FWIW, I was part of the gang talking with Adam about using Hashcash as a postage stamp as an anti-spam measure. I implemented an extension to Postfix using Hash

FWIW, I was part of the gang talking with Adam about using Hashcash as a postage stamp as an anti-spam measure. I implemented an extension to Postfix using Hashcash as an anti-spam method (camram/two penny blue). The compute inflationary pressure you mentioned was only one of the challenges in deploying such a system. FWIW, none of the challenges were barriers; they were the kind of challenges you get when you take something beautiful, theoretical, and pure and try to make it real. Some may not be aware that Hashcash value is as a decentralized rate limiter that can be added to almost any protocol. Experience with Hashcash taught us that it's essential to have a dynamic pricing scheme based on the reputation of the contact originator. In the email context, when a message sender connects to a receiving server, the receiving server should be able to tell the sender the size of the stamp needed based on the reputation created by previous messages. From my perspective, the two main challenges in rate-limiting HTTP requests are embedding the required Hashcash stamp size and measuring the reputation of the request initiator. I think Anubis is a good first start, but the fact that it uses a fixed size stamp, a small one, does not have a reputation database, and from what I can tell, does not have a robust detector of good versus bad players. These shortcomings will make it challenging to provide adequate protection without interfering with good players. I'll spare you my design note rambling, but I think from 3-page requests, one can gather enough information to determine the size of the Hashcash stamp for future requests.