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.

Gronsfeld Cipher Tool

You must enter the message.

You must enter the encryption key.

Auto Solve Options


You can decode (decrypt) or encode (encrypt) your message with your key. If you don't have any key, you can try to auto solve (break) your cipher.

  • Standard Mode v s Autokey Variant: The Autokey mode is a stronger variant of the cipher, where letters of the plaintext become part of the key. It eliminates the periodic repeats otherwise seen in polyalphabetic ciphers.
  • Language: The language determines the letters and statistics used for decoding, encoding and auto solving.
  • Min/Max Key Length: This is the search range for keys when auto solving a cipher.
  • Iterations: The more iterations, the more time will be spent when auto solving a cipher.
  • Max Results: This is the maximum number of results you will get from auto solving.
  • Spacing Mode: This is about the spaces (word breaks) in the text. In most cases it should be set to Automatic. In case a specific letter (for instance X) is used as word separator, set it to Substitute.

Note: Auto Solve will try in the mode you select (Standard Mode or Autokey mode). Standard mode is the most common, but if you don't know the mode, you should try both.


Not seeing the correct result? Try Auto Solve or use the Cipher Identifier Tool.

Auto Solve results

Score Key Text

Still not seeing the correct result? Then try experimenting with the Auto Solve settings or use the Cipher Identifier Tool.


  • 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 Gronsfeld Cipher

Code-breaking is not only fun, but also a very good exercise for your brain and cognitive skills. Why don’t you try breaking this example cipher:

tqkul os wu opbiwm woonp zkhz ib tra gfaglk chnt la lalkv kgnpku ane cxxl iodxdnk ib oq mgcrtj kgnpku dnew erb grn giygim gqk zhjz nptd xl fvarjmh fuu qgyl on yrhuzy