How to Solve the Gronsfeld Cipher

A polyalphabetic cipher is a shift cipher whose shift keeps moving. A key word steps along under the message, and each letter is shifted by whichever key letter happens to be beneath it.

How to recognise it

Letter frequency looks flat. No letter dominates the way E dominates English, and the same ciphertext letter stands for different plaintext letters in different places.

Gronsfeld Vigenère with a number for a key. Shift each letter by the next digit in turn.

A worked example

This is a shift cipher whose shift keeps changing. That is why letter frequency looks flat and nothing obvious jumps out. This one is Gronsfeld. The key: key 31415. Vigenère with a number for a key. Shift each letter by the next digit in turn. Line the key 31415 up under the message and undo each letter's own shift. The opening letters come back as T R E. The answer was TREASURE MAP.

Working one out from scratch

You need the key. With it, line the key up under the message, repeating it as often as it takes, and undo each letter's own shift. Without it, look for a repeated block of ciphertext — the gap between repeats is usually a multiple of the key length.