+The choice is a design decision, taking into account space needed, time to create and use, expected use patterns, etc.
+
+## Thoughts?
+
+---
+
+# How much to store?
+
+Keeping the decipherment close to encipherment seems aesthetically better to me.
+
+Giving the ability to do division is the most obvious (to me).
+
+As there are only a few possible modular bases, might as well calculate the whole table at startup.
+
+## Now implement affine decipherment.
+
+Check both enciphering and deciphering work. Round-trip some text.
+---
+
+# Counting from 0 or 1
+
+When converting letters to numbers, we're using the range 0-25.
+
+Another convention is to use numbers in range 1-26.
+
+Implement this.
+
+* You'll need another parameter: