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Friday, 3 May 2019

Permissioned Blockchain - (Byzantine General Problem)

Byzantine Generals Problem




Three Byzantine Generals Problem : Lieutenant Faulty

Round 1 :
→ Commander correctly sends same message to Lieutenants

Round 2 :
→ Lieutenant1 correctly echoes to Lieutenant2
→ Lieutenant2 incorrectly echoes to Lieutenant1



  • Lieutenant1 received differing message
  • By integrity condition, Lieutenant1 bound to decide on Commander message
  • What if Commander is faulty ??
  • This contradicts the agreement condition
  • No solution possible for three generals including one faulty
Four Byzantine Generals Problem : Lieutenant Faulty
  • Lieutenant1 decides on majority (Retreat, Attack, Retreat) = Retreat
  • Lieutenant2 decides on majority (Attack, Retreat,Retreat)=Retreat
  • Lieutenant3 decides on majority (Retreat,Retreat,Attack)=Retreat

Round 1 :
→ Commander sends differing message to Lieutenants
Round 2 :
→ Lieutenant1, Lieutenant2 and Lieutenant3 correctly echo the message to others

Byzantine Generals Model
  • N number of process with at most f faulty
  • Receiver always knows the identity of the sender
  • Fully connected
  • Reliable communication medium
  • Synchronous system

Lamport-Shostak-Pease Algorithm



  • Base Condition : 
           Broadcast (N, t-=0)
            - N : number of processes
            - t : algorithm parameter
  • Commander decides on its own value

  • All lieutenants broadcast their values to the other lieutenants except the senders


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