Vigenere Cipher

Introduction

The Vigenere Cipher, invented by the French cryptographer Blaise de Vigenere in the 16th century, is a method of encrypting and decrypting messages. It is a polyalphabetic substitution cipher, which means that it uses multiple cipher alphabets to encrypt the plaintext.
The Vigenere Cipher operates by using a keyword, typically a word or phrase, as the basis for encryption. The keyword is repeated to match the length of the plaintext message. Each letter in the keyword is then used to determine the shift value for the corresponding letter in the plaintext.
To encrypt a message, each letter of the plaintext is shifted by the corresponding letter in the keyword. Decryption works in a similar way. Each letter of the cipher text is shifted back by the corresponding letter in the keyword.

Encryption:  C = (P + K)mod26

Decryption:  P = (C - K)mod26

where, P → Plaintext
            C → Ciphertext
            K → Shift/Key

Example: Using Vigenere Cipher encrypt the message “DITUNIVERSITY” with a Key “HELLO”.

1. Write down the plaintext message: DITUNIVERSITY
2. To get the new key repeat the given key, i.e. “HELLO” until it match the length of the plaintext message.
3. Now, each letter in the keyword is then used to determine the shift value for the corresponding letter in the plaintext.
   • D I T U N I V E R S I T Y
     H E L L O H E L L O H E L
4. Now, we will encrypt out plaintext.
   • Encryption:  C = (P + K)mod26
   • DITUNIVERSITY → KMEFBPZPCGPXJ
5. Thus, the encrypted message is “KMEFBPZPCGPXJ”

Security

The security of Vigenere cipher depends on the length of the key. If the key has only a length of one (the passphrase is only one letter) then the Vigenere cipher would be identical to just using the Caesar cipher. The Vigenere cipher, though historically significant, is considered weak by modern cryptographic standards due to its vulnerability to frequency analysis attacks, known plaintext attacks, and reliance on key length and randomness for security. Its susceptibility to revealing patterns in the plaintext and the lack of forward secrecy make it unsuitable for secure communication in contemporary contexts. For robust security, it's recommended to use modern encryption algorithms like AES with strong key management practices.

Advantages

1. It is relatively easy to implement compared to more complex modern encryption algorithms, making it accessible for educational purposes and historical reenactments.
2. The key length in the Vigenere cipher can vary, allowing for a degree of customization and flexibility in encryption.
3. The use of multiple alphabets in the encryption process makes it more resistant to simple frequency analysis compared to monoalphabetic ciphers.

Disadvantages

1. The Vigenere cipher is vulnerable to frequency analysis attacks, where patterns in the ciphertext reveal information about the key and plaintext.
2. If an attacker knows or can guess part of the plaintext and its corresponding ciphertext, they can deduce information about the key and decrypt the rest of the message.
3. Proper key management is crucial for the security of the Vigenere cipher. If the key is short, repetitive, or easily guessable, it significantly weakens the encryption.