http://easygpg2.i2p/egpgtechnical.html
When he decrypts the message with EasyGPG, he’s prompted for the shared secret. He tries the secret he and Alice shared. The message is authenticated, so Bob believes the message came from Alice, but, as noted above, he can’t prove it to anyone else because he can’t prove that the address is Alice’s or that the secret that authenticated it was agreed to by Alice.