  This paper presents a new consensus protocol based on verifiable delay
function. First, we introduce the concept of verifiable delay puzzle (VDP),
which resembles the hashing puzzle used in the PoW mechanism but can only be
solved sequentially. We then present a VDP implementation based on the
continuous verifiable delay function. Further, we show that VDP can be combined
with the Nakamoto consensus in a proof-of-stake/proof-of-delay hybrid protocol.
We analyze the persistence and liveness of the protocol, and show that compared
to PoW, our proposal consumes much less energy; compared to BFT leader-election
based consensus algorithms, our proposal achieves better resistance to
long-range attacks and DoS attacks targeting the block proposers.
