BA-06 · LESSON 05

Witness programs

KEY DEFINITIONS

Terms used in this lesson

Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
P2WPKH: Pay to Witness Public Key Hash: a native SegWit output for one public-key hash.
P2WSH: Pay to Witness Script Hash: a native SegWit output that commits to a witness script.
Hash160: SHA-256 followed by RIPEMD-160, producing the 20-byte hash used by several Bitcoin spending conditions.
SHA-256: A cryptographic hash function used throughout Bitcoin to produce fixed-size 256-bit digests.
Taproot: A Bitcoin output and spending design that can combine a public-key path with privately committed alternative scripts.
P2SH: Pay to Script Hash, a legacy Bitcoin output template that commits to a redeem script.
P2TR: Pay to Taproot, the Bitcoin output type introduced with Taproot.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
Witness transaction ID: An identifier that commits to a transaction including its witness data.

SegWit moves authorization data into witness structures and versions the program semantics.

35 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 · witness programA witness program selects a versioned spending ruleSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
OUTPUT TYPEP2WPKH
A 20-byte program commits to HASH160 of a compressed public key.
02
SCRIPTPUBKEYOP_0 PUSH20 <key hash>
The witness version and program length determine the consensus interpretation.
03
WITNESSSignature and public key
Witness items are serialized outside the legacy transaction body but remain covered by the wtxid.
04
COMMITMENT CHECKHASH160(pubkey) = program
A wrong script or public key fails before its authorization condition can pass.
05
EXECUTIONVerify P2PKH-style condition
SegWit defines the transaction digest and resource accounting used during signature and Script validation.

Rule shown: Version 0 with a 20-byte program is P2WPKH; version 0 with a 32-byte program is P2WSH. The witness is separate from the base transaction serialization.

STAGE 1 OF 5OUTPUT TYPE: P2WPKH

A 20-byte program commits to HASH160 of a compressed public key.

Governing relationship: Version 0 with a 20-byte program is P2WPKH; version 0 with a 32-byte program is P2WSH. The witness is separate from the base transaction serialization. Complete path: 1. OUTPUT TYPE, P2WPKH. A 20-byte program commits to HASH160 of a compressed public key. 2. SCRIPTPUBKEY, OP_0 PUSH20 <key hash>. The witness version and program length determine the consensus interpretation. 3. WITNESS, Signature and public key. Witness items are serialized outside the legacy transaction body but remain covered by the wtxid. 4. COMMITMENT CHECK, HASH160(pubkey) = program. A wrong script or public key fails before its authorization condition can pass. 5. EXECUTION, Verify P2PKH-style condition. SegWit defines the transaction digest and resource accounting used during signature and Script validation.

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

BEFORE

A version-0 output contains 0 <32-byte SHA256(recoveryScript)>.

MECHANISM

A native witness output has a version opcode followed by a canonical data push. The version and program length select defined semantics.

AFTER

The witness program commits to data whose interpretation depends on the witness version and program length.

Text equivalent: Starting from A version-0 output contains 0 <32-byte SHA256(recoveryScript)>. The mechanism begins when A native witness output has a version opcode followed by a canonical data push. The version and program length select defined semantics. The resulting lesson is The witness program commits to data whose interpretation depends on the witness version and program length.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Recognize the program

A native witness output has a version opcode followed by a canonical data push. The version and program length select defined semantics.

Complete path: 1. Recognize the program: A native witness output has a version opcode followed by a canonical data push. The version and program length select defined semantics. 2. Keep scriptSig empty for native spends: Native SegWit inputs use an empty scriptSig. Nested P2SH witness spends use scriptSig only to reveal the witness program. 3. Supply a witness stack: P2WPKH carries a signature and public key. P2WSH carries satisfaction items plus the witness script as its final item. 4. Verify the commitment: P2WPKH checks HASH160 of the key against the 20-byte program. P2WSH checks SHA-256 of the witness script against the 32-byte program. 5. Apply version-specific rules: Nodes that understand the version apply its consensus rules. Versioning was designed so future witness semantics can be introduced by consensus upgrades.

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

ACCEPTED PATH

Witness version and program length select P2WPKH, P2WSH, P2TR, or upgradeable semantics; native scriptSig, program commitment, witness order, and version rules must all match.

CHANGED CONDITION

Invalid v0 length, nonempty native scriptSig, wrong witness-script SHA256, malformed stack, or failed revealed script rejects the spend.

Important distinction: SegWit witness data is optional metadata that full nodes ignore. Witness data is excluded from the legacy txid, but upgraded nodes validate it as required authorization and include it in wtxid and block witness commitments.

Text equivalent: The accepted path is enforced by Witness version and program length select P2WPKH, P2WSH, P2TR, or upgradeable semantics; native scriptSig, program commitment, witness order, and version rules must all match. If the required condition changes, Invalid v0 length, nonempty native scriptSig, wrong witness-script SHA256, malformed stack, or failed revealed script rejects the spend.

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Learn the idea

A witness program is an output script consisting of a version and a program byte string. SegWit version 0 defines P2WPKH for a 20-byte program and P2WSH for a 32-byte program. Version 1 with a 32-byte program defines Taproot under later rules. Witness data is carried separately from scriptSig and is interpreted according to the version.

GUIDED EXPLANATION1/5 ideas inspected

Recognize the program

A native witness output has a version opcode followed by a canonical data push. The version and program length select defined semantics.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“SegWit witness data is optional metadata that full nodes ignore.”

Witness data is excluded from the legacy txid, but upgraded nodes validate it as required authorization and include it in wtxid and block witness commitments.

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.