COMPARISON BETWEEN BITCOIN, RIPPLE, ETHEREUM, AND OPEN BLOCKCHAIN

In what follows, we briefly discuss the security and privacy provisions of Ripple, Ethereum, and Open Blockchain in relation to the well-investigated Bitcoin system.

Similar to Bitcoin, Ripple, Ethereum, and Open Blockchain relies on ECDSA signatures to ensure the authenticity and nonrepudiation of transactions in the system. Furthermore, since Ripple and Ethereum are open systems (like Bitcoin), all transactions and their orders of execution are publicly available. This ensures the detection of any double-spending attempt (and of malformed transactions). Open Blockchain, on the other hand, is a closed, permissioned enterprise blockchain, where transactions can only be seen by the registered participants.

Consensus in Ripple and Open Blockchain are achieved by requiring that the validating servers check the log of all transactions in order to select and vote for the correct transactions in the system. In this way, these systems adopt a voting scheme across all validating servers (one vote per each validating server). As mentioned earlier, Open Blockchain relies on the PBFT consensus protocol, which ensures security even if 33% of the validators are Byzantine. On the other hand, the consensus layer in Ripple is closer requires that the transactions for which (80% of) the validators agree upon are considered to be valid [30]. Ripple Labs claims that it is easy to identify colluding validators and recommend that users choose a set of heterogenous validators that are unlikely to collude. Note that if validators in Ripple refuse to come to a consensus with each other, this is detectable by other validators, which then pronounce the network broken. In this case, the only way to resolve the problem would be to manually analyze the signed validations and proposals to see which validators were being unreasonable and for all honest participants to remove those validators from the UNLs (i.e., from the lists of validators they try to come to a consensus with). As far as we are aware, there is no formal security treatment of the correctness of Ripple's consensus protocol; this protocol has recently received some criticism [31,32]. In [28], Armknecht et al. show that the current choice of parameters does not prevent the occurrence of forks in the system.

In contrast, transaction security in Bitcoin and Ethereum is guaranteed by means of proof-of-work which replaces the vote per validating server notion of Ripple and Open Blockchain, with a vote per computing power of the miners that are solving the PoW. Unlike Ripple and Open Blockchain, once transactions are confirmed in the global ledger (i.e., once transactions receive six confirmation blocks), it is computationally infeasible to modify these transactions [33] as long as the majority of the computing power in the network is honest. In contrast, in Ripple and Open Blockchain, if at any instant in time the majority of the validating servers becomes malicious, then they can rewrite the entire history of transactions in the system. For instance, at the time of writing, there are only a handful of Ripple validating servers that are mostly maintained by the Ripple Labs; if these servers are compromised, then the security of Ripple is at risk.