# Gronsfeld cipher decoder and encoder

The Gronsfeld cipher, also called Bronckhorst cipher, was invented by José de Bronckhorst (earl of Gronsfeld) in 1744. He was a Belgian diplomate who invented it to protect his communications. The Gronsfeld cipher is a polyalphabetic cipher, a series of Caesar ciphers, where the shift is determined by numbers (between 0 and 9). It is similar to the Vigenère cipher, but the key uses digits instead of letters.

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## Decode Gronsfeld cipher

Note that you can type in either the ciphertext or plaintext area.

## Decryption/encryption settings

## Features

- The Gronsfeld cipher is a polyalphabetic substitution cipher, very similar to the Vigenère cipher.
- It was invented around 1744 by the Earl of Gronsfeld, José de Bronckhorst.
- A Gronsfeld cipher works like a series of Caesar ciphers. The secret key determines how many places each letter should be shifted. For example, if the secret key is 1234, the shift will be 1, 2, 3, 4, 1, 2, 3, 4, etc.
- The Gronsfeld cipher is vulnerable to letter frequency analysis, just like the Vigenère cipher. It has more limited key strength than Vigenère cipher, because the shift only be between 0-9, so unless the key is long it can be cracked by brute force methods.

Gronsfeld ciphers, and variants of it, are frequently used in CTFs, geocaching mystery caches, and logic puzzles.

## Sample text

MRSFGHBSRQCLKRRTW

The ciphertext above represents "JOSE DE BRONCKHORST" encrypted using the key 3301.