Bellaso Cipher

Encryption uses an alphabet, the one of Bellaso was latin with 20 letters (U=V), a key to generate N alphabets from the first one and a cipher key.

Example: Take the alphabet ABCDEFGHILMNOPQRSTVX, CHIAVEALPHABET the key to generate 5 alphabets and CHIAVE the ciphering key. To crypt DCODE ME.

The message is split into words. For the nth word of the message, get the nth letter of the key (modulo key length) and substitute using the alphabet for the nth letter.

Example: Word 1 : DCODE, Word 2 = ME
1st letter of the key : C, alphabet for C = CHIAVDFGMN/ELPBTOQRSX, DCODE becomes OEDOC
2nd letter of the key : H, alphabet for H = CHIAVDFGMN/XELPBTOQRS, ME becomes RH
The message is encrypted OEDOC RH

Decryption is identical to encryption.

To decrypt, take distinct letters of the generating key and split it in half

Example: CHIAVEALPHABET becomes CHIAVELPBT so CHIAV and ELPBT

The two parts are filled with remaining letters in order to make it reversible

Example: CHIAVDFGMN/ELPBTOQRSX

To generate the next N-1 alphabets, keep the first part but make a rotation of i times n characters of the second part.

Example: i=1, n=1, ELPBTOQRSX becomes XELPBTOQRS
i=2, n=1, ELPBTOQRSX becomes SXELPBTOQR

For each letter of the alphabet 1 is associated one of the N generated alphabet, in order, ie, the first letter is associated to the first alphabet, the second letter to the second alphabet, etc.

Example: For N=5 alphabets, C,H,I,A,V,D,F,G,M,N,E,L,P,etc. are associated, respectively to alphabets 1,2,3,4,5,1,2,3,4,5,1,2,3,etc.

The ciphered message has a smaller index of coincidence than to the language of the plaintext.

In its original version, only 20 characters are used (latin alphabet, U=V)

In its original version, the message use a word-separator (space).

It is hard. However it is possible to find the number N of alphabet by analyzing frequency of one word out of N.

A book from Giovanni Battista Bellaso describing the process is dated 1553.