You may be interested in looking into some of the research that has been done in other areas on Byzantine consensus. Depending on the system model, solutions th

You may be interested in looking into some of the research that has been done in other areas on Byzantine consensus. Depending on the system model, solutions that are much more efficient than the Bitcoin block chain (but have other tradeoffs) exist. For example, check out Castro and Liskov (http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.127.6130 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.127....). Personally, I think some people in the Bitcoin community have been a bit dishonest in claiming that bitcoin's protocol solves a consensus problem which wasn't solvable before. Lower bounds on Byzantine consensus in any practical system model exist which makes it very attractive, for efficiency reasons, to solve a subset of that problem for things like database replication in trusted systems. Lower bounds for consensus vary from f+1 (for f faults) to 2f+1 to 3f+1 depending on what kinds of faults you admit. Still it's very interesting to think about.