Variant Beaufort Cipher

Encryption is a variant of the Beaufort cipher, itself being a variant of Vigenere, so 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: DCODE corresponds to 3,2,14,3,4 and KEY to 10,4,24

Encryption consists in subtracting the key to the plain text. Calculation is made letter after letter (do not subtract letters but their associated value 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 key K (=10) to the first letter of the plain message D (=3) : 3-10=-7. As the result is negative, add 26. -7+26=19. 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 : 2-4=-2+26=24. Keep going with the third letters O and Y : 14-24=-10+26=16. At the 4th step, arrived at the end of the key, repeat the key (or go to the beginning, its the same), subtract the first letter of the key K (where K is the 4th letter of the key if it has been repeated) to the 4th letter of the plaintext D : 3-10=-7+26=19, 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 gives the cipher text.

Example: 19,24,16,19,0 becomes with 19:T, 24:Y, 16:Q, 19:T, 0:A, the cipher message TYQTA.

Variant Beaufort cipher adds the message to the key, this is equivalent to encoding with Vigenere.

The ciphered message has an index of coincidence between 0.4 and 0.5, generally smaller than the one of the language of the plain text.

Techniques to decode Beaufort are similar to the ones for Vigenere. dCode can analyze the text in order to find the probable key lengths (Kasiski, etc.) and uses frequency analysis techniques to find the key.