Scenario 1: Have you ever played a game on your phone or video game console where you collect different items like gold, health packs, or weapons? Imagine these items are like special coins called Bitcoin. Just like in the game, you can send these coins to your friends if you know their usernames. But you don't have to worry about anyone stealing your coins as long as you keep your secret password safe. Now, let's say you want to see how many coins your friend has to see if you have...