Blockchain is a distributed ledger that is shared and maintained by all participants on the network based on a consensus protocol. The most widely used consensus mechanism is Proof-of-Work (PoW), which has been deployed in public blockchain networks like Bitcoin and Ethereum. In PoW, block generation requires solving a cryptographic math puzzle whose solution is easy to verify but extremely hard to solve. The participants in the blockchain network exhaust their computing resources to solve the puzzle. Here, block generation is called mining and participants are called miners.
Once a miner successfully mines a block, he becomes eligible to receive a reward. For this reason, miners competitively participate in the mining process. To reduce the average block generation time, individual miners aggregate their computing powers into a mining pool, where all participating miners solve the puzzle together and share the reward. In a mining pool, the pool manager distributes less difficult partial PoW puzzles (pPoW) than the original full PoW puzzle (fPoW) to individual miners. If enough miners solve pPoW, some of these solutions are likely to become the solution to fPoW because each solution to pPoW has a probability of yielding a solution to the fPoW. However, mining pools undermine the decentralization and security of Blockchain. Especially, if a single mining pool owns more than half of the entire network mining capacity, this pool can generate the blocks faster than any other pool, reaping all rewards and choices of transactions to confirm (51% attack).
Meanwhile, in order to be used in bitcoin practically, any solution to the centralized mining problem must preserve the existing blockchain; preserve large investments many miners have made and are planning to make in their equipment; provide a seamless transition from the existing system to the new one, providing adjustable knobs that can be fine-tuned for a desired trade-off that fits the community’s needs.
However, none of the proposals satisfies all the requirements above. In other words, preceding techniques require some changes in the design of the cryptocurrency and so, they are not compatible with the current bitcoin system.
Kim Kyu Chol, a section head at the Faculty of Information Science and Technology, has proposed a hybrid PoW consensus protocol in order to discourage centralization and tackle the 51% attack. In the proposed scheme, miners can generate the block by solving either an original outsourceable bitcoin puzzle or a non-outsourceable puzzle.
The main feature of his system is that it is fully compatible with current bitcoin designs, i.e., it can be implemented right now, because it preserves both the existing blockchain and investments which have been made in mining hardware. In contrast to the current bitcoin system and any other preceding protocols, his scheme presents two puzzles, but is still single-tiered.
You can find further details in his paper “Single-Tiered Hybrid PoW Consensus Protocol to Encourage Decentralization in Bitcoin” in “Security and Communication Networks” (SCI).
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