What is the Checkerboard Cipher?
The checkerboard cipher uses a keyed Polybius square. In addition, two five-letter keywords, with no repeated letters between the two, are chosen to act as coordinates for the Polybius square. One word is written along the left of the square, and the other is written along the top. The coordinates for each letter of the plaintext are found, and these coordinates are written out as the ciphertext. For a more secure version, use two keywords for the left and two for the top of the square, all of which have no repeated letters between them.