In the vast landscape of blockchain technology, Bitcoin is undoubtedly a monumental milestone. For many, blockchain seems to be all about Bitcoin. But when we look deeper into its core, we find a more intricate structure: blockchain technology is merely the “flesh” of Bitcoin, while the three-layer structure of PH (Polynomial Hierarchy) in computational complexity theory serves as the “skeleton” supporting this grand architecture.
To understand this, we must first touch on a core concept in computer science — computational complexity theory. This theory classifies computational problems based on the resources (time or space) needed to solve them. PH (Polynomial Hierarchy) is an important tier within this theory, describing classes of problems defined by polynomial-time Turing machines and alternating quantifiers.
Bitcoin’s power and uniqueness lie precisely in how its design cleverly embeds this PH three-layer structure:
1. Layer One: UTXO (Unspent Transaction Output) — Verifiable Outputs
This corresponds to the “NP” (nondeterministic polynomial time) class in the PH hierarchy. A UTXO represents a spendable, unspent unit of Bitcoin in the digital world. Its existence is verifiable: anyone can easily check whether a UTXO is real and unspent. However, to create a new UTXO (i.e., conduct a valid transaction) requires solving a computational problem, which is the starting point of a Bitcoin transaction.
2. Layer Two: PoW (Proof-of-Work) — Provable Workload
This corresponds to “coNP” problems, or more accurately, the solution of an “NP-hard” problem. The proof-of-work mechanism in mining requires miners to invest massive computational resources to find a nonce (a random number) that meets specific conditions. Verifying whether this “work” is completed (i.e., whether a block is valid) is extremely easy and fast. But finding that valid nonce is an extremely difficult and unpredictable computational task — this is the essence of PoW. This process ensures network security and defends against malicious attacks.
3. Layer Three: The Longest Chain — Irreversible Consensus
This corresponds to a higher level in the PH hierarchy, as it involves selecting the optimal path under uncertainty. The Bitcoin network achieves consensus and resolves forks through the “longest chain rule.” Miners always continue mining on the chain with the highest accumulated work they know of. The length of this chain represents the total accumulated computational effort and is thus considered the “correct” and most secure chain. Attempting to alter historical transactions would require recomputing and surpassing the PoW on the current longest chain — a task that’s nearly impossible computationally — thereby ensuring finality and immutability.
These three interlinked and progressive layers of computational complexity together form the unbreakable “skeleton” of Bitcoin.
When we talk about blockchain, we usually refer to the structure of Transactions (Tx), Blocks, and Chains. This structure provides a way to package, link, and timestamp transaction data in a distributed ledger. However, this structure is like an empty shell — it requires specific mechanisms to give it life.
It is the PH three-layer skeleton of Bitcoin that breathes life and meaning into this structure:
Thus, the commonly referenced blockchain Tx/Block/Chain organizational structure is filled and reinforced by the PH skeleton layers of UTXO/PoW/Longest Chain, respectively.
This is the fundamental reason why Bitcoin is so “powerful” — and the essential difference between it and most “altcoins.” Many cryptocurrency projects also adopt blockchain technology and have the Tx/Block/Chain “flesh” structure, but they often lack — or fail to deeply understand and effectively apply — the PH computational complexity theory as their “skeleton.”
These altcoins may innovate on consensus mechanisms (e.g., PoS, DPoS) or enhance programmability (e.g., smart contracts), but in terms of underlying security, degree of decentralization, and resilience against large-scale attacks, they often fall short of the PH-based three-layer skeleton that underpins Bitcoin. While they may achieve the functionality of a “distributed ledger” (trusting code instead of human intermediaries), they cannot provide the same level of “intermediary-free finality” and “immutability.”
In other words, they possess only the visible structure and application layer — the “flesh” of blockchain — but lack the robust mathematical and logical skeleton provided by computational complexity theory and proven over time. This makes them more vulnerable to influence from centralized entities and potentially insecure when faced with real-world challenges.
Bitcoin’s uniqueness lies in the fact that it is not merely a technical invention but a masterpiece of engineering and cryptography, whose underlying logic is deeply rooted in the solid foundation of computational complexity theory. This makes it not just a “digital currency,” but a profound social experiment — a truly intermediary-free infrastructure for freedom.
