Diffie-Hellmann key exchange is vulnerable to "Man-in-the-Middle" attacks. Unless you performed the initial key exchange through some secure channel {say, InfraRed} then you can't be sure someone isn't pretending to you to be the person you called, and to the person you called to be you.
Also, without access to the source code, you can never, ever be sure that an encryption system is even half secure.