Pay-to-contract tweak fields for PSBT

  BIP: 372
  Layer: Applications
  Title: Pay-to-contract tweak fields for PSBT
  Authors: Maxim Orlovsky 
  Status: Draft
  Type: Specification
  Assigned: 2022-01-16
  License: BSD-2-Clause
  Requires: BIP-174

Introduction

Abstract

This document proposes additional fields for BIP 174 PSBTv0 and BIP 370 PSBTv2 that allow for pay-to-contract (P2C) key tweaking data to be included in a PSBT of any version. These will represent extra-transaction information required for the signer to produce valid signatures spending previous outputs.

Copyright

This BIP is licensed under the 2-clause BSD license.

Background

Key tweaking is a procedure for creating a cryptographic commitment to a message using elliptic curve properties. The procedure uses the discrete log problem (DLP) to commit to an extra-transaction message. This is done by adding to a public key (for which the output owner knows the corresponding private key) a hash of the message multiplied by the generator point G of the elliptic curve. This produces a tweaked public key containing the commitment. Later, in order to spend an output containing the P2C commitment, the same commitment should be added to the corresponding private key.

This type of commitment was originally proposed as a part of the pay to contract concept by Ilja Gerhardt and Timo Hanke in [1] and later used by Eternity Wall [2] for the same purpose. Since that time, multiple different protocols for P2C have been developed, including OpenTimeStamps [3], Elements sidechain P2C tweaks [4] and LNPBP-1 [5], used for constructing Peter Todd's single-use-seals [6] in client-side-validation protocols like RGB.

Motivation

P2C outputs can be detected onchain and spent only if the output owner not only knows the corresponding original private key, but also is aware of a P2C tweak applied to the public key. In order to produce a valid signature, the same tweak value must be added (modulo group order) to the original private key by a signer device. This represents a challenge for external signers, which may not have any information about such commitment. This proposal addresses this issue by adding relevant fields to the PSBT input information.

The proposal abstracts details of specific P2C protocols and provides a universal method for spending previous outputs containing P2C tweaks, applied to the public key contained within any standard form of the scriptPubkey, including bare scripts and P2PK, P2PKH, P2SH, witness v0 P2WPKH, P2WSH, nested witness v0 P2WPKH-P2SH, P2WSH-P2SH and witness v1 P2TR outputs.

Design

P2C-tweaked public keys are already exposed in the PSBT_IN_REDEEM_SCRIPT, PSBT_IN_WITNESS_SCRIPT, PSBT_IN_TAP_INTERNAL_KEY and PSBT_IN_TAP_LEAF_SCRIPT fields; the only information signer is needed to recognize which keys it should sign with is from which of the original keys they were generated. This is achieved by introducing a new `PSBT_IN_P2C_TWEAK` field, which has the original key as a field key and the tweak as a field value. The signer will recognize the keys which are available to it, apply the tweak to them and see in which scripts it was used -- and use this information to apply tweaks for the corresponding private keys and produce valid signatures.

Specification

The new per-input type is defined as follows:

Security considerations

The scope of this proposal is deliberately kept narrow; it addresses only spending of transaction outputs containing P2C tweaks - and does not address construction of new P2C commitments or transactions containing them in their outputs.

Rationale

Compatibility

The proposal is compatible with the existing consensus rules and does not require any modification to them.

The proposed P2C PSBT fields provide sufficient information for creating valid signatures for spending the following output types containing tweaked public keys: - bare scripts, - P2PK, - P2PKH, - P2SH, - witness v0 P2WPKH and P2WSH, - nested witness v0 P2WPKH-P2SH and P2WSH-P2SH

Post-0 witness versions, including taproot outputs and future witness versions, may not be supported or covered by this BIP and may require the addition of new fields to the PSBT inputs.

Reference implementation

WIP

Acknowledgements

The author is grateful to Andrew Poelstra, who provided an initial set of ideas and information with his previous work on the topic, on which this standard was designed.

Test vectors

TBD

References

[1] Ilja Gerhardt, Timo Hanke. Homomorphic Payment Addresses and the Pay-to-Contract Protocol. arXiv:1212.3257 [cs.CR]. arxiv.org/pdf/1212.3257.pdf

[2] Eternity Wall. Sign-to-contract. blog.eternitywall.com

[3] Peter Todd. OpenTimestamps: Scalable, Trust-Minimized, Distributed Timestamping with Bitcoin. petertodd.org

[4] Adam Back, Matt Corallo, Luke Dashjr, et al. Enabling Blockchain Innovations with Pegged Sidechains (commit5620e43). Appendix A. blockstream.com/sidechains.pdf

[5] Maxim Orlovsky, Rene Pickhardt, Federico Tenga, et al. Key tweaking: collision-resistant elliptic curve-based commitments. LNPBP-1 Standard. LNPBP-1 on GitHub

[6] Peter Todd. Single-use-seals. LNPBP-8 Standard. LNPBP-8 on GitHub