Playfair cipher

The Playfair cipher or Playfair square, is a manual symmetric encryption technique and was the first literal digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair who promoted the use of the cipher. The Playfair cipher uses a 5 by 5 table containing a keyword or phrase. Memorization of the keyword and 4 simple rules are all that’s required to create the 5 by 5 table and use the cipher. For this mission, we will do one better and use a 6 by 6 table.

For the key table, we should use ASCII letters in lowercase ("abcdefghijklmnopqrstuvwxyz") and digits ("0123456789"). They are have the following order:

```"abcdefghijklmnopqrstuvwxyz0123456789"

```

To generate the key table, the spaces in the table must be filled with the letters contained in the keyword (dropping any duplicate letters and digits), then the remaining spaces are filled with the rest of the letters and digits of the alphabet in order. The key is written in the top rows of the table, from left to right. The keyword together with the conventions for filling in the 6 by 6 table constitute the cipher key.

To encrypt a message, we will need to prepare a block of text. Upper case letters get transposed into lower case of letters, we’d break the message into digraphs (groups of 2 letters) and skip white spaces and punctuation symbols. The result would turn a message like "Hello World!" into "he ll ow or ld", and would get mapped out in the key table. The two letters of the digraph are considered to be the opposite corners of a rectangle in the key table. Note the relative position of the corners of this rectangle. Then apply the following 4 rules, in order, to each pair of letters in the plaintext:

• Prepare text: convert to lowercase, remove all non-useable symbols (white spaces, punctuation etc) and break the message into digraphs. If both letters are the same, add an "x" after the first letter (for double "x" use "z" as completion character) and shift following digraphs. If needed, append a "z" to complete the final digraph (or "x" if the last letter is "z"). For example "pp dr ..." will become "px pd r..." before encoding and "xx zz ..." will became "xz xz z...".
• If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively (wrapping around to the left side of the row if a letter in the original pair was on the right side of the row).
• If the letters appear on the same column of your table, replace them with the letters immediately below respectively (wrapping around to the top side of the column if a letter in the original pair was on the bottom side of the column).
• If the letters are not on the same row or column, replace them with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair. The order is important – the first letter of the encrypted pair is the one that lies on the same row as the first letter of the plaintext pair.

To decrypt, use the inverse (opposite) of the last 3 rules and you will get the processed (cut version).

For example, the keyword is "checkio101". Then the key table will be looked as

```c h e k i o
1 0 a b d f
g j l m n p
q r s t u v
w x y z 2 3
4 5 6 7 8 9

```

Let's the message is "Fizz Buzz is x89 XX." After using rule 1 (text preparation) we will get - "fi zx zb uz zi sx 89 xz xz".
- "fi" => "do";
- "zx" => "2y";
- "zb" => "7m";
- "uz" => "t2";
- "zi" => "2k";
- "sx" => "ry";
- "89" => "94";
- "xz" => "y2";
- "xz" => "y2".
And the encoded message is "do2y7mt22kry94y2y2".

You should write two functions - "encode" and "decode". Each function receives a message (ciphered or opened) and keyword. The "encode" function processes and encrypts a message. The "decode" function decrypts the encoded message (of course in the processed version).

Input: Two arguments. A message as a string (unicode) and a keyword as a string (unicode).

Output: The encoded or decoded message (related to function).

Example:

```encode("Fizz Buzz is x89 XX.", "checkio101") == "do2y7mt22kry94y2y2"
decode("do2y7mt22kry94y2y2", "checkio101") == "fizxzbuzzisx89xzxz"
```

How it is used: This mission gives you a little practice in working with data structures and shows you how to use positional ciphers. Also you can use this cipher for your spy correspondence should you have no computer access.

Precondition: 0 < len(key) ≤ 36

Sources: Wikipedia