  This paper presents IBFT, a simple and elegant Byzantine fault-tolerant
consensus algorithm that is used to implement state machine replication in the
\emph{Quorum} blockchain. IBFT assumes a partially synchronous communication
model, where safety does not depend on any timing assumptions and only liveness
depends on periods of synchrony. The algorithm is deterministic, leader-based,
and optimally resilient - tolerating $f$ faulty processes out of $n$, where $n
\geq 3f+1$. During periods of good communication, IBFT achieves termination in
three message delays and has $O(n^2)$ total communication complexity.
