Let’s say that you send coins to someone, and that person wants to verify that it is actually you who have sent the coins. That person will probably not approve you just showing them the public address that you sent the coins from, and you should neither show your private keys to anybody else. Luckily, there is a function where you can have a custom signature that can verify your ownership but doesn’t reveal your private keys. Even more luckily, it happens to be a function that we now offer in the COINiD Wallet; sign message.