Bitcoin is essentially a solution to the
Byzantine General's problem. Satoshi came up with a way to develop consensus over a trustless network. This was done by creating a dynamically-varying
Proof-of-Work system whereby "miners" come up with solutions to a complicated algorithm and are rewarded for it with the exact token used as the currency itself.
Everything else was postulated and ready to go as far as digital currency was concerned prior to Bitcoin. This paper is just one example of how such a digital currency could be secured, but it still relied on a central server or service which could be targeted.