Polybius Square Cipher
Key
Polybius Square Cipher 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. [Wikipedia]
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).
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