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In a public instance, one would expect the more miners the merrier, as this improves the overall mining diversity and therefore we get better security.

Private blockchains, as in the Eris case, tend not to rely on proof of work mining, and use some form of proof of stake instead.

My question is: could I run a private blockchain (say using Eris), say, with just two participating nodes? Or is there any inherent lower bound below which security can become compromised?

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Theoretically that would work, practically it likely wouldn't work every well.

We recommend a minimum bound of seven validator nodes with stake distributed amongst the various actors within the network (depending on what the application is trying to achieve). This will allow a tolerance of two nodes to be either malicious or offline at any one time.

The tendermint consensus engine will not move the chain forward unless there is agreement amongst > 66% of the stake bonded as to the application's state root.

Tendermint chains have a strong finality meaning they do not fork. They either move forward or do not depending on whether there is > 66% agreement amongst the validators as to the application's state root.

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  • With 5 nodes, we could tolerate 2 nodes being malicious as well rite? – Minisha Mar 27 '18 at 3:14
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    Not with Tendermint's consensus algo. You'd need 7 nodes to tolerate 2 byzantine. – Casey Kuhlman Apr 12 '18 at 19:47
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Eris currently uses Tendermint as a consensus algorithm. It uses a Byzantine Fault Tolerance algorithm which is secure as long as two-thirds of validators are honest. Thus, it doesn't make much sense to use just two nodes. Blockchains in general don't really apply to just two parties, since each node could just fork the chain without too much work.

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