Ah, I see what you're getting at. Yes. You can think of any given computer aa having a fixed amount of compute budget for these types of acceleration-resistant hashing algorithms. Let's say the scraper can perform 10,000 hashing operations per second total on their machine, and needs an average of 1,000 hashing operations to solve the PoW. It's a minor detail, but note that these PoW challenges non-deterministically vary in the number of hashing operations needed to produce a valid hash, not dissimilar to bitcoin mining, where a hash with a certain number of 0s prefixed is sought, and the scraper essentially has to brute force through all possible inputs until an input that produces a valid hash is found.
In a well-designed PoW systems, there is a per-site prefix or suffix that is required to be prepended or appended to these random inputs, and it may change not only between websites, but even between PoW sessions on the same website, and should not be predictable - only being disclosed to the client at the time the PoW challenge is issued. In such a case, the scraper cannot simply precompute a bunch of valid hashes that work across multiple sites, nor a bunch of valid hashes that will always be good for even one site, the scraper operator will need to compute these hashes (with a limited budget to do so) upon initiating each PoW session.
alt Hacker News
Ah, I see what you're getting at. Yes. You can think of any given computer aa having a fixed amount of compute budget for these types of acceleration-resistant hashing algorithms. Let's say the scraper can perform 10,000 hashing operations per second total on their machine, and needs an average of 1,000 hashing operations to solve the PoW. It's a minor detail, but note that these PoW challenges non-deterministically vary in the number of hashing operations needed to produce a valid hash, not dissimilar to bitcoin mining, where a hash with a certain number of 0s prefixed is sought, and the scraper essentially has to brute force through all possible inputs until an input that produces a valid hash is found.
In a well-designed PoW systems, there is a per-site prefix or suffix that is required to be prepended or appended to these random inputs, and it may change not only between websites, but even between PoW sessions on the same website, and should not be predictable - only being disclosed to the client at the time the PoW challenge is issued. In such a case, the scraper cannot simply precompute a bunch of valid hashes that work across multiple sites, nor a bunch of valid hashes that will always be good for even one site, the scraper operator will need to compute these hashes (with a limited budget to do so) upon initiating each PoW session.