You are incorrect on your first point. This is an example of the birthday paradox[1]. With 2^512 possible keys (~10^154) even if everyone on earth (~10^9 peopl

You are incorrect on your first point. This is an example of the birthday paradox[1]. With 2^512 possible keys (~10^154) even if everyone on earth (~10^9 people) got a billion new IDs every second (~10^18 per person per year) it would be more than a hundred thousand billion billion billion billion billion years (~10^50 year) before there was an even chance of a single ID collision (~10^77 IDs => 50% chance of 1 collision). So I think we can safely assume there will not be a collision. In fact it is almost always the case in cryptography that there will be some risk of failure but as long as that risk is very small the system is considered secure. This is way way way beyond the normal standards of security (say 10^30 or so). Many of your other points are a matter of opinion. There will be always tradeoffs between usability and security. This system appears to offer similar or perhaps even improved usability to existing two-factor systems with enhanced security in some respects but similar weaknesses in other respects. I would love to hear the opinions of cryptographers and security experts. [1] http://en.wikipedia.org/wiki/Birthday_attack#Mathematics http://en.wikipedia.org/wiki/Birthday_attack#Mathematics