  Decentralized systems built around blockchain technology promise clients an
immutable ledger. They add a transaction to the ledger after it undergoes
consensus among the replicas that run a Proof-of-Stake (PoS) or Byzantine
Fault-Tolerant (BFT) consensus protocol. Unfortunately, these protocols face a
long-range attack where an adversary having access to the private keys of the
replicas can rewrite the ledger. One solution is forcing each committed block
from these protocols to undergo another consensus, Proof-of-Work(PoW)
consensus; PoW protocol leads to wastage of computational resources as miners
compete to solve complex puzzles. In this paper, we present the design of our
Power-of-Collaboration (PoC) protocol, which guards existing PoS/BFT
blockchains against long-range attacks and requires miners to collaborate
rather than compete. PoC guarantees fairness and accountability and only
marginally degrades the throughput of the underlying system.
