Re: Book on Pre-MATH for cryptography and cryptanalysis. Reply

David Kahn's "The Codebreakers" (unabridged hardcover edition, not the
dumbed-down paperback edition) does a good job of presenting some
mathematical methods of analysis that can be understood at a high-
school level, for example Kasiski's method and the index of coincidence.

That reminds me that in addition to abstract algebra, a good foundation
in probability and statistics is helpful. A standard Community-College
level statistics text would suffice for that.