The Polybius Square is essentially identical to the simple substitution cipher, except that each plaintext character is enciphered as 2 ciphertext characters. It can ususally be detected if there are only 5 or 6 different characters in the ciphertext.
This algorithm offers very little communication security, and can be easily broken even by hand, especially as the messages become longer (more than several hundred ciphertext characters).
Example
Here is a quick example of the encryption and decryption steps involved with the Polybius Square. The text we will encrypt is "defend the east wall of the castle".
Keys for the Polybius Square usually consist of a 25 letter 'key square'. e.g. (the letters along the top and side can be chosen arbitrarily):
A B C D E
A| p h q g m
B| e a y l n
C| o f d x k
D| r c v s z
E| w b u t i
plaintext: d e f e n d t h e e a s t w a l l o f t h e c a s t l e
ciphertext: CCBACBBABECC EDABBA BABBDDED EABBBDBD CACB EDABBA DBBBDDEDBDBA
