Tool to decrypt/encrypt with Beaufort automatically. Beaufort Cipher is a variant of the Vigenere Cipher: rather than add the key to the plaintext, Beaufort subtract the plaintext to the key.

Beaufort Cipher - dCode

Tag(s) : Poly-Alphabetic Cipher

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Encryption is a variant of the Vigenere cipher, it uses a key (and an alphabet).

__Example:__ Encrypt the plain text `DCODE` with the key `KEY` and the latin alphabet `ABCDEFGHIJKLMNOPQRSTUVWXYZ`

For each letter corresponds the value of its rank in the alphabet starting from `0=A,1=B,...,25=Z`

__Example:__

Letter | D | C | O | D | E |

Value | 3 | 2 | 14 | 3 | 4 |

Encryption consists in subtracting the plain text to the key. Calculation is made letter after letter (subtraction of letters values in the alphabet).

If the result is negative, add `26` to the result (where 26 is the length of the alphabet).

The key is repeated as necessary in order that it fits the length of the plain text: `KEYKEYKEYK...`

__Example:__ Subtract the first letter of the plain message `D` (=3) to the first letter of the key `K` (=10) : 10-3=`7`. Keep this result and continue with the next letters: the second letter of the plain message `C` and the second letter ot the key `E` : 4-2=`2`. Keep going with the third letters `O` and `Y` : 24-14=`10`. At the 4th step, at the end of the key, repeat it (or go to the beginning, its the same), subtract the 4th letter of the plaintext `D` to the first letter of the key `K` (where `K` is the 4th letter of the key if it has been repeated) : 10-3=`7`, and to finish `E` and `E` so 4-4=`0`.

Each result is a number between 0 and 25, to which a correspondence with a letter of the same rank in the alphabet get the cipher text.

__Example:__ `7,2,10,7,0` becomes with 7:H, 2:C, 10:K, 7:H, 0:A, the cipher message `HCKHA`.

The Beaufort cipher has for particularity that the decryption is identical to the encryption: subtract the cipher message to the key.

The ciphered message has an indice of coincidence between 0.04 and 0.05, generally smaller than the one of the language of the plain text.

Beaufort is a French town in Savoie, known for its cheese, any reference to these elements can be a clue.

Techniques for Beaufort automatic decryption are similar to techniques for Vigenere (frequency analysis, probable key lengths, Kasiski method, etc.) to find the key.

Sir Francis Beaufort, british amiral made this method published after his death by its brother in 1857. However, some writings indicate that this variant was known since 1710.

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beaufort,vigenere,cheese,variant,key,subtraction

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