Classic Cryptography: Polybius Chequerboard

Polybius was the name of the Greek who invented a system of converting alphabetic characters into numeric characters. It was devised to enable messages to be easily signalled using torches.

#	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

Doesn’t that work nicely? Try the following message:
Ciphertext: 31 44 43 22 24 11 53 32 45 – 42 34 – 12 54 33

Each letter may be represented by two numbers by looking up the row the letter is in and the column. For instance h=23 and r=42.

The idea was that a message may be transmitted by holding different combinations of torches in each hand. The chequerboard has other important characteristics, namely the reduction in the number of different characters, the conversion to numbers and the reduction of a symbol into two parts which are separately manipulable. As such chequerboards form the basis for many more ciphers.

Varients of this idea are used to convert a single character into units which may be manipulated seperately, this can lead to some very strong ciphers, for instance the adfgvx cipher.