Autokey Cipher Simulator

Explore the Autokey Cipher and experiment with different key sequences to encrypt and decrypt messages. Visualize classical cryptography principles interactively with our simulator.

Autokey Cipher

Cryptography surrounds our digital lives, protecting the secrets we share every day. Have you ever wondered how a message can be made unreadable to anyone but the intended recipient? The Autokey Cipher, a classical encryption technique, uses a dynamic key derived from the message itself to encode text. With our interactive simulator, you can explore this cipher directly. Enter a message, experiment with keys, observe how plaintext transforms into ciphertext, and uncover the principles behind secure communication. Start your journey into the fascinating world of classical cryptography — experiment with the Autokey Cipher today!

Mathematical description

The Autokey Cipher is a classical substitution cipher that uses a key composed of a keyword followed by the plaintext itself. Encryption and decryption are performed using modular arithmetic.

Encryption formula:
$ C_i = (P_i + K_i) \mod 26 $
Where:

$C_i$ = Ciphertext letter at position $i$
$P_i$ = Plaintext letter at position $i$
$K_i$ = Key letter at position $i$ (keyword followed by plaintext letters)

Decryption formula:
$ P_i = (C_i – K_i) \mod 26 $
Where the key for the first letters is the keyword, and the remaining key letters are taken from the decrypted plaintext.

Example:

Plaintext: HELLO
Keyword: KEY
Key sequence: KEYHE

Encryption steps:
H + K → R
E + E → I
L + Y → J
L + H → S
O + E → S

Ciphertext: RIJSS

Decryption steps:
Using the keyword KEY and the ciphertext RIJSS:
R – K → H
I – E → E
J – Y → L
S – H → L
S – E → O

Recovered Plaintext: HELLO

Simulator

Dive into the art of classical cryptography with our interactive Autokey Cipher simulator!

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FAQs on Autokey Cipher

Qus 1. What is the Autokey Cipher?

The Autokey Cipher is a classical substitution cipher in which each letter of the plaintext is shifted according to a key that is derived from the plaintext itself. This makes it more secure than simple ciphers like the Caesar cipher.

Qus 2. How does the Autokey Cipher work (principle)?

The principle of the Autokey Cipher is to use a key that starts with a predefined keyword followed by the plaintext letters themselves. Each plaintext letter is encrypted by shifting it according to the corresponding key letter using modular arithmetic:
$ C_i = (P_i + K_i) \mod 26 $
Where $C_i$ is the ciphertext letter, $P_i$ is the plaintext letter, and $K_i$ is the key letter.

Qus 3. How is decryption performed in the Autokey Cipher?

Decryption reverses the process. The first part of the key is known (the keyword), and subsequent key letters are taken from the decrypted plaintext:
$ P_i = (C_i – K_i) \mod 26 $

Qus 4. What are the common uses of the Autokey Cipher?

Historically, the Autokey Cipher was used for secret military and diplomatic communications because it improves security over simple substitution ciphers. Today, it is mainly used for educational purposes and in classical cryptography exercises.

Qus 5. Who invented the Autokey Cipher and when?

The Autokey Cipher was invented by Blaise de Vigenère in the 16th century as an improvement over the Caesar Cipher. It became widely studied as a classical encryption method.

Qus 6. What are the advantages and disadvantages of the Autokey Cipher?

Advantages: More secure than simple ciphers, resists frequency analysis better.
Disadvantages: Still vulnerable to certain cryptanalytic attacks, requires secure sharing of the keyword.