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Bitcoin solved the problem of double spending in a distributed system. Cryptocurrencies without this specific feature have been around.

Your paper avoids this issue: "In order to prevent multiple spending in off-line payments, we need to rely on physical security."

Basically they envisioned that for their system to work, you'd need really good DRM that prevented people from spending money twice. The NSA envisioned that most people wouldn't break the DRM so the counterfeiting wouldn't be any worse then it is now.

Yet, you should know how hard (or impossible) it is to prevent people from copying and distributing stuff. That's where the blockchain shines. It prevents this problem by having an eventually-consistant public ledger. No drm needed.

In abstract sense SHA256 keys are bitcoin DRM system. We assume that it's not going to be possible to break SHA256.