I actually researched this long ago - 2004 - when I was a student at Tech and gave a presentation to the Atlanta Radio Club about cryptographically secure authentication that would be Part 97 friendly. Nothing came of it, but I did later post the write up to my blog it 2009: http://blog.rietta.com/blog/2009/08/17/authentication-withou...

My opinion at the time was: " What Does Part 97 Say?

Section 97.113 (4) '…messages in codes or ciphers intended to obscure the meaning thereof, except as otherwise provided herein…' (emphasis added).

Based on the above quote, we can use any method at our disposal to provide for secure authentication which does not obscure the meaning of communications. As we start using more computing environments and bring the Internet to ham radio, we have to make sure that service is not provided to non-licensed users. In voice space, it is generally easy to spot a non-ham, but when everyone is using the same software there is not a similarly intuitive way to distinguish between the licensed user and the unlicensed user. "