlying
language of the plaintext. Trying to find this permutation is essentially the same problem encountered when analysing a
columnar transposition cipher: multiple anagramming.

See also

Topics in cryptography

Classical cryptography

Ciphers: ADFGVX | Affine | Alberti | Atbash |
Autokey | Bifid | Book | Caesar | Four-square | Hill | Keyword | Nihilist | Permutation | Pigpen | Playfair | Polyalphabetic | Polybius | Rail Fence | Reihenschieber | Reservehandverfahren | ROT13 | Running key | Scytale |
Smithy code | Solitaire | Straddling checkerboard | Substitution |
Tap Code | Transposition | Trifid | Two-square | VIC
cipher | Vigenère

Cryptanalysis: Frequency analysis |
Index of coincidence

Misc: Cryptogram | Bacon | Polybius square | Scytale | Straddling checkerboard | Tabula recta

Cryptography

History of cryptography | Cryptanalysis | Cryptography portal | Topics in cryptography

Symmetric-key algorithm | Block cipher | Stream cipher | Public-key cryptography | Cryptographic hash function | Message authentication code | Random numbers

This entry is from Wikipedia, the leading
user-contributed encyclopedia. It may not have been reviewed by
professional editors (see full disclaimer)

Donate to Wikimedia

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Wikipedia information about permutation cipherThis article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Permutation cipher". More from Wikipedia

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Mentioned Inpermutation cipher is mentioned in these AnswerPages:Hasty Pudding Ciphertransposition cipherTopics in cryptographyList of mathematics articles (P)block cipherMore>

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About "permutation cipher":Find the permutation for the following ciphertext encrypted with a permutation cipher:
BTOOLLYDWGHOEENRAONMSHGAEOBNOERFTERM THIS
QUESTION COULD BE BASED ON PERMUTATION OR COMBINATION. Suppose you knew
a message had been coded using a Substitution cipher. If it took you
one second to... I am attempting to solve a numeric cipher/code in which the letter should translate or arrange into words.

The code is 253112522, and it should translate into...
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