What makes the Autokey cipher different from Vigenère?

Both add a key letter to each plaintext letter. But Vigenère repeats a short keyword, which creates exploitable patterns. The Autokey cipher appends the plaintext to the keyword, so the key is as long as the message and never repeats.

How is the key stream built?

The key stream begins with your keyword (the primer). After that, each plaintext letter is appended to the key as it is processed. When decrypting, each recovered plaintext letter is appended instead, which keeps the streams identical.

Why must I decrypt progressively?

Because the key after the primer is the plaintext itself, decryption recovers one letter at a time and feeds it back into the key to decode the next letter. This tool handles that chaining automatically.

What happens to spaces and punctuation?

Non-letter characters pass through unchanged and do not advance the key stream, so the keyword stays aligned with the letters of the message. Case is preserved.

Is the Autokey cipher secure?

It is stronger than plain Vigenère because the key does not repeat, but it is still a classical cipher that modern cryptanalysis can break. Use it for puzzles, education and historical interest rather than real secrets.

Autokey Cipher Encoder & Decoder

The Autokey cipher is a polyalphabetic substitution cipher devised by Blaise de Vigenère, and it is a notable improvement on the better-known Vigenère cipher that bears his name. Its defining trick is that the encryption key extends itself with the plaintext, so the running key is exactly as long as the message and never falls into a repeating pattern.

How it works

Every letter becomes a number from 0 (A) to 25 (Z). The cipher builds a key stream: it starts with your keyword (called the primer) and then, as each plaintext letter is enciphered, appends that plaintext letter to the stream. Encryption uses the Vigenère addition rule:

C = (P + K) mod 26

where K is the current key-stream value. Because the primer is short but the plaintext continues the key, there is no short repeat for an attacker to exploit.

Decryption must run progressively. The first letters are decrypted with the keyword primer using P = (C - K) mod 26. Each recovered plaintext letter is then appended to the key stream and used to decrypt the following letter. This chaining is automatic in the tool, so you only supply the original keyword.

Only letters are enciphered and advance the key; spaces, digits and punctuation pass through untouched and keep the alignment intact.

Example

With keyword KEY and plaintext HELLO, the key stream is KEYHE — the primer KEY followed by the first two plaintext letters HE. Adding letter by letter:

P:  H  E  L  L  O
K:  K  E  Y  H  E
C:  R  I  J  S  S

Decrypting RIJSS with keyword KEY recovers H, appends it to the key, recovers E, and so on, rebuilding HELLO.

Notes

The Autokey cipher resists Kasiski examination because the key has no period, but error propagation means a single wrong letter garbles the rest of the message.
Pick a primer that is not a common word to avoid easy guessing of the opening letters.
For a reciprocal classical cipher where encryption equals decryption, see the Beaufort cipher.