When we think about the money is it’s really a promise from a trusted bank. If the promise it says if someone brings this money to the bank, the bank will exchange it for something of real value. Today, we think of the paper itself as having real value. But in the early days, what the paper meant was you could go to the bank and exchange it for something that everyone would recognize as real value, say a bar of gold–almost as valuable as a gold star. If that’s what money means, well then we should be able to do the same thing with cryptography. Let’s see how that would work. Alice would go to Indivisible Prime Bank, which everyone knows is very trustworthy, and she would give to the bank a $100 in some other currency, let’s assume the bank to take US dollars. The bank will read the message that says, “The bank owes the bearer of this note $100.” The bank will send Alice a signed version of that message. The message, along with its signature, which is signed by the bank using the bank’s private key, signing a hash of that message. This signature proves that it is a valid IOU from this bank, and now Alice has something representing currency. Now Alice could spend the currency. She could buy some tacos from Bob’s Taco Emporium. Give him the currency. Pretty expensive tacos. And Bob could take the note–this signed message from the bank– and give it to the bank and ask the bank to deposit into his account. Now the question is how well would the scheme that I’ve described work? Let’s assume that everyone in the universe trusts Indivisible Primes and knows that bank’s public key.