[hashcash] Re: RPOW - Reusable Proofs of Work

  • From: Jonathan Morton <chromi@xxxxxxxxxxxxxxxxxxxxx>
  • To: hashcash@xxxxxxxxxxxxx
  • Date: Sun, 22 Aug 2004 18:11:17 +0100

Because RPOWs are only created from equal-value POWs or RPOWs, they are
as rare and "valuable" as the hashcash that was used to create them.
But they are reusable, unlike hashcash.

The new concept in the server is the security model.  The RPOW server
is running on a high-security processor card, the IBM 4758 Secure
Cryptographic Coprocessor, validated to FIPS-140 level 4.

This is a very neat idea. My concern, however, is in the rather limited capacity of each server card. How much does each card cost, and is it possible to fit more than one in a machine? How easy is it, in practice, to add (a batch of) cards to the trusted server network?

These factors are extremely limiting to the scalability of the system - recall that some MTAs routinely handle many mails a second, and would need multiple RPOW cards to service their own load if it became widely used. This means RPOW, in it's present form, is almost certainly unsuited to any application which requires a high global volume of transactions - including both e-mail and "P2P load balancing" which you suggest. Such applications might also have problems with laundering, if the scalability were solved.

An interesting and practical application, however, would be to allow users of low-end machines to "save up" hashcash in RPOW tokens, and redeem it when needed by asking the server to generate regular hashcash with certain properties, providing an appropriate value RPOW token as "payment".

This would have less scalability problems than using RPOW as a complete hashcash replacement, and would considerably extend the low-end scalability of hashcash itself. That's important - the poorest Internet users are potentially the worst affected by the recipient-end costs of spam, yet can least afford the hardware to run hashcash efficiently.

--------------------------------------------------------------
from:     Jonathan "Chromatix" Morton
mail:     chromi@xxxxxxxxxxxxxxxxxxxxx
website:  http://www.chromatix.uklinux.net/
tagline:  The key to knowledge is not to rely on people to teach you it.


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