Re: namedroppers, continued

["Steven M. Bellovin" <smb at research.att.com>]

In message <Pine.LNX.4.44.0212071209090.2775-100000 at commander.av8.net>, Dean An
derson writes:
>This seems clever, however, it will also take significant computational
>effort to verify the computational effort was actually done. Even if a
>class of functions are found that are "easier" to verify than to compute,
>they will no doubt still take up a significant fraction of time.

In fact, that's the easy part.  You could demand that the sender 
compute 1,000,000 HMACs of the text, the envelope, the time of day, and 
a counter.  The verifier could check 100 randomly-chosen ones -- if any 
fail, there's a forgery.  (Well, you probably wouldn't want those 
values, since 1,000,000 HMACs would be a lot of data to transmit.  But 
you get the general idea.)

		--Steve Bellovin, http://www.research.att.com/~smb (me)
		http://www.wilyhacker.com ("Firewalls" book)