Basic Cryptography – The Vigenere Cipher

This doesn't really explain Vigenere well, since what's left out is where the key comes from:

* If key is the same length as plaintext, and truly random, that's a One-Time Pad.

* If the message is appended to the seed key, that's an Autokey Cipher.

* If the key is repeated (what the PHP does but not made clear), that's classical Vigenere.

It also doesn't describe breaking them:

* OTPs are unbreakable.

* You can break Autokey by using crib words (e.g. "the") at different locations of the ciphertext, selecting the most phonetically plausible and working backwards to recover the earlier plaintext.

* Vigenere can be attacked by trying the same shift for every Nth letter, or using cribs.

Quote: "Our decryption function is basically the same, but in reverse" and follows with a full decrypt function.

A very simple optimization is to use encrypt function to decrypt the encrypted message by providing the "inverse key" as parameter, which is a 2 line function altogether. Implementation of said implementation is left as an exercise for the reader.

For a course several years ago, I wrote a compact/efficient implementation of a Vigenere cipher:

    private static char encipherCharacter(char message_char, char key_char){
      return (char)(((int)(message_char) + (int)(key_char) - 194)%26 + 97);
    }
  
    public static String encipher(String plaintext, String key){
      String ciphertext = "";
      
      for(int i = 0; i < plaintext.length(); i++)
        ciphertext += encipherCharacter(plaintext.charAt(i), key.charAt((i % key.length())));
        
      return ciphertext;
    }

I'd love to see other implementations, if anyone has some to share.

https://gist.github.com/ctrezevant/8ca9e163d702af245ded35d97...

This reminds of the times when I was at school and was solving the weekly online challenges for National Cipher Challenge (UK). My program which cracked Vigenere cipher was one of my first projects when I was learning programming. My solution depended on the fact that I knew a few words which will occur in the plaintext, and I would just try these words in all possible positions in the text and get the possible keys.

Why on Earth would you pass plaintext punctuation characters through to the ciphertext?

It's not a very nice implementation. It creates a long key as long as the message by appending the key to itself as many times as necessary.

Could have just used a mod operator to cycle around the key.

Please help solve Kryptos: https://en.m.wikipedia.org/wiki/Kryptos