Question 1009

Counting Traitors

A permissioned ledger runs a PBFT-style protocol and must keep working correctly when up to 2 replicas are Byzantine, meaning they may lie or send different messages to different peers. What is the minimum total number of replicas?

A

9 replicas, since each Byzantine node must be outvoted by four independent honest nodes

B

7 replicas, since Byzantine agreement needs at least 3f + 1 nodes to tolerate f arbitrary faults

C

5 replicas, since 2f + 1 nodes leave an honest majority that can outvote the faulty ones

D

4 replicas, since f + 1 correct nodes are enough to outnumber the f faulty ones on any decision