Distributed Network Systems and the Byzantine Generals Problem
In distributed network systems, achieving consensus involves addressing the Byzantine Generals Problem. It has been shown in the context of Decentralized Consensus Formation and the Byzantine Generals Problem that if more than one-third of the nodes are malicious, reaching consensus is impossible.
Even when the Byzantine Generals Problem arises, the property that allows a network to agree on the correct value is referred to as BFT (Byzantine Fault Tolerance).
Challenges of Distributed Consensus in Bitcoin
One of the key technical challenges in building a decentralized electronic currency system like Bitcoin is achieving distributed consensus. The question arises: does the Bitcoin blockchain⛓️ exhibit BFT?
In conclusion, while the Bitcoin blockchain does not provide a theoretical solution to the Byzantine Generals Problem, it offers a practical one. This is because it operates under conditions not present in traditional models, which allow it to function effectively in practice.
Two Key Conditions Supporting Bitcoin’s Consensus Mechanism
- The Concept of Incentives
The first condition is the concept of incentives. To prevent fraudulent transactions and maintain the blockchain, nodes (miners) that create blocks through mining are rewarded with Bitcoin (a combination of block rewards and transaction fees)*1. Since Bitcoin functions as a currency, participating nodes are incentivized to act honestly.
*1 Maintaining the blockchain here refers to PoW (Proof of Work), which involves expending significant computational resources (performing hash calculations). - Probabilistic Consensus
The second condition is probabilistic consensus. In the Bitcoin blockchain, there is no specific starting or ending point for achieving consensus. Instead, consensus is reached over an extended period, with actual operations often taking around an hour*2. Even after this period, nodes cannot be absolutely certain whether a specific transaction or block has been permanently written to the blockchain. However, over time, the visible state converges to the final consensus state, and the probability of divergence between these two states decreases exponentially.
*2 Since blocks are generated approximately every 10 minutes, it is commonly believed that a block becomes valid if 6 subsequent blocks are added after it.
It is important to note that the number “6” is not based on theoretical grounds but is rather an empirical estimate.
Bitcoin Blockchain and Avoiding the Byzantine Generals Problem
The key to the Bitcoin blockchain avoiding the Byzantine Generals Problem in traditional distributed consensus lies in these differences in the model. It is described as “avoiding” rather than “solving” the Byzantine Generals Problem because, as mentioned earlier, a theoretical solution has not been provided.
In fact, to claim that a theoretical solution exists, the probability of consensus being overturned must be zero. In the Bitcoin blockchain, however, consensus achieved through PoW is not final at the time a block is created. Instead, the mechanism ensures that the probability of overturning the consensus converges to zero over time.