Vigenère Cipher

Yaser Rahmati | یاسر رحمتی

The Vigenère cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. It's more secure than the simple Caesar cipher, where each letter in the plaintext is shifted by a fixed amount. The Vigenère cipher uses a keyword to determine the shift for each letter.

1. Understanding the Vigenère Cipher

  • Plaintext: This is the original message that you want to encrypt.

  • Ciphertext: This is the encrypted message.

  • Key: The key is a word or phrase that determines how the letters in the plaintext are shifted. The key is repeated or truncated to match the length of the plaintext.

2. How the Cipher Works

  • Each letter in the plaintext is shifted by a number of positions defined by the corresponding letter in the key.

  • The alphabet is usually considered circular, meaning after 'Z', it wraps around to 'A'.

  • For example, if the letter in the key is 'B', the plaintext letter is shifted by 1 position (B is the 2nd letter of the alphabet, so the shift is 1).

3. Encryption Process

  • Step 1: Write down the plaintext.

  • Step 2: Write the key repeatedly below the plaintext, aligning it with the plaintext letters.

  • Step 3: For each letter in the plaintext, find the corresponding letter in the key. Determine the shift based on the key letter.

  • Step 4: Shift the plaintext letter by the key's shift value to find the ciphertext letter.

  • Step 5: Write down the ciphertext letter.

Example

Let's encrypt the plaintext "HELLO" using the key "KEY".

  • Plaintext: HELLO

  • Key: KEYKE (repeated to match the length of the plaintext)

Now, align the plaintext with the key:

PlaintextHELLO

Key

K

E

Y

K

E

Next, shift each letter in the plaintext according to the corresponding key letter:

  1. H with K (K is the 11th letter, so shift H by 10 positions): H -> R

  2. E with E (E is the 5th letter, so shift E by 4 positions): E -> I

  3. L with Y (Y is the 25th letter, so shift L by 24 positions): L -> J

  4. L with K (K is the 11th letter, so shift L by 10 positions): L -> V

  5. O with E (E is the 5th letter, so shift O by 4 positions): O -> S

  • Ciphertext: RIJVS

4. Decryption Process

  • Decrypting is essentially the reverse of the encryption process.

  • Instead of shifting forward, you shift backward by the key's shift value.

  • Using the ciphertext "RIJVS" and the key "KEYKE" to decrypt:

  1. R with K (K is the 11th letter, so shift R back by 10 positions): R -> H

  2. I with E (E is the 5th letter, so shift I back by 4 positions): I -> E

  3. J with Y (Y is the 25th letter, so shift J back by 24 positions): J -> L

  4. V with K (K is the 11th letter, so shift V back by 10 positions): V -> L

  5. S with E (E is the 5th letter, so shift S back by 4 positions): S -> O

  • Decrypted Text: HELLO

5. Security Considerations

  • The Vigenère cipher is more secure than a Caesar cipher because the key introduces variability in the shifts.

  • However, it's still vulnerable to certain attacks, especially if the key is short or the attacker knows some parts of the plaintext.

6. Frequency Analysis Attack

  • One way to break the Vigenère cipher is through frequency analysis.

  • If the key is short, the cipher behaves like several Caesar ciphers, which can be cracked by analyzing the frequency of letters in each segment of the ciphertext.

7. Key Length and Security

  • A longer key makes the cipher more secure.

  • The key should be as long as the plaintext for maximum security (in this case, it would be a one-time pad, which is theoretically unbreakable).

Summary

The Vigenère cipher is a classical encryption technique that improves upon the Caesar cipher by using a keyword to vary the shifts for each letter in the plaintext. Although more secure, it is not invulnerable and can be cracked with enough effort, particularly if the key is short or the plaintext is predictable.

Keywords

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