ExampleAlice and Bob wants to make a deal. Bob wants to make sure Alice is truly Alice and not any other person. Alice send Bob her wallet public key.
Bob went to Electrum and clicked on tools and clicked on encrypt/decrypt message:
Bob write his message, input Alice public key and clicked on encrypt to see the encryption like this example:
Bob will send Alice the encrypted that you see in the image above. Alice will go to her wallet, click on tools and encrypt/decrypt message. She will input her public key and the encrypted and click on decrypt to see the message just like you can see in the image below:
If Alice is able to decrypt the message, that means Bob wants make deal with the real Alice.
Other methods that can be used that is common on this forum is signing message with the bitcoin address that Alice has stated on this forum before. Another one is the use of PGP encrypted message.