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diff --git a/bip-p2c.mediawiki b/bip-p2c.mediawiki new file mode 100644 index 0000000..e7e9c61 --- /dev/null +++ b/bip-p2c.mediawiki @@ -0,0 +1,190 @@ +<pre> + BIP: ? + Layer: Applications + Title: Pay-to-contract tweak fields for PSBT + Author: Maxim Orlovsky <orlovsky@lnp-bp.org>, + Andrew Poelstra <apoelstra@wpsoftware.net> + Discussions-To: <bitcoin-dev@lists.linuxfoundation.org> + Comments-URI: <to be assigned> + Status: Draft + Type: Standards Track + Created: 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 key tweaking data data to be included in a PSBT +of any version. These will represent an 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 some +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 on 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 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 +has been developed, including OpenTimeStamps [3], Elements sidechain P2C tweaks +[4] and LNPBP-1 [5], used in 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 just knowns the corresponding original private key, but also is aware about +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 channelge 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 universal +method for spending previous outpus 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 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 of 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 `02` +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 +performs that distinguishment. The type of the key is defined by the input type, +and adding additional PSBT field type 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 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 +addresses construction of a 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 value +(like in some sidechains, single-use-seal applications like RGB etc). Creation +of such outputs much allow hardware devices to understand the structure of such +extra-transaction data, which may be in different formats and constantly +involve. Thus, this should be addresses with a separate standards (or be a +vendor-based). The current proposal only touches the question of spending an +output which contained previously created P2C commitment, which does not creates +a new commitment and does not provides that kind of risk of extra-blockchain +value loses.</ref> + + +==Rationale== + +<references/> + + +==Compatibility== + +The proposal is compatible with the existing consensus rules and does not +require any of their modifications. + +The proposed P2C PSBT fields provides sufficient information for creating a +valid signatures for spendings of 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 addition of new +fields to the PSBT inputs. + + +==Reference implementation== + +WIP + + +==Acknowledgements== + +TBD + + +==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> +[2] Eternity Wall's "sign-to-contract" article. + <https://blog.eternitywall.com/2018/04/13/sign-to-contract/> +[3] Peter Todd. OpenTimestamps: Scalable, Trust-Minimized, Distributed + Timestamping with Bitcoin. + <https://petertodd.org/2016/opentimestamps-announcement> +[4] Adam Back, Matt Corallo, Luke Dashjr, et al. Enabling Blockchain + Innovations with Pegged Sidechains (commit5620e43). Appenxix A. + <https://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> +[6] Peter Todd. Single-use-seals. LNPBP-8 Standard. + <https://github.com/LNP-BP/LNPBPs/blob/master/lnpbp-0008.md> |