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.