Progressive Key
Description
The Progressive Key cipher can be used with any of the Periodic substitution ciphers; Vigenère, Beaufort, Variant Beaufort and Porta. The difference to the Periodic ciphers is that the Progressive Key cipher is double encrypted making it slightly more secure. Encipherment follows the same procedure as the Periodic cipher to encipher text but a further encipherment is performed using a progressing key for each cipher group. For example if a key of 3 is used the first enciphered group is enciphered again using the key A, the second group using the key D, the third group G and so on.
To encipher text using the Vigenère cipher the key letter is found in the left column of the table and determines the row, and the plaintext letter is found in the top row and determines the column. The intersection of these reveals the cipher letter. So if the key is P and the plaintext letter is I the cipher letter is X. If a progression key of 3 is used and the plaintext letter is in the 2nd cipher group, it is enciphered again using the Vigenère with the key D producing the final ciphertext A.
Example
Encipher the following Winston Churchill quote "History will be kind to me for I intend to write it” using the keyword POLITICS and Progression key 3.
Type: Vigenère Progressive Key
Keyword: POLITICS
Progression Key: 3
Key: POLITICS POLITICS POLITICS POLITICS POLITICS
Plaintext: historyw illbekin dtomefor iintendt owriteit
First cipher: WWDBHZAO XZWJXSKF SHZUXNQJ XWYBXVFL DKCQMMKL
Progressive Key: A D G J M
Ciphertext: WWDBHZAO ACZMAVNI YNFADTWP GFHKGEOU PWOCYYWX
Vigenère Cipher
History will be kind to me for I intend to write it.
WWDBHZA OACZ MA VNIY NF AD TWP G FHKGEO UP WOCYY WX.
Beaufort Cipher
History will be kind to me for I intend to write it.
SUHLVJW EWAD WO FJYU LJ KR DSF C DLUUOK KL USMMI SN.
Variant Beaufort Cipher
History will be kind to me for I intend to write it.
SUHLVJW EQUX QI ZDSI ZX YF RGT K LTCCWS SN WUOOK UP.
Porta Cipher
History will be kind to me for I intend to write it.
BCNPSNX NBEC EM AISH PZ AK GQY L LZTJNA ON VSGQC DQ.
Solving
Solving method: Brute force search.
After selecting the Progressive Key cipher, the type needs to be selected from the Cipher Settings drop down menu on the toolbar - Vigenère, Beaufort, Variant or Porta.
Vigenère Table
Beaufort Table
Variant Beaufort Table
Porta Table