Transposition Cipher, Python

Kids who are motivated to learn a programming language will love Al Sweigart's books.  He's written four Python programming books, including a beginner game programming book and an intermediate game programming book.  They are freely downloadable, or you can buy printed copies.

I'm going through his Hacking Secret Ciphers with Python, which is a fun introduction to using Python to encrypt and decrypt messages.

This transposition code was pretty cool.  I want to teach this!  I also want to turn it into a Scratch program!