How to Solve the Autokey 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.

Free tool Have one to decode right now? Use the Autokey Cipher tool →

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.

Autokey The key starts as GOLD, then the message itself takes over as the key. Decode the first letters and the rest unravels.

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 Autokey. The key: primer GOLD. The key starts as GOLD, then the message itself takes over as the key. Decode the first letters and the rest unravels. Line the key GOLD 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.

Autokey days we’ve run

These have closed, so each one carries the full solution. Play them for practice — archive days never touch a streak.

• Cipher #28 · 27 Sep 2026

  A review day — the cipher's name was withheld, so step one was working out which of the week's techniques was in play. This is a shift cipher whose shift keeps changing. That is why letter frequency looks flat and nothing obvious jumps out. This one is Autokey. The key: primer ANVIL. The key starts as ANVIL, then the message itself takes over as the key. Decode the first letters and the rest unravels. Line the key ANVIL up under the message and undo each letter's own shift. The opening letters come back as N O B. The answer was NOBODY ELSE KNOWS.

• Cipher #26 · 25 Sep 2026

  This is a shift cipher whose shift keeps changing. That is why letter frequency looks flat and nothing obvious jumps out. This one is Autokey. The key: primer ANVIL. The key starts as ANVIL, then the message itself takes over as the key. Decode the first letters and the rest unravels. Line the key ANVIL up under the message and undo each letter's own shift. The opening letters come back as C O D. The answer was CODE DEADRECKON.