DARPA looked into the Bitcoin and Ethereum blockchain implementations, and has documented some weaknesses, like it would only take 4 bad actors to take control of Bitcoin due to the low number of validators currently.
This is no klaxon warning about the failure of cryptocurrencies; this is just documenting what happens to distributed networks when everyone on the network doesn't keep their software up to date. These tools rely on their users following some basic rules - like updating their versions - that, when abandoned, create weaknesses for anybody.
In other words; when you design a new network, the same old networking problems still exist, PLUS the new ones you inadvertently hard-coded into your design.
DARPA didn't look into shit, they paid Trail of Bits to do it. Here's how they did it. I wonder why deployed Ethereum smart contracts are at least 56% similar to each other. It's almost like contracts have boilerplate in them. I wonder what percentage of code is the same between any given .xls file? Now do SWIFTOf all Bitcoin traffic, 60% traverses just three ISPs.
The Ethereum ecosystem has a significant amount of code reuse: 90% of recently