12. Conclusion
We have proposed a system for electronic transactions without relying on trust.  We started with
the usual  framework of coins made from digital  signatures,  which provides strong control  of
ownership,  but  is  incomplete  without  a way to  prevent  double-spending.   To solve  this,  we
proposed a peer-to-peer network using proof-of-work to record a public history of transactions
that  quickly becomes  computationally  impractical  for  an attacker  to  change  if  honest  nodes
control a majority of CPU power.  The network is robust in its unstructured simplicity.  Nodes
work all at once with little coordination.  They do not need to be identified, since messages are
not routed to any particular place and only need to be delivered on a best effort basis.  Nodes can
leave  and  rejoin  the  network  at  will,  accepting  the  proof-of-work  chain  as  proof  of  what
happened while they were gone.  They vote with their CPU power, expressing their acceptance of
valid blocks by working on extending them and rejecting invalid blocks by refusing to work on
them.  Any needed rules and incentives can be enforced with this consensus mechanism.