Pay-to-contract tweak fields for PSBT

The five-minute read

One action, walked through

1. Open BIP 372 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 type is defined as follows: {| ! Name ! ! ! Description ! ! Description ! Versions Requiring Inclusion ! Versions Requiring Exclusion ! Versions Allowing Inclusion |- | P2C Key Tweak | PSBT_IN_P2C_TWEAK = 0x19 | | 33 bytes of compact public key serialization specifying to which keys the P2C tweak may be applied (i.e.
3. Name the object that changes: P2C, PSBT_IN_P2C_TWEAK, MUST.
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 372 at the status the text itself states: Draft.
• The object that has to change is P2C, PSBT_IN_P2C_TWEAK, MUST. 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 draft. It is not a live consensus rule just because it has a number. 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 Pay-to-contract tweak fields for PSBT?
• Can the same signature be replayed on another chain, account, or message?
• Which primitive does the text require: P2C, PSBT_IN_P2C_TWEAK, MUST?

Terms

BIP 372

The public text titled Pay-to-contract tweak fields for PSBT.

Draft

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 type is defined as follows: {| ! Name ! ! ! Description ! ! Description ! Versions Requiring Inclusion ! Versions Requiring Exclusion ! Versions Allowing Inclusion |- | P2C Key Tweak | PSBT_IN_P2C_TWEAK = 0x19 | | 33 bytes of compact public key serialization specifying to which keys the P2C tweak may be applied (i.e.
• this MUST be a value of a public key before the tweak is applied).
• BIP-340 keys are serialized by appending 0x02 byte.

How the mechanism is specified

• Taken from the specification, the next constraint is: this MUST be a value of a public key before the tweak is applied).
• Locate P2C, PSBT_IN_P2C_TWEAK, MUST in BIP 372 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 draft. It is not a live consensus rule just because it has a number.

Why a venture studio still reads it

Use BIP 372 when a pitch says 'Pay-to-contract tweak fields for PSBT' without saying whether the rule is consensus, policy, or an interface. The number is the citation. The pitch is not.