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LNPBP-12: TapRet

LNPBP: 0012
Vertical: Bitcoin protocol
Title: Tapret: taproot-based OP_RETURN deterministic bitcoin commitments
Author: Dr Maxim Orlovsky <[email protected]>,
Peter Todd
Comments-URI: https://github.com/LNP-BP/discussions
Status: Draft
Type: Standards Track
Created: 2022-04-01
Finalized: not yet
License: CC0-1.0
Related standards: LNPBP-4, BIP-341

Abstract

The proposal provides standard mechanism for creating, structuring & verifying deterministic bitcoin commitments, which puts the commitment in a leaf tapscript with OP_RETURN opcode in the taproot script tree.

Background

TBD

Motivation

TBD

Design

Tapret commitment is structured as an OP_RETURN-based script, containing the commitment to the LNPBP-4 message, constructed under multiple protocols. The commitment script (tapret leaf script) always consists of 64 bytes.
The leaf with the tapret leaf script is always put into the same depth of the taproot script tree, and this depth is 1 (in 0-based indexing of depth levels, where depth 0 corresponds to the merkle tree root). The commitment is always put into the rightmost node of the tree by using consensus ordering of the nodes, as it is defined in BIP-341 merkle path construction (lexicographic ordering). This precise definition of the possible place of the commitment allows to prove the uniqueness of the commitment, i.e. the absence of any alternative tapret commitment in the same tree.
If at depth 7 in the rightmost position of the tree another taproot node is present (either leaf script or branch), an additional branch node is created with both tapret leaf script and the parent of the rightmost depth-7 node becoming its children. This new branch node is added instead of the original parent at depth 6, efficiently shifting the previous subtree from that position one level down.
If the original taproot script tree does not have depth 7 nodes on its right-side (in terms of consensus lexicographic ordering of each branch child hashes), a subtree (tapret subtree) consisting of repeated tapret script leaves is created and inserted at the depth of the last node of the tree on the right-side merkle path. The height of the subtree is selected to put the taproot script leaves at depth 7.
Since addition of the new node will change the merkle hashes of all its parents, and the merkle hash value is used in the lexicographic ordering of the tree, this operation with 1 - 1/(2 ^ 7) = 99.22% probability will make just inserted tapret subtree merkle root to be non-rightmost node of the modified tree. Since the deterministic nature of the tapret commitments requires ability to prove the absence of any alternative commitment in the same tree, two components are used to keep the determinism of the commitment position and ability to prove its uniqueness.
First, a special one-byte variable (nonce) is added to the taproot leaf script, which allows "mining" the hash value in a way that the added subtree will appear at the right-side of the tree. At depth 7 there might be only 2^7=128 possible tree position, and 256 iterations (per one nonce value) should be enough to solve the issue.
Second, if it was not possible to solve the issue with the nonce, a special uniqueness proof is produced, which ensures that none of the nodes at depth 7 on the right side of the tree does not contain alternative commitment. This proof includes leaf script (for leaf nodes) or two child node hashes (for branch nodes) for each of the nodes right to the tapret leaf script.
The tapret commitment is put into a transaction output scriptPubkey as a modified BIP-341 output key value, which is produced from the same internal key and new merkle root of the taproot script tree containing the embedded tapret commitment. The number of the transaction output with the commitment, if multiple taproot outputs are present in the same transaction, must be deterministically defined by an upper-level protocol using the present tapret commitment scheme.
Internal key value, merkle proof and uniqueness proof are combined to construct tapret proof, passed to the validators as off-chain data for client-side validation.

Specification

Tapret proof

Tapret proof must consist of the following data, serialized exactly in the given order:
  1. 1.
    32 bytes: internal key value, representing x-only public key serialized according to BIP-341;
  2. 2.
    1 byte: nonce value used in constructing tapret script;
  3. 3.
    1 byte: number n of merkle proof elements (see below), must be in range 0..=7, otherwise the proof must be considered invalid;
  4. 4.
    n*32 bytes: sequence of merkle proof elements, ordered by their depth in the tree. Each of the elements consists of 32 bytes representing hash of the taproot script tree hashing partner;
  5. 5.
    1 byte: number u of uniqueness proof elements (see below), must be less or equal to 2^(n+1) - 2, otherwise the proof must be considered invalid;
  6. 6.
    sequence of m uniqueness proofs, each consisting of: 6.1. 1 byte: proof type, either 0x00, 0x01 or 0x02. The proof type defines length of the proof data and their semantic meaning: 6.1. the proof type 0x00 (called empty) means that node is absent at the depth 7. The proof type is followed by one byte indicating the depth of the last script leaf present in the tree, followed by the structure described in pt. 6.3. below; 6.2. for the proof type 0x01 (called branch) two 32-byte hash values are given; these hash values represents hashes of taproot script tree nodes; 6.3. for the proof type 0x02 (called leaf) a following structure is used: 6.3.1. 1 byte containing script leaf version, 6.3.2. 2 bytes containing script length l in little endian encoding, 6.3.3. l bytes of raw leaf script data.

Tapret verification

First, a tapret script leaf is constructed. The version of the script must be 0xC0 (tapscript) and the script must be equal to the byte sequence (in hex) FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF01<nonce>6A20 followed by a 32-byte serialization of LNPBP-4 multiprotocol commitment. The provided byte sequence represent tapscript consisting of 28 OP_INVALIDOPCODE, followed by OP_PUSHBYTES_1, nonce byte (taken from tapret proof), OP_RETURN and OP_PUSHBYTES_32 op-codes[^why-64-bytes].
Then, tapret subtree is constructed from the tapret script leaves. The tree should be a valid taproot script tree under BIP-341 standard, consisting of 8-n depth levels, where at depth = 7 (for zero-based depth indexing, representing merkle root level) the subtree must contain 2^(7-n) tapret script leaves. For the edge case of n=7 the subtree must consist of a single tapret script leaf which TapLeaf-tagged hash will be the hash of the subtree merkle root.
The merkle root hash of the tapret subtree is used together with the provided node hash partner values from the perkle proof data of the tapret proof to receive the final merkle tree root value. Combined with the value of the internal key from the tapret proof according to BIP-341 algorithm it must produce exactly the same output key value as present in the scriptPubkey of the transaction output containing the commitment; otherwise validation procedure fails.
Finally, the validity of the uniqueness proof must be checked. The uniqueness proof data are used to reconstruct each and all the right-side merkle tree hashing partners, which must match the exact number and values of the hasing partners from the merkle proof, which values were grater than the value of the parent of the tapret leaf script at that depth. If this procedure fails, the validation procedure must fail.

Tapret construction

TBD

Compatibility

TBD

Reference implementation

Rationale

Tapret proof size estimation

Due to the use of nonce "mining" mechanism, in the most of cases the proof size should not exceed 290 bytes. Since the most of the taproot script trees will not have a depth more than 1 or 2, the actual size in these cases will be 66 or 98 bytes.

Why 64 byte tapscript

This allows distinguishing of tapret leaf script from a data used in production of taproot branch hash, such that the proof of the absence of an alternative tapret commitment can be validated by simple comparison of the first of the child node hashes to the tapret leaf script prefix.

Why script depth 1

This helps to keep client-side-validated proof size smaller.

Acknowledgements

TBD

References

TBD
This document is licensed under the Creative Commons CC0 1.0 Universal license.

Test vectors

TBD