In message <Pine.LNX.4.44.0212071209090.2775-100000@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)