Vigenère Cipher
Definition:
- The Vigenère cipher is a method for encrypting alphabetic text.
- It's a polyalphabetic substitution cipher, meaning it uses multiple Caesar ciphers with varying shifts.
- Encryption relies on a keyword, which determines the shift amount for each letter of the plaintext.
Example:
- Plaintext: ATTACK AT DAWN
- Key: SECRET
Encryption process:
- Match each letter of the plaintext with the corresponding letter of the key (repeated if necessary).
- ATTACK AT DAWN
- SECRETSECRETS
- Use the Vigenère square to find the cipher text letter. The row index is the plaintext letter, and the column index is determined by the key letter.
- Ciphertext: LXTFNL LPNFNLP
Applications (Historical):
- The Vigenère cipher was once considered a very secure method due to its complex encryption.
- It was used for military communication in various conflicts, including the American Civil War.
Limitations:
- The Vigenère cipher is vulnerable to Kasiski Examination, which can reveal the key length.
- With the key length known, cryptanalysis techniques can be applied to break the cipher.
Related Algorithms:
- Caesar Cipher: A basic substitution cipher with a single shift value for the entire message.
- Autokey Cipher: A variant of the Vigenère cipher where the key is derived from the plaintext itself.
Note: In modern cryptography, the Vigenère cipher is not considered secure due to its vulnerabilities. More robust encryption algorithms are used for secure communication.
Python code:
def vigenere_cipher(text, key, mode='encrypt'):
result = []
key = [ord(k) - 65
for k in key.upper()]
key_length =
len(key)
for i, char in
enumerate(text):
if
char.isalpha():
shift =
key[i % key_length]
shift_base
= 65 if char.isupper() else 97
shift =
shift if mode == 'encrypt' else -shift
result.append(chr((ord(char) - shift_base + shift) % 26 + shift_base))
else:
result.append(char)
return
''.join(result)
# Usage
message = "HELLO, WORLD!"
key = "KEY"
encrypted_message = vigenere_cipher(message, key, 'encrypt')
decrypted_message = vigenere_cipher(encrypted_message, key,
'decrypt')
print(f"Encrypted: {encrypted_message}")
print(f"Decrypted: {decrypted_message}")
Also see:
Ceaser Cipher
