PSBT Version 2

The five-minute read

One action, walked through

1. Open BIP 370 and read the status line before the examples.
2. Write down the rule in one sentence. A fair version of that sentence is: PSBT Version 2 (PSBTv2) only specifies new fields and field inclusion/exclusion requirements.
3. Name the object that changes: PSBT, PSBT_GLOBAL_UNSIGNED_TX, PSBT_GLOBAL_VERSION.
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 370 at the status the text itself states: Deployed.
• The object that has to change is PSBT, PSBT_GLOBAL_UNSIGNED_TX, PSBT_GLOBAL_VERSION. 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 PSBT Version 2?
• Can the same signature be replayed on another chain, account, or message?
• Which primitive does the text require: PSBT, PSBT_GLOBAL_UNSIGNED_TX, PSBT_GLOBAL_VERSION?

Terms

BIP 370

The public text titled PSBT Version 2.

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

• PSBT Version 2 (PSBTv2) only specifies new fields and field inclusion/exclusion requirements.
• PSBT_GLOBAL_UNSIGNED_TX must be excluded in PSBTv2.
• PSBT_GLOBAL_VERSION must be included in PSBTv2 and set to version number 2 '''What happened to version number 1?''' Version number 1 is skipped because PSBT Version 0 has been colloquially referred to as version 1.

How the mechanism is specified

• Taken from the specification, the next constraint is: PSBT_GLOBAL_UNSIGNED_TX must be excluded in PSBTv2.
• Locate PSBT, PSBT_GLOBAL_UNSIGNED_TX, PSBT_GLOBAL_VERSION in BIP 370 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 370 when a pitch says 'PSBT Version 2' without saying whether the rule is consensus, policy, or an interface. The number is the citation. The pitch is not.