"Defense Advanced Research Projects Agency (DARPA), engaged Trail of Bits—a security research organization—to investigate the blockchain. Trail of Bits focused on Bitcoin and Ethereum, the two leading cryptocurrencies in the global market.
Trail of Bits says that it only takes four entities to disrupt Bitcoin and only two to disrupt Ethereum. Additionally, 60% of all Bitcoin traffic moves through just three ISPs. Outdated and unencrypted software and blockchain protocols were also identified by the organization."