def get_verifier():
    """
        Wait for key verification to occur, then return the verifier string.
        When this fires, we have seen at least one validly-encrypted message
        from our peer, indicating that we have established a shared secret
        key with some party who knows (or correctly guessed) the wormhole
        code.

        The verifier string (bytes) can be displayed to the user (perhaps as
        hex), who can manually compare it with the peer's verifier, to obtain
        more confidence in the secrecy of the established key.

        If we receive an invalid encrypted message (such as what would happen
        if an attacker tried and failed to guess the wormhole code), this
        will instead errback with a ``WrongPasswordError``.

        :return: the verifier string, after a valid encrypted message has
                 arrived
        :rtype: ``Deferred[bytes]``
        """
