Polybius Checkerboard Cypher

The Polybius Checkerboard (otherwise known as the Greek square) was named for the ancient Greek historian to whom its invention is due. The original set up is quite simple, but variations on it can be quite hard to break. One variation was used by a spy in the American Civil War, and it was never broken by the enemy.

Below is the basic version.

	1	2	3	4	5
1	a	b	c	d	e
2	f	g	h	ij	k
3	l	m	n	o	p
4	q	r	s	t	u
5	v	w	x	y	z

Yes, I did write out all of the html for that table, feel free to use it if you wish.

To use this cypher one substitutes the row number and the column number, for the letter. For example:

"A Study in Purple" becomes

11 43 44 45 14 54 24 33 35 45 42 35 31 15 or

1143444514542433354542353115 or

11434 44514 54243 33545 42353 11500 or even

1 1434 445 14 5424 333 5454 23531 15

So obviously one of the variations of the Polybius Checkerboard is spacing. Other variations include: scrambling the letters and/or numbers of the square and using different alphabets (this is called a "Greek Square" because it was originally used the Greek alphabet). I thought it would be neat for short messages to also be encoded mathematically, for example:

Take the square root of 1,143,444,514,542,433,354,542,353,115 then divide it by 999,999,999, then subtract 330,000 from it, and the number becomes 8,148.564434 (much more portable in my opinion). It would be impossible to crack without knowing what steps to take to reverse the function. One could even take 8,148.564434, divide it by as many people as one wants to give the code to, and have them only be able to come up with the right message when all of the individuals add their values. The variation possibilities are endless! Here is an exercise in breaking a Polybius Checkerboard, use the square above, I only used a spacing variation.

...213442-442315-243454-3421-54235123-2443-54344542-4344421533224423.