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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Gronsfeld Cipher
Gronsfeld Decoder
Gronsfeld Encoder
Answers to Questions (FAQ)
How to encrypt using 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.
| 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.
How to decrypt Gronsfeld cipher?
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
| 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.
How to recognize Gronsfeld ciphertext?
The message has an index of coincidence of about 0.04 to 0.05 (similar to Vigenere).
How to decipher Gronsfeld without the key?
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).
What are the variants of the Gronsfeld cipher?
Gronsfeld variants are the same as Vigenere variants.
Source code
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Gronsfeld Cipher on dCode.fr [online website], retrieved on 2024-04-18,
