This is a very nice little book on ciphers and secret writings that one can work though to get some exposure to the ways that people might encrypt messages to prevent others from reading them. The book is designed to be read by "children" but I expect that many adults will find it an enjoyable read. In that it presents simple and entertaining methods to encode and decode messages. Information about this book can be found here.
The first section of the book deals with encryption and decryption techniques that are used to make directly reading of the encoded message impossible. The algorithms in this section would make enjoyable programming exercises for someone first learning a programming language. In fact my original intention was to code some of these methods up to be able to encode and decode much larger sections of text. If you code any of these techniques up and wouldn't mind sharing your code email me and I'll include links to it from this page (with acknowledgments of course). Later sections of the book present ways to make any type of printing "invisible" (using chemical compounds) and then present techniques that can be used to make the text visible again. Information in that section could be used for some fun after school experiments.
In addition to the detailed explanations of methods to encode and decode messages the book presents simple encoded messages for children to decode. Most of these puzzles are the encoded answers to riddles or jokes which makes attempting to solve them very enjoyable. Most of the puzzles are really just easy applications of the methods discussed in the text. As I read this book with my oldest daughter we wrote down our solutions as we solved the puzzles and put them in book (pdf) form.
After many years of giving this material away for free (in PDF format) I've decided to try to sell copies of it on Amazon. Please look here for the most recent version of the solution manual.As always, I am interested in hearing about any errors that might exist in this material.