AutokeyCipher

AutokeyCipher#

The AutokeyCipher class implements the Autokey cipher, a polyalphabetic substitution cipher that uses a keyword and the plaintext itself for encryption, making it a variation of the Vigenère cipher.

class AutokeyCipher(key: str)#

Creates a new AutokeyCipher instance with the specified keyword.

Parameters:

key (str) – The keyword used for encryption. The key is followed by the plaintext itself for encryption.

Attention

AutokeyCipher is a basic symmetric cipher, which should be used ONLY for educational purposes, and NOT in production. Proceed accordingly.

Introduction#

The Autokey cipher is a form of polyalphabetic substitution cipher, which means that it encrypts text by using multiple substitutions based on a keyword. It is similar to the Vigenère cipher but with an important difference: the key used for encryption is not a fixed word or phrase. Instead, the key is generated by appending the plaintext itself to the end of the keyword. This makes the cipher less vulnerable to frequency analysis, but it also introduces some weaknesses if the key is too short or predictable.

Mathematical Details#

In the Autokey cipher:

  • The plaintext is encrypted by shifting each letter based on the corresponding letter of the key.

  • The key is formed by appending the plaintext to the keyword, starting from the first letter of the plaintext.

  • The encryption formula is:
    \[C_i = (P_i + K_i) \mod 26\]
    where:

    • \(C_i\) is the ciphertext letter,

    • \(P_i\) is the plaintext letter,

    • \(K_i\) is the key letter (the letters from the keyword followed by the plaintext itself).

  • The decryption formula is:
    \[P_i = (C_i - K_i) \mod 26\]

    where \(K_i\) is the key letter used for decryption.

Usage#

# Example usage of Playfair Cipher to encrypt and decrypt a message
from cryptosystems import AutoKeyCipher
cipher = AutoKeyCipher("key")
ciphertext = cipher.encrypt("Hello World")
print(ciphertext) # 'Rijss Hzfhr'
plaintext = cipher.decrypt(ciphertext)
print(plaintext) # 'Hello World'

Methods#

encrypt(plaintext: str) → str#

Encrypts the given plaintext using the Autokey cipher.

Parameters:

plaintext (str) – The plaintext message to be encrypted.

Returns:

The encrypted ciphertext.

Return type:

str

decrypt(ciphertext: str) → str#

Decrypts the given ciphertext using the Autokey cipher.

Parameters:

ciphertext (str) – The ciphertext message to be decrypted.

Returns:

The decrypted plaintext.

Return type:

str

Notes#

  • Key: The key is formed by combining the given keyword with the plaintext itself. If the plaintext is longer than the keyword, the keyword is repeated.

  • Security: The Autokey cipher is less vulnerable to frequency analysis than the Vigenère cipher because the key changes with each letter of the plaintext. However, if the key is known or predictable, it is still susceptible to cryptanalysis.

  • Application: The Autokey cipher is useful for educational purposes and cryptographic exercises but is not considered secure for modern applications, especially with short or predictable keys.