This may sound complex, but it is actually the soul of Bitlayer's security model.
Core dilemma: Making an 'abacus' verify a 'supercomputer'
First, remember a basic fact: The Bitcoin network itself is like an extremely limited abacus. You can make it do addition and subtraction, verify a signature, no problem. But to make it run a complex Ethereum smart contract or verify the state transition of an entire L2 block, that would be sheer nonsense.
So, how does Bitlayer make this 'abacus' adjudicate whether the work of a 'supercomputer' (L2 execution layer) is correct?
The answer is: It does not directly verify the entire work, but creates a protocol that forces the cheater to prove at which step they cheated.
This is the essence of the entire challenge-response game.
Step one: Prepare the 'crime scene'—compile computations and build the graph
Before any L2 state submission occurs, a massive amount of preparatory work has already been completed.
Atomicity of computation: First, Bitlayer's protocol thoroughly deconstructs the L2 state transition function (State Transition Function, STF)—the possible computations. This process is much deeper than you might think. It does not break down into high-level language functions or assembly instructions, but directly disassembles into the most basic logic gates (Logic Gates), like the 'NAND gate' that can combine to create any computation. This means that a complex DeFi transaction, from the protocol's perspective, is viewed as a combination of millions or even billions of the most basic logical operations of 'input A, input B, output C'.
Building the 'web of fate': Next, the protocol will construct a massive and complex directed acyclic graph (DAG) of Bitcoin transactions based on these atomic operation steps. This is not a flowchart, but a network made up of tens of thousands of real, legitimate, yet unbroadcast Bitcoin transactions. The network includes the following key paths:
Optimistic path (Happy Path): A very short path. If the L2 state submitted by the Operator goes unchallenged within the specified time, he can unlock his staked BTC through a transaction on this path.
Pessimistic path (Unhappy Path): This is an extremely large branching network. It has predefined a complete set of transaction-based responses for every possible dispute. Each node and each branch corresponds to a challenge and a response.
Pre-signed, locking fate: Once this massive transaction graph is generated, it will be co-signed by the Operator and an 'Attesting Committee' (composed of highly staked validators). Once signed, this 'web of fate' is activated. Each transaction within it has been legitimized, awaiting specific conditions to be met (like the appearance of a certain input UTXO) before being broadcast on-chain. This means that from this moment on, all actions of the Operator will be constrained by this web, and he can no longer create transactions outside of any rules to defend himself.
Step two: The game begins—fraud, challenge, and bisection
Now, suppose a cheating Operator submits an incorrect L2 state assertion. A vigilant Watcher discovers the problem.
Initiate challenge: The Watcher broadcasts a specific Bitcoin transaction that consumes a UTXO declared in the Operator's transaction, thereby initiating the pessimistic path. This is akin to shouting in the game: 'You cheated!'
Forced to respond with 'Bisection Game': At this moment, the Operator is dragged into a preset script. He cannot remain silent, or else his stake will be automatically forfeited after a timeout. He also cannot freely argue, as he can only follow the path dictated by the transaction graph.
The core of this path is a game called'Bisection Game'.Watcher: 'You claim to have executed 1 million logical operations, with the final result being Z, but the correct result should be Y. Please tell me the intermediate state at your 500,000th step.'
The Operator must reveal the computation state at the 500,000th step by unlocking a specific transaction in the graph.
The Watcher checks this intermediate state.
If the state at the 500,000th step is incorrect, the Watcher will say: 'Okay, the problem lies in the first half. Now please tell me the state at the 250,000th step.'
If the state at the 500,000th step is correct, the Watcher will say: 'Okay, the problem lies in the second half. Now please tell me the state at the 750,000th step.'
Locking in a single error point: This process will peel back like an onion, narrowing the scope of contention by half each round. After dozens of rounds of interaction (log n rounds, highly efficient), they can pinpoint the contention from 'the entire L2 block is wrong' down to 'the result of the 345,678th NAND gate operation is wrong.'
Step three: The ultimate judgment of the 'abacus'
When the dispute is narrowed down to the level of a single logic gate, a miracle happens.
Verifying the operation of a 'NAND gate' (for example, inputting 0 and 1, the output should be 1) is extremely simple. This simple logic can easily be written into a Bitcoin script.
The final 'Disprove Transaction' does this: the Watcher submits the input of this locked single logic gate (like 0 and 1) as data to the Bitcoin network. The transaction script will execute this 'NAND gate' operation on-chain, obtaining the correct result (1), and then compare it with the result claimed by the Operator (which a cheater might claim is 0).
IF a NAND b == claimed_result THEN ... ELSE ...
Once the script detects a mismatch, the transaction will be validated. The output of this transaction will be to seize the Operator's stake, part of which will reward the Watcher, and the other part will be destroyed or enter the community treasury.
This is the moment of 'gotcha'. The complex computations of the entire L2 have never run on the Bitcoin chain. The Bitcoin chain acted as the final judge at the last moment, performing the simplest verification of an atomic, indisputable mathematical fact.
Through this exquisitely clever design, Bitlayer truly achieves the effective use of Bitcoin's native security to ensure the validity of Turing-complete computation, which is a key step toward the Holy Grail of Bitcoin L2 technology.
