HASHCASH

HashCash is different from most crypto-currencies currently in vogue. Unlike Bitcoin, Ethereum and other blockchain based systems, HashCash is a real digital cash system based on blind signatures, as introduced by David Chaum in 1983... And with that, we have a private digital cash system that prevents both counterfeiting and double spending... Imagine a sheet of paper that we want to have signed by a signer, Sam, without revealing to Sam what's written on it. This can be accomplished by placing a sheet of carbon paper on top of the paper to be signed, inserting them both into an envelope and sealing it, and then having Sam sign on the envelope. The signature will be copied onto the paper through the carbon paper above it, and we will have the signature on the paper without having revealed its contents to Sam. This is a blind signature.