Digital money failed for years on two unsolved problems. Double-spending seemed to require a central operator, and most digital currencies were claims on issuers and not money proper.

Chaum's DigiCash threaded privacy and double-spending through blind signatures, then collapsed in 1998 when its operator went bankrupt. Nakamoto joined a permissionless ledger to consensus rules each operator enforces locally, with chain selection by accumulated work. Holders possess the units themselves.

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