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Permutation cipher

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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.

[edit] 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.

[edit] 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.

[edit] See also

Topics in cryptography




Classical cryptography
v â€Ã³ d â€Ã³ e




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
v â€Ã³ d â€Ã³ e




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







Retrieved from "http://en.wikipedia.org/wiki/Permutation_cipher"
Categories: Classical ciphers | Permutations






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This page was last modified 03:36, 11 March 2007.
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