Tool to decrypt/encrypt using Gronsfeld. Gronsfeld Cipher is a variant of the Vigenère cipher, the difference being that the key is a number.

Gronsfeld Cipher - dCode

Tag(s) : Poly-Alphabetic Cipher

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Tool to decrypt/encrypt using Gronsfeld. Gronsfeld Cipher is a variant of the Vigenère cipher, the difference being that the key is a number.

**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.

Plain letter | G | R | O | N | S | F | E | L | D |

Key (repeated) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | 1 |

Cipher Letter | H | T | R | R | T | H | H | P | E |

Example: The encrypted message is HTRRTHHPE.

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

Example: The encrypted message EEREG with the key 123

Ciphertext Letter | E | E | R | E | G |

Key (Repeated) | 1 | 2 | 3 | 1 | 2 |

Decrypted Letter | D | C | O | D | E |

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.

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Source : https://www.dcode.fr/gronsfeld-cipher

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