First the encipherer constructs a Polybius square using a mixed alphabet. This is used to convert both the plaintext and a keyword to a series of two digit numbers. These numbers are then added together in the normal way to get the ciphertext, with the key numbers repeated as required.
Example
Consider the Polybius square created using the keyword ZEBRAS:
12345
1ZEBRA
2SCDFG
3HIKLM
4NOPQT
5UVWXY
with a plaintext of "DYNAMITE WINTER PALACE" and a key of RUSSIAN. This expands to:
PT: 23 55 41 15 35 32 45 12 53 32 41 45 12 14 43 15 34 15 22 12
KEY: 14 51 21 21 32 15 41 14 51 21 21 32 15 41 14 51 21 21 32 15
CT: 37 106 62 36 67 47 86 26 104 53 62 77 27 55 57 66 55 36 54 27
