  PermitBFT establishes a permissioned byzantine ledger in the partially
synchronous networking model. For n replicas, PermitBFT tolerates up to f < n/3
byzantine replicas. It is the first BFT protocol to achieve a latency of just 2
message delays despite tolerating byzantine replicas throughout the "fast
track", as long as they are not the leader. The design of PermitBFT relies on
two fundamental concepts. First, in PermitBFT the participating nodes do not
wait for a distinguished leader to act and subsequently confirm its actions,
but send permits to the next leader proactively. Second, PermitBFT achieves a
separation of the decision powers that are usually concentrated on a single
leader node. A leader in PermitBFT controls which transactions to include in a
new block, but not where to append the block in the block graph.
