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The date is March 28, 1997, when the 2,000-or-so subscribers of the Cypherpunks mailing list receive an email with the above header in their inbox. Back’s Hashcash was actually not the first solution of its kind. By the early 1990s, the promise of the internet, and the advantages of an electronic mailing system in particular, had become obvious to techies paying attention. Still, internet pioneers of the day came to realize that email, as this electronic mailing system was called, presented its own challenges. Pricing via Processing or Combatting Junk Mail. Indeed, as email rose in popularity, so did spam. A solution was needed, early internet users agreed — and a solution is what Dwork and Naor’s paper offered.
The duo proposed a system where senders would have to attach some data to any email they send. This data would be the solution to a math problem, unique to the email in question. Specifically, Dwork and Naor proposed three candidate puzzles that could be used for the purpose, all based on public-key cryptography and signature schemes. Adding a solution to an email wouldn’t be too difficult, ideally requiring only a couple of seconds of processing power from a regular computer, while its validity could easily be checked by the recipient.
But, and this is the trick, even a trivial amount of processing power per email adds up for advertisers, scammers and hackers trying to send thousands or even millions of messages at once. Spamming, so was the theory, could be made expensive and, therefore, unprofitable. Users would have to literally show that their computer performed work, to prove that they spent real-world resources. A nifty solution, but perhaps too far ahead of its time. The proposal never made it very far beyond a relatively small circle of computer scientists.
The ideologically driven crowd started to organize through a mailing list centred around privacy-enhancing technologies. Over the years, Adam Back — who earned his Ph. 1996 — established himself as one of the more active participants on this list, at times contributing dozens of emails in a single month. T-shirts with an encryption protocol printed on them, intended to help point out the absurd decision by the U. Like many, Back was not aware of Dwork and Naor’s proof-of-work proposal.
The Cypherpunks mailing list grew significantly in about half a decade. What started out as an online discussion platform for a group of people that initially gathered at one of their startups in the Bay Area became a small internet phenomenon with thousands of subscribers — and often more emails on a single day than anyone could reasonably keep track of. It was around this time — 1997, close to the list’s peak popularity — that Back submitted his Hashcash proposal. Hashing is a cryptographic trick that takes any data — whether it’s a single letter or an entire book — and turns it into a seemingly random number of predetermined length. As you can see, merely inserting one comma into the sentence completely changes the hash.
The only way to find out was to actually hash both sentences. Hashcash applies this mathematical trick in a clever way. All this metadata, including the nonce, is then hashed, so the resulting hash looks a bit like one of the random numbers above. Instead, the binary version of the hash must start with a predetermined number of zeroes.
Like Dwork and Naor’s solution, this requires computational resources: it’s a proof-of-work system. Back explained on the Cypherpunks mailing list. 100 MIP years which is going to be more compute than they’ve got. Notably, Back’s proof-of-work system is more random than Dwork and Naor’s. The duo’s solution required solving a puzzle, meaning that a faster computer would solve it faster than a slow computer every time. But statistically, Hashcash would still allow for the slower computer to find a correct solution faster some of the time. By analogy, if one person runs faster than another person, the former will win a sprint between them every time.
But if one person buys more lottery tickets than another person, the latter will statistically still win some of the time — just not as often. Where one of the most powerful features of digital products is the ease with which they can be copied, proof of work was essentially the first concept akin to virtual scarcity that didn’t rely on a central party: it tied digital data to the real-world, limited resource of computing power. And scarcity, of course, is a prerequisite for money. Back argued on the mailing list. Hashcash is free, all you’ve got to do is burn some cycles on your PC. It is in keeping with net culture of free discourse, where the financially challenged can duke it out with millionaires, retired government officials, etc on equal terms. Unlike money, it could not be re-spent elsewhere.