BA-04 · LESSON 07

PSBT

A Partially Signed Bitcoin Transaction, called a PSBT, carries one unsigned transaction plus the metadata signers need to review, sign, combine, and finalize it.

Key terms in this lesson 4 definitions

PSBT: Partially Signed Bitcoin Transaction: a format for passing an unsigned or partly signed transaction between constructors and signers.
UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.
Sighash: The exact transaction digest and signature mode that determine which fields a Bitcoin signature authorizes.
Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.

Self-paced Intermediate 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 · psbt signing flowA PSBT carries an unsigned transaction and the context needed to sign itSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
GLOBAL MAPUnsigned transaction
The global section identifies the complete transaction that every signer is being asked to authorize.
02
INPUT MAPUTXO, script, and key origin
Each input map supplies the amount, locking condition, derivation, and other context needed to compute and review the signature hash.
03
OUTPUT MAPPayment and change metadata
Output derivation data lets a wallet identify change without asking the signer to trust the online coordinator.
04
SIGNERVerify then add partial signature
The private key stays with the signer while the PSBT returns a signature bound to the reviewed transaction.
05
FINALIZECombine and extract transaction
The coordinator combines valid signatures, constructs final witnesses, and extracts the broadcast transaction only when every input is complete.

Rule shown: Each signer verifies the unsigned transaction and input metadata independently; missing or contradictory previous-output data must stop signing.

STAGE 1 OF 5GLOBAL MAP: Unsigned transaction

The global section identifies the complete transaction that every signer is being asked to authorize.

Governing relationship: Each signer verifies the unsigned transaction and input metadata independently; missing or contradictory previous-output data must stop signing. Complete path: 1. GLOBAL MAP, Unsigned transaction. The global section identifies the complete transaction that every signer is being asked to authorize. 2. INPUT MAP, UTXO, script, and key origin. Each input map supplies the amount, locking condition, derivation, and other context needed to compute and review the signature hash. 3. OUTPUT MAP, Payment and change metadata. Output derivation data lets a wallet identify change without asking the signer to trust the online coordinator. 4. SIGNER, Verify then add partial signature. The private key stays with the signer while the PSBT returns a signature bound to the reviewed transaction. 5. FINALIZE, Combine and extract transaction. The coordinator combines valid signatures, constructs final witnesses, and extracts the broadcast transaction only when every input is complete.

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

BEFORE

One input requires two signatures from keys A, B, and C. The PSBT proposes one payment and one verified change output.

MECHANISM

A coordinator selects inputs and fixes outputs, amounts, version, sequence, and locktime. This global transaction is the object all signers must review.

AFTER

PSBT transports structured context and signatures. It does not make an unsafe proposal safe or replace signer verification.

Text equivalent: Starting from One input requires two signatures from keys A, B, and C. The PSBT proposes one payment and one verified change output. The mechanism begins when A coordinator selects inputs and fixes outputs, amounts, version, sequence, and locktime. This global transaction is the object all signers must review. The resulting lesson is PSBT transports structured context and signatures. It does not make an unsafe proposal safe or replace signer verification.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 6

Create the unsigned transaction

A coordinator selects inputs and fixes outputs, amounts, version, sequence, and locktime. This global transaction is the object all signers must review.

Complete path: 1. Create the unsigned transaction: A coordinator selects inputs and fixes outputs, amounts, version, sequence, and locktime. This global transaction is the object all signers must review. 2. Attach input context: Each input receives enough prior-output and script information to calculate its amount, spending condition, and signature hash. 3. Attach output context: Key origins and scripts can identify wallet-controlled change. A signer should not infer change merely from output order. 4. Add partial signatures: Each signer checks the full proposal and adds only the signatures it controls. Unknown or proprietary fields may be preserved through coordination. 5. Finalize and extract: When required signatures and scripts are present, a finalizer constructs each scriptSig or witness and extracts the broadcast transaction. 6. Combine compatible copies: Separate PSBT copies for the same unsigned transaction can be merged. Copies describing different unsigned transactions must not be combined as if they were one session.

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

ACCEPTED PATH

All copies must share one unsigned transaction; input values/scripts, sighash context, change origins, valid signatures, and final witness must satisfy policy before extraction.

CHANGED CONDITION

A tx-B signature cannot combine with tx-A, false change blocks signing, and a PSBT container itself is never broadcast as a consensus transaction.

Important distinction: A PSBT is safe to sign because it is not yet a broadcast transaction. A PSBT can encode a theft transaction. If enough signers approve it, finalization makes that transaction broadcastable.

Text equivalent: The accepted path is enforced by All copies must share one unsigned transaction; input values/scripts, sighash context, change origins, valid signatures, and final witness must satisfy policy before extraction. If the required condition changes, A tx-B signature cannot combine with tx-A, false change blocks signing, and a PSBT container itself is never broadcast as a consensus transaction.

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Learn the idea

PSBT stands for Partially Signed Bitcoin Transaction. It is a key-value container that carries an unsigned transaction and the metadata different constructors and signers need. A signature hash, often shortened to sighash, is the exact transaction digest selected by the applicable signature mode. Inputs can include prior-output data, scripts, key origins, sighash type, and partial signatures. Outputs can include derivation data that helps identify change.

GUIDED EXPLANATION1/6 ideas inspected

Create the unsigned transaction

A coordinator selects inputs and fixes outputs, amounts, version, sequence, and locktime. This global transaction is the object all signers must review.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“A PSBT is safe to sign because it is not yet a broadcast transaction.”

A PSBT can encode a theft transaction. If enough signers approve it, finalization makes that transaction broadcastable.

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