Time for a little mathemagic! Choose any five cards from a pack and show four of them to your partner. How can they work out the fifth?
Find the frequency distribution for ordinary English, and use it to help you crack the code.
How can Agent X transmit data on a faulty line and be sure that her message will get through?
456 has 14 factors.
The two methods used to encrypt the message are commutable, so it doesn't matter which method you decipher first.