The strongest technology that would assure Alice that Bob is the sender of a message involves public key cryptography. If Alice knows Bob's public key, she can encrypt a message to him, and he will decrypt it with his private key, verifying that the message came from her. This can be accomplished using a central keyserver that maintains an association between a participant's name and their public key. But this approach is insecure, because a malicious participant (let's call her Mallory) can tamper with the central keyserver to get her own public key instead of Bob's, and then encrypt a message to Alice under that name.
A more secure approach involves a centralized directory service that attests to participants' identities. One such entity is called a Certificate Authority.
