Unconditional Byzantine Agreement with Good Majority

We present a protocol which achieves Byzantine Agreement (BA) if less than half of the processors are faulty and which does not rely on unproved computational assumptions such as the unforgeability of digital signatures. This is the first protocol which achieves this level of security. Our protocol needs reliable broadcast and secret channels in a precomputation phase. For a security parameter k, it achieves BA with an error probability exponentially small in k, whereas all computations are polynomial in k and the number of processors, n. The number of rounds is linear in k and independent of n. The length of the precomputation phase is linear in n and proportional to the number of BAs based on it. As a subprotocol, we present a coin flipping protocol on the same assumptions.