<pre>
  BIP: 372
  Layer: Applications
  Title: Pay-to-contract tweak fields for PSBT
  Authors: Maxim Orlovsky <orlovsky@lnp-bp.org>
  Status: Draft
  Type: Specification
  Assigned: 2022-01-16
  License: BSD-2-Clause
  Requires: BIP-174
</pre>

==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 <tt>scriptPubkey</tt>, 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
<tt>PSBT_IN_REDEEM_SCRIPT</tt>, <tt>PSBT_IN_WITNESS_SCRIPT</tt>,
<tt>PSBT_IN_TAP_INTERNAL_KEY</tt> and <tt>PSBT_IN_TAP_LEAF_SCRIPT</tt> 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:

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><keydata></tt> Description
! <tt><valuedata></tt>
! <tt><valuedata></tt> Description
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| P2C Key Tweak
| <tt>PSBT_IN_P2C_TWEAK = 0x19</tt>
| <tt><pubkey></tt>
| 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.<ref>'''Why compressed public keys are not distinguished from BIP-340
public keys''' We follow the logic of BIP32 key derivation, which does not
distinguish them. The type of the key is defined by the input type,
and adding additional PSBT field types will just create the need for handling
errors when the input type does not match the provided key type.</ref>
| <tt><tweak></tt>
| The 32 byte value which MUST be added to a private key to produce a correct
ECDSA and/or Schnorr signature ("key tweak"). Signers SHOULD remove this field
after <tt>PSBT_IN_PARTIAL_SIG</tt> is constructed.
|
|
| 0, 2
| BIP-P2C
|}


==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.<ref>'''Why only spending of P2C tweaked outputs is covered'''
P2C tweaks commit to external data, some of which may represent certain values
(like in some sidechains, single-use-seal applications like RGB, etc). Creation
of such outputs may allow hardware devices to understand the structure of such
extra-transaction data, which may be in different formats and constantly
evolve. Thus, this should be addressed with separate standards (or be
vendor-based). The current proposal only touches the question of spending an
output that contained a previously created P2C commitment, which does not create
a new commitment and does not provide that kind of risk of extra-blockchain
value losses.</ref>


==Rationale==

<references/>


==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]. [https://arxiv.org/pdf/1212.3257.pdf arxiv.org/pdf/1212.3257.pdf]

[2] Eternity Wall. ''Sign-to-contract.'' [https://blog.eternitywall.com/2018/04/13/sign-to-contract/ blog.eternitywall.com]

[3] Peter Todd. ''OpenTimestamps: Scalable, Trust-Minimized, Distributed Timestamping with Bitcoin.'' [https://petertodd.org/2016/opentimestamps-announcement petertodd.org]

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

[5] Maxim Orlovsky, Rene Pickhardt, Federico Tenga, et al. ''Key tweaking: collision-resistant elliptic curve-based commitments. LNPBP-1 Standard.'' [https://github.com/LNP-BP/LNPBPs/blob/master/lnpbp-0001.md LNPBP-1 on GitHub]

[6] Peter Todd. ''Single-use-seals. LNPBP-8 Standard.'' [https://github.com/LNP-BP/LNPBPs/blob/master/lnpbp-0008.md LNPBP-8 on GitHub]


