kinds of computation?

[dmolnar]

On Sat, 24 Mar 2001, David Honig wrote:

> What the world needs now 
> 
> is a membrane keyboard, used only for entering keys, 
> which can be folded into a credit card and stored
> in your wallet.  If not hung around your neck.

This reminds me of something I've had in the back of my mind for a while
(thanks to the Oxygen project over at MIT).  
Is it useful to divide "available computation power" into these
categories:

		1) untrusted and "impersonal" 
		2) untrusted but personalized
		3) trusted and "impersonal"
		4) trusted and personal

Here "personalized" is a purposely vague notion which is supposed to
capture such things as, say, a Palm Pilot with your passphrases saved on
it. That would be a "trusted and personal" computation device. A "trusted
but impersonal" device might be something you have good reason to believe
works correctly, but doesn't have any special information about you. 

I'm not clear on whether it's useful to distinguish between 1) and 2).
Probably that depends on what is shoved into the word "personalized." 

The point of dividing computation into categories is that 4) is very rare,
often computationally weak, but a little goes a long way. At least, given
the right protocols. My brain is trusted and personal, but it can't do
much more than remember a password. My Palm Pilot and my brain together
can execute SRP or PAK or whatever - but my Palm Pilot doesn't need to
remember my password. (it can be "trusted but impersonal", contrary to the
example above). 

I'd be interested to hear where else this sort of categorisation has
popped up, or whether people think it's useful. 

thanks, 
-David