This procedure proves the presence of the principal but gives no assurance that the principal is actually at the other end of the channel; it is possible that an adversary controls the channel and is relaying messages from the principal. To provide this assurance, the principal should encrypt some unambiguous identification of the channel with his private key as well, thus certifying that he is at one end. If the channel is secured by encryption, the encryption key identifies it. Since the key itself must not be disclosed, a one-way hash (see Appendix B) of the key should be used instead.

Another problem with retina scans is that individuals concerned about potential health effects sometimes object to use of the technology.