This is the fifth and final (for now) mission inspired by classical cryptography. In this mission we'll try to break the Playfair cipher using known plaintext attack. To learn more about this cipher, check out Playfair Cipher mission.

Playfair Cipher is a manual substitution cipher invented in 1854. Unlike other substitution ciphers that existed at the time, Playfair doesn't encrypt single letters - instead it works with pairs of letters, or bigrams . Let's remind ourselves of the encryption algorithm:

Playfair cipher used a 5x5 key table filled with 25 letters of English alphabet in random order (letter J is omitted). Message is broken up into pairs of letters (each pair must consist of two different letters, otherwise letter X is inserted between them). Then each bigram is encrypted according to following rules:

1. If both letters are in the same row of key table, each is replaced by the letter to it's immediate right (the last column wraps around to the first);

2. If the letters appear in the same column - each is replaced with the one below it (the bottom row wraps around to the top);

3. If the letters are at diagonally opposite corners of a rectangle, they are replaced with letters at other two corners. The order...