How to calculate a multiple-character checksum for authentication using a tabula recta? I am not sure if this is possible to do using a tabula recta, but I would like to know how to calculate a multiple-character checksum of a text.  For a single-character checksum, it's easy.  You just begin on the table at the first letter of text, go down to the second, left/right to the third, up/down to the fourth, left/right to the fifth, etc., then make a 90-degree turn and keep going until you hit the last letter of text.

How would one calculate a double-character checksum?  In other words, the sum of the text with a modulus of 676 in base 26 [A-Z].
I just realized that you could use a 676x676 table with 2-character entries.  But hopefully there is an easier way.