http://ssd.iykpqm7jiradoeezzkhj7c4b33g4hbgfwelht2evxxeicbpjy44c7ead.onion/ps/module/deep-dive-end-end-encryption-how-do-public-key-encryption-systems-work
Signing is also one of several methods that can make messages tamper-proof. If someone tried to change your message from “I promise to pay Aazul $100” to “I promise to pay Ming $100,” they would not be able to re-sign it using your private key. A signed message guarantees it originated from the correct source and wasn't messed with in transit.