The Bifid Cipher
-------------------

Introduction:
----------------

The Bifid cipher, developed by Felix Delastelle in 1901, is a
combination of the Polybius square (please see Polybius Cipher article)
with transposition.

How it works:

Firstly, you must be familiar with the Polybius Cipher. I have written
an article on this specific cipher previously. In order to create a
Bifid Cipher, one must first draw up a random Polybius square such as
the one below:

   | 1 | 2 | 3 | 4 |  5  |
-------------------------
1 | S | E | B | A |  G |
-------------------------
2 | W | H | K | C | M |
-------------------------
3 | N | O |  F |  P | L |
-------------------------
4 |X/Z| Y | Q | D | V |
-------------------------
5 |   I  | R | T |  J  | U |
-------------------------

Now that we have the square, we are going to jot down the numbers that
correspond to each letter in the message right below. Lets say for instance that our message to be encrypted is: "Stroke the monkey head". That would be written like so:

s  t  r  o  k  e  t  h  e  m  o  n  k  e  y  h  e  a  d
1 3  2 2  3  2  3  2 2   5   2 1  3  2  2  2  2  4  4
1 5  5 3  2  1  5  2 1   2   3 3  2  1  4  2  1  1  4

Now, take the second row of numbers, add it to the first at the very end
like so:

1 3 2 2 3 2 3 2 2 5 2 1 3 2 2 2 2 4 4 1 5 5 3 2 1 5 2 1 2 3 3 2 1 4 2 1 1 4

Re-organize the numbers to be in sets of two (you will always have sets
of two because you just doubling the first row. If not, you have
forgotten a number). The new string of numbers will appear like so:

13 22 32 32 25 21 32 22 24 41 55 32 15 21 23 32 14 21 14

Now, take each new number and find the corresponding letter on the
original Polybius Square that you used, and write down the new
incomprehensible string. The cipher text should look like this:

N H K K R E K H Y A U K I E K X/Z E X/Z


Deciphering the cipher:
-----------------------------

Deciphering this cipher is as simple as doing all the steps in reverse.
Firstly, and most obviously, it is a requirement that your have the
exact same Polybius Square that was used to create the cipher. So, in
order to decipher it, you write down the letters for each number, split
the number string in half and place the second half below the first, and
then find the corresponding letter by working vertically down the
columns of numbers you have created.

I hope this has been interesting and insightful.


Peace,

~o invas10n o~
[Ask The Experienced, Not The Learned]