Bruce Schneier is a security consultant and a leading expert on codes and ciphers.
In the late 1990’s, he created a powerful cipher code system using a deck of ordinary playing cards that he called the “solitaire encryption algorithm.” Although invented for a novel, Solitaire (called “Pontifex” in the book) is actually a very secure method of developing a cipher key.
By putting a deck of 52 cards (plus two Jokers) into any given starting order, a person could manipulate that order in a very specific manner to arrive at a unique “key.” That key could then be combined with a simple substitution alphabet (where A = 1, B = 2, C = 3, etc).
The result appears to a casual observer to be a random series of numbers. However, anyone that knows the original starting order can reproduce the key and use that key to reverse-engineer the message.
It’s complicated and somewhat time consuming to implement Solitaire, but when it’s done correctly, the cipher is considered very secure. Like most ciphers, Solitaire isn’t perfect, but most experts agree it would require sophisticated analysis to crack.
And you thought only magicians kept secrets with their playing cards!