> why shouldn't the visitor keep that proof to themselves and use it instead Because they can't, if the website operator designs their JavaScript correctly. I

> why shouldn't the visitor keep that proof to themselves and use it instead Because they can't, if the website operator designs their JavaScript correctly. In detail: If Alice goes to Bob's website, Bob tells Alice to find a hash with 20 leading zeros for a Bitcoin block that says "Send the newly printed bitcoins and transaction fees to Bob." (It will take Alice ~2^20 guesses to find such a block, so Bob picked the number 20 such that those ~2^20 guesses happen in a couple seconds for normal humans with normal web browsers on normal devices.) Supposing the actual Bitcoin blockchain needs a hash with 50 leading zeros, one in every 2^30 Alice's will mine a valid block (worth ~$300k at current Bitcoin prices). If Alice finds a block with 50 leading zeros and then tries to change the block to say "Send the newly printed bitcoins and transaction fees to Alice," her new block will have a different hash (that is very unlikely to have 2^50 leading zeros), and neither the website nor the blockchain will accept it. Sure, Alice could change the block at the beginning before starting the search. But if she finds a block with 20 leading zeros that says "Send the newly printed bitcoins and transaction fees to Alice," Bob won't accept it for access to his website. The only way Alice gets anything for a block that sends the coins to herself is from the Bitcoin blockchain if she finds a 2^50 block -- at that point, Alice is just mining Bitcoins for herself and not interacting with Bob's website at all. (If a 1 in a billion chance to win $300k is too risky for Bob's liking, he can get a lower payout with a higher probability by using a different proof-of-work blockchain and/or a mining pool.)