The Greek Square Cipher or the Polybius Checkerboard

This week’s cypher is based on a system developed by Polybius, a Greek historian and cryptographer, about 2,000 years ago. 

43   23   11   42   15   /   34   45   42   /    43   24   32   24   31   11   42   24   44   24   15   43   /   13   15   31   15   12   42   11   44   15   /   34   45   42   /  

14   24   21   21   15   42   15   33   13   15   43

The key to this code is made by setting up a 6 x 6 square. Leaving the first square blank, number 1 – 5 down and across. Then fill in the remaining squares with the alphabet. Since the English alphabet has 26 letters, it will not fit. I and J occupy the same space, number 24. To encode something, determine first the row then the column number for each letter.  A = 11, Z = 55, S = 43.

To determine if a Greek Square is being used, notice that all letters have two numbers and no number is higher than 55.