“A blockchain based system is as secure and robust as its consensus model. ”
The security of the consensus model is perhaps the most crucial aspect that requires close attention when
choosing a blockchain platform. The consensus mechanism maintains the sanctity of the data recorded on the
blockchain. The blockchain system will safeguard the transaction and block order thereby safeguarding all the
key properties of blockchain, such as immutability and auditability only when the underlying assumptions are
correct and the consensus model can uphold the state of the blockchain under failure and adversarial conditions.
Poor choice of a consensus mechanism can render the blockchain platform useless thereby compromising the
data recorded on the blockchain. Below are some of the issues that can result when the consensus mechanism
fails.
Blockchain Fork – A blockchain fork can result in different nodes in the system converging on different blocks
as being part of the blockchain. In Bitcoin, though temporary forks may exist due to network latencies, the
protocol is designed such that all nodes will eventually converge on a single chain. A blockchain fork can wreak
havoc on applications leading to completely inconsistent view of data recorded on the blockchain thereby forcing
applications to behave in an unpredictable manner. The Stellar network, which originally forked code from Ripple, experienced a fork in the Stellar blockchain due to a misconfiguration
Consensus Failure – Certain consensus algorithms may not guarantee the ability to reach consensus. For e.g.
if the consensus algorithm requires a super-majority vote from a certain percentage of nodes, failing to reach
this number because of node or network failures, non-compliant nodes or as a result of valid honest nodes not
being able to make a decision due to inconsistent messages received from other nodes, may result in consensus
failure.
Dominance – Consensus round outcomes can be manipulated by a single or group of entities if it is not designed
to be resilient against Sybil attacks, where one or handful of nodes can generate millions of identities that they
control. Having such dominance allows the dominating group to confirm the transactions and blocks as per their
rules, even include transactions that can double-spend the cryptocurrency. Dominance can also be achieved by
other means, such as controlling 51% of mining power in a PoW network
Cheating – Validating nodes either individually or in collusion can independently maintain parallel forks in the
blockchain of fraudulent transactions or altered reality that can been provided as proof to the auditor or external
third party. The consensus and blockchain reading mechanism has to ensure that such attacks cannot be carried
out on the blockchain platform.
Poor Performance – Based on the design of the consensus algorithm, it may require more time under certain
conditions for consensus to converge. These conditions could be dynamic where other nodes have turned
malicious or a network partition may delay messages that are exchanged between nodes, etc. This may manifest
as inconsistently high latencies in applications.
