Taproot Fields for PSBT

The five-minute read

One action, walked through

1. Open BIP 371 and read the status line before the examples.
2. Write down the rule in one sentence. A fair version of that sentence is: The new per-input types are defined as follows: {| ! Name ! ! ! Description ! ! Description ! Versions Requiring Inclusion ! Versions Requiring Exclusion ! Versions Allowing Inclusion |- | Taproot Key Spend Signature | PSBT_IN_TAP_KEY_SIG = 0x13 | None | No key data '''Why is there no key data for PSBT_IN_TAP_KEY_SIG '''The signature in a key path spend corresponds directly with the pubkey provided in the output script.
3. Name the object that changes: PSBT_IN_TAP_KEY_SIG, PSBT_IN_FINAL_SCRIPTWITNESS, PSBT_IN_TAP_SCRIPT_SIG.
4. Ask what an old client, an old contract, or an offline counterparty does. If the document is silent, the silence is part of the design.

The argument, unpacked

What has to be true

• You are implementing BIP 371 at the status the text itself states: Deployed.
• The object that has to change is PSBT_IN_TAP_KEY_SIG, PSBT_IN_FINAL_SCRIPTWITNESS, PSBT_IN_TAP_SCRIPT_SIG. A neighbouring document with a similar name is not this one.
• Wallet support is not the same as consensus validity.

What happened after the paper

The source marks this deployed. That is a statement about the text, not a promise that every wallet or node has shipped it. Later documents can narrow, replace, or ignore this one. Cite the number you mean.

What to check before you use the idea

• What is covered by the signed bytes in Taproot Fields for PSBT?
• Can the same signature be replayed on another chain, account, or message?
• Which primitive does the text require: PSBT_IN_TAP_KEY_SIG, PSBT_IN_FINAL_SCRIPTWITNESS, PSBT_IN_TAP_SCRIPT_SIG?

Terms

BIP 371

The public text titled Taproot Fields for PSBT.

Deployed

The document's own label for how finished the text is. It is not a market fact.

The problem the paper names

A signature that does not commit to what the user saw can be replayed into a different spend, a different chain, or a different message.

What the design proposes

• The new per-input types are defined as follows: {| ! Name ! ! ! Description ! ! Description ! Versions Requiring Inclusion ! Versions Requiring Exclusion ! Versions Allowing Inclusion |- | Taproot Key Spend Signature | PSBT_IN_TAP_KEY_SIG = 0x13 | None | No key data '''Why is there no key data for PSBT_IN_TAP_KEY_SIG '''The signature in a key path spend corresponds directly with the pubkey provided in the output script.
• Thus it is not necessary to provide any metadata that attaches the key path spend signature to a particular pubkey.
• | | The 64 or 65 byte Schnorr signature for key path spending a Taproot output.

How the mechanism is specified

• Taken from the specification, the next constraint is: Thus it is not necessary to provide any metadata that attaches the key path spend signature to a particular pubkey.
• Locate PSBT_IN_TAP_KEY_SIG, PSBT_IN_FINAL_SCRIPTWITNESS, PSBT_IN_TAP_SCRIPT_SIG in BIP 371 and apply it to one transaction or call.
• Then check the failure the class of rule always has: a node that did not upgrade, a reverted call, a replayed signature, or a peer that does not speak the message.

What this page does not treat as proven

• A signature scheme is not a privacy system. It hides the signer only if the protocol says so.
• Wallet support is not the same as consensus validity.
• The source marks this deployed. That is a statement about the text, not a promise that every wallet or node has shipped it.

Why a venture studio still reads it

Use BIP 371 when a pitch says 'Taproot Fields for PSBT' without saying whether the rule is consensus, policy, or an interface. The number is the citation. The pitch is not.