Gronsfeld Cipher

Gronsfeld encryption uses the Vigenere method, the difference being that the key is directly numeric, no need to calculate the rank of the letters of the key in the alphabet. However, the corresponding shifts have to be applied, so the Gronsfeld method approaches a multi-shift encryption.

Example: The message to be encrypted GRONSFELD with the key 1234. Add 1 to G to get H (the letter 1 rank after G is H in the alphabet), then add 2 to C or E (the letter 2 ranks after C is E), and so on.

Example: The encrypted message is HTRRTHHPE.

Decryption requires to know the decryption key (and the alphabet used if necessary). The decryption is identical to Vigenere, but with a numeric key.

Example: The encrypted message EEREG with the key 123

The plain message is then DCODE.

The message has an index of coincidence of about 0.04 to 0.05 (similar to Vigenere).

Cryptanalysis techniques used for Vigenere are also applicable to Gronsfeld. These techniques also accelerated since the use of a numeric key limits the number of combinations (each character of the key has only 10 possibilities against 26 with Vigenere).

Gronsfeld variants are the same as Vigenere variants.