In Dead Heat, Book 2 of Outlaw Ridge, Texas, which I'm working on now, I knew I wanted to have Sabrina Snow, the heroine of the book, leave a coded message for John Galloway.
The message is to be something he'll understand by knowledge of their relationship, but also it's to have information encoded that a casual reader would not think twice about, but which John will realize is a code.
So I wrote the secret letter then went about encoding it.
To that end, I watched Codebreakers: Science of Secrecy, a fascinating documentary series on Amazon Prime that's based on The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography by Simon Singh.
I'll confess. I was so taken with the documentary series that I bought Mr. Singh's book too.
I ended up using a combination of code and ciphers--there is a difference between the two. I used a simple Kama-sutra cipher and referenced it in the letter so John would know there was more to the letter than the words on the paper.
I did this by including the number 45 in the letter. In the context of the sentence that contains 45, John will know that the sentence is meaningless.
He'll also know that Number 45 is the Art of Secret Writing which the Kama-sutra recommends women should learn. (And you just thought the Kama-sutra was about sex positions!)
Why will John know this? Because he and Sabrina are spies!
Oh, writing this book is so much fun!
Being a writer is a never-ending education--if you're lucky.