In Phase 3, the sender builds up a hash value containing session key B, the sender's name, the receiver's name, and the text "phase 3." This hash value is then sent to the destination user. The details of this process are discussed in Sender Code Example.
The destination user accepts the hash value from the sender and validates it by creating a hash of its own and comparing the two hash values. If the hash values do not match, then the protocol should be terminated and the communication link severed.
If the two hash values do match, this tells the receiver that the sender is presently online and in real-time communication. This is primarily because the hash value contains session key B, which was sent out encrypted with the sender's public key. Only the real sender could have decrypted the session key and built the hash value.