Gronsfeld cipher is a polyalphabetic substitution cipher that uses digits as a key, which is used to do multiple alphabetic shifts on the plaintext to encrypt it to ciphertext.
For example, a gronsfeld key of 0123 is the same as a vigenere key of ABCD. The key is repeated to match the plaintext length.
It is also called, Bronckhorst cipher.
Gronsfeld cipher is very similar to vigenere cipher.
What is the difference between gronsfeld cipher and vigenere cipher:
Gronsfeld cipher uses a key number for encryption while vigenere cipher uses a keyword.
A brief history of gronsfeld cipher:
It was invented by a Belgian diplomate, jose de bronckhorst in 1744.
Gronsfeld cipher can easily be converted to a vigenere cipher by substitution of the key: 0 for a, 1 for b, and so on.
Gronsfeld Cipher
In this guide I will be discussing the following:
How to solve the Gronsfeld cipher.1) How to solve the Gronsfeld cipher
How to encrypt using Gronsfeld cipher:
Take the letters of the plaintext one by one, and apply a shift corresponding to the number in the key.
Here is an example:
Plaintext: GRONSFELD.
Key: 1234.
I start by shifting the G of 1 position in the alphabet, becoming H, then the R shifts of 2 positions and becomes T, and so on.
Plain letterGRONSFELDKey (repeated)123412341Cipher LetterHTRRTHHPE
Thus, ciphertext: HTRRTHHPE.
How to decrypt Gronsfeld cipher:
You need to know the decryption key and the alphabet used to successfully decode the message.
Here is an example:
Ciphertext: dqgfu
Key: 123.
Ciphertext LetterDQGFUKey (Repeated)12312Decrypted LetterCODES
Thus, plaintext: Codes.
How to recognize Gronsfeld ciphertext:
The message has an index of coincidence of about 0.04 and 0.05.
How to decipher Gronsfeld without key:
The same methods used to break a vigenere cipher can also be applied to Gronsfeld.
But it's easier to crack gronsfeld since the use of a numeric key limits the number of combinations.
