Trifid Cipher - Description and Cryptanalysis

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On this page: [Introduction] [Example] [Javascript Example] [Cryptanalysis] [Code] [References]

Introduction

The Trifid cipher combines substitution with transposition and fractionation . It was invented by F�ix Delastelle. Delastelle was a Frenchman who invented several ciphers including the bifid, trifid, and the four-square ciphers.

Trifid is very similar to Bifid, except that instead of a 5 by 5 keysquare (in bifid) we use a 3 by 3 by 3 key cube.

The Algorithm

Keys for the Trifid cipher usually consist of a 27 letter 'key cube'. e.g.

Javascript Example of the Trifid Cipher

The keysquare in this example has been written on a single line. To convert to the cube shape,
Plaintext
defend the east wall of the castle

keysquare =

Period =

Ciphertext

Cryptanalysis

... Coming Soon ...

Code

I have included here some C code that does encryption and decryption of the Bifid cipher. It is only meant to show the working of the algorithm, not be a final polished solution.
Bifid_encrypt.c Bifid_decrypt.c
There is also some code that utilises a markov model to automatically solve a Bifid cipher. Note that very short ciphertexts may not be cryptanalysed properly. The longer the ciphertext the better, several hundred characters ideal.
Bifid_crack.c

References

[1] Wikipedia has a good description of the encryption/decryption process, history and cryptanalysis of this algorithm

[2] Kahn, D (1973) The CodeBreakers. Macmillan: New York

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