BA-06 · LESSON 08

Script debugging

KEY DEFINITIONS

Terms used in this lesson

Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
P2SH: Pay to Script Hash, a legacy Bitcoin output template that commits to a redeem script.
SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
Tapscript: The version of Bitcoin Script used by a Taproot script-path spend.
Preimage: The original secret data whose hash equals a previously committed value.
CHECKSIG: The Bitcoin Script operation that checks whether a signature is valid for the required public key and transaction message.

Experts trace every stack transition, identify the failing predicate, and distinguish malformed witnesses from false conditions.

65 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · script stackWitness data is evaluated by a deterministic stack machineSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered validation pipeline

01
WITNESSStack items
The spender supplies signatures, preimages, selectors, or other required items.
02
PROGRAMLocking script
The prior output determines the program and rule version to execute.
03
EXECUTEScript debugging
This lesson follows the stack effects and failure boundary for Script debugging.
04
FIRST FAILUREOpcode and state
Invalid encoding, underflow, failed verification, or disabled behavior stops execution.
05
FINAL STACKTrue or false
Consensus accepts the input only when the applicable Script rules succeed.

Rule shown: A spend succeeds only if every opcode is valid and the final stack satisfies the spending condition.

STAGE 1 OF 5WITNESS: Stack items

The spender supplies signatures, preimages, selectors, or other required items.

Governing relationship: A spend succeeds only if every opcode is valid and the final stack satisfies the spending condition. Complete path: 1. WITNESS, Stack items. The spender supplies signatures, preimages, selectors, or other required items. 2. PROGRAM, Locking script. The prior output determines the program and rule version to execute. 3. EXECUTE, Script debugging. This lesson follows the stack effects and failure boundary for Script debugging. 4. FIRST FAILURE, Opcode and state. Invalid encoding, underflow, failed verification, or disabled behavior stops execution. 5. FINAL STACK, True or false. Consensus accepts the input only when the applicable Script rules succeed.

1. State transitionWhat exists, what changes, and what results

BEFORE

A toy script expects <sig> <pubkey> OP_CHECKSIG, but the initial stack is [pubkey, sig].

MECHANISM

Record raw transaction, prior output amount and script, network, block or mempool context, and software version. Signature checks cannot be debugged from Script text alone.

AFTER

The first observed failure is more useful than a broad guess about keys or nodes.

Text equivalent: Starting from A toy script expects <sig> <pubkey> OP_CHECKSIG, but the initial stack is [pubkey, sig]. The mechanism begins when Record raw transaction, prior output amount and script, network, block or mempool context, and software version. Signature checks cannot be debugged from Script text alone. The resulting lesson is The first observed failure is more useful than a broad guess about keys or nodes.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Freeze the transaction context

Record raw transaction, prior output amount and script, network, block or mempool context, and software version. Signature checks cannot be debugged from Script text alone.

Complete path: 1. Freeze the transaction context: Record raw transaction, prior output amount and script, network, block or mempool context, and software version. Signature checks cannot be debugged from Script text alone. 2. Select the correct interpreter: Determine legacy, P2SH, SegWit version 0, or Tapscript rules and applicable flags before tracing. 3. Build the initial stack: Decode scriptSig or witness items without silently reordering them. Mark signatures, scripts, control data, selectors, and preimages. 4. Trace every executed opcode: After each operation, record stack items and branch state. Stop at the first error rather than explaining later code that never ran. 5. Classify the result: Report success, false final result, execution error, consensus invalidity, or policy-only rejection with the exact evidence.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Debugging pins transaction, prior output, interpreter version, flags, and initial byte stack, then records every executed transition and the first failed rule.

CHANGED CONDITION

Correcting one defect invalidates the previous trace and requires a full rerun because the new first failure may be false Script, consensus, or policy.

Important distinction: If the final stack is false, the witness must be malformed. A well-formed witness can execute completely and fail a comparison or signature check. Malformation, false predicates, and policy rejection are different diagnoses.

Text equivalent: The accepted path is enforced by Debugging pins transaction, prior output, interpreter version, flags, and initial byte stack, then records every executed transition and the first failed rule. If the required condition changes, Correcting one defect invalidates the previous trace and requires a full rerun because the new first failure may be false Script, consensus, or policy.

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Learn the idea

Script debugging means reproducing validation one operation at a time. A malformed witness has the wrong structure or encoding, a false condition executes normally but ends false, and a policy rejection can occur before or after Script checks without implying consensus failure. Good debugging records transaction context, initial stack, executed branch, stack state, and exact failing rule.

GUIDED EXPLANATION1/5 ideas inspected

Freeze the transaction context

Record raw transaction, prior output amount and script, network, block or mempool context, and software version. Signature checks cannot be debugged from Script text alone.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“If the final stack is false, the witness must be malformed.”

A well-formed witness can execute completely and fail a comparison or signature check. Malformation, false predicates, and policy rejection are different diagnoses.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.