BIP 0387

BIP: 387
 Layer: Applications
 Title: Tapscript Multisig Output Script Descriptors
 Author: Pieter Wuille <pieter@wuille.net>
 Ava Chow <me@achow101.com>
 Comments-Summary: No comments yet.
 Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0387
 Status: Draft
 Type: Informational
 Created: 2024-04-17
 License: BSD-2-Clause
==Abstract==
This document specifies multi_a() and sortedmulti_a() output script descriptors.
Like BIP 383's multi() and sortedmulti(), both functions take a threshold and one
or more public keys and produce a multisig script. The primary distinction is that multi_a()
and sortedmulti_a() only produce tapscripts and are only allowed in a tapscript context.
==Copyright==
This BIP is licensed under the BSD 2-clause license.
==Motivation==
The most common complex script used in Bitcoin is a threshold multisig.
These expressions allow specifying multisig scripts as a descriptor.
==Specification==
Two new script expressions are defined: multi_a() and sortedmulti_a().
Both expressions produce the scripts of the same template and take the same arguments.
They are written as multi_a(k,KEY_1,KEY_2,...,KEY_n).
k is the threshold - the number of keys that must sign the input for the script to be valid.
KEY_1,KEY_2,...,KEY_n are the key expressions for the multisig. k must be less than or equal to n.
multi_a() and sortedmulti_a() expressions can only be used inside of a tr() descriptor.
The maximum number of keys is 999.
The output script produced also depends on the value of k. If k is less than or equal to 16:
KEY_1 OP_CHECKSIG KEY_2 OP_CHECKSIGADD ... KEY_n OP_CHECKSIGADD OP_k OP_NUMEQUAL
if k is greater than 16:
KEY_1 OP_CHECKSIG KEY_2 OP_CHECKSIGADD ... KEY_n OP_CHECKSIGADD k OP_NUMEQUAL
===sortedmulti_a()===
The only change for sortedmulti_a() is that the x-only public keys are sorted lexicographically prior to the creation of the output script.
This sorting is on the keys that are to be put into the output script, i.e. after all extended keys are derived.
===Multiple Extended Keys</tt>===
When one or more of the key expressions in a multi_a() or sortedmulti_a() expression are extended keys, the derived keys use the same child index.
This changes the keys in lockstep and allows for output scripts to be indexed in the same way that the derived keys are indexed.
==Test Vectors==
Valid descriptors followed by the scripts they produce. Descriptors involving derived child keys will have the 0th, 1st, and 2nd scripts listed.
* tr(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1,multi_a(1,KzoAz5CanayRKex3fSLQ2BwJpN7U52gZvxMyk78nDMHuqrUxuSJy))
** 5120eb5bd3894327d75093891cc3a62506df7d58ec137fcd104cdd285d67816074f3
* tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,multi_a(1,669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))
** 5120eb5bd3894327d75093891cc3a62506df7d58ec137fcd104cdd285d67816074f3
* tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(2,[00000000/111'/222]xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0))
** 51202eea93581594a43c0c8423b70dc112e5651df63984d108d4fc8ccd3b63b4eafa
* tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,sortedmulti_a(2,[00000000/111'/222]xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0))
** 512016fa6a6ba7e98c54b5bf43b3144912b78a61b60b02f6a74172b8dcb35b12bc30
* tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,sortedmulti_a(2,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0/0/*))
** 5120abd47468515223f58a1a18edfde709a7a2aab2b696d59ecf8c34f0ba274ef772
** 5120fe62e7ed20705bd1d3678e072bc999acb014f07795fa02cb8f25a7aa787e8cbd
** 51201311093750f459039adaa2a5ed23b0f7a8ae2c2ffb07c5390ea37e2fb1050b41
* tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(2,xprv9s21ZrQH143K31xYSDQpPDxsXRTUcvj2iNHm5NUtrGiGG5e2DtALGdso3pGz6ssrdK4PFmM8NSpSBHNqPqm55Qn3LqFtT2emdEXVYsCzC2U/2147483647'/0,xprv9vHkqa6EV4sPZHYqZznhT2NPtPCjKuDKGY38FBWLvgaDx45zo9WQRUT3dKYnjwih2yJD9mkrocEZXo1ex8G81dwSM1fwqWpWkeS3v86pgKt/1/2/*,xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi/10/20/30/40/*'))
** 5120e4c8f2b0a7d3a688ac131cb03248c0d4b0a59bbd4f37211c848cfbd22a981192
** 5120827faedaa21e52fca2ac83b53afd1ab7d4d1e6ce67ff42b19f2723d48b5a19ab
** 5120647495ed09de61a3a324704f9203c130d655bf3141f9b748df8f7be7e9af55a4
Invalid descriptors
* Unsupported top level: multi_a(1,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0)
* Unsupported sh() context: sh(multi_a(1,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))
* Unsupported wsh() context: wsh(multi_a(1,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))
* Invalid threshold: tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(a,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))
* Threshold of 0: tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(0,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))
* Uncompressed pubkey: tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(1,04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235))
* Threshold larger than keys: tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(3,L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1,5KYZdUEo39z3FPrtuX2QbbwGnNP5zTd7yyr2SC1j299sBCnWjss))
==Backwards Compatibility==
multi_a() and sortedmulti_a() descriptors use the format and general operation specified in 380.
As these are wholly new descriptors, they are not compatible with any implementation.
However, the scripts produced are standard scripts, so existing software are likely to be familiar with them.
==Reference Implementation==
multi_a() and sortedmulti_a() descriptors were implemented in Bitcoin Core in https://github.com/bitcoin/bitcoin/pull/24043 and have been available since version 24.0.