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102 lines
7.0 KiB
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102 lines
7.0 KiB
Plaintext
<pre>
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BIP: 387
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Layer: Applications
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Title: Tapscript Multisig Output Script Descriptors
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Author: Pieter Wuille <pieter@wuille.net>
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Ava Chow <me@achow101.com>
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Comments-Summary: No comments yet.
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Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0387
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Status: Draft
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Type: Informational
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Created: 2024-04-17
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License: BSD-2-Clause
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</pre>
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==Abstract==
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This document specifies <tt>multi_a()</tt> and <tt>sortedmulti_a()</tt> output script descriptors.
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Like BIP 383's <tt>multi()</tt> and <tt>sortedmulti()</tt>, both functions take a threshold and one
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or more public keys and produce a multisig script. The primary distinction is that <tt>multi_a()</tt>
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and <tt>sortedmulti_a()</tt> only produce tapscripts and are only allowed in a tapscript context.
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==Copyright==
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This BIP is licensed under the BSD 2-clause license.
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==Motivation==
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The most common complex script used in Bitcoin is a threshold multisig.
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These expressions allow specifying multisig scripts as a descriptor.
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==Specification==
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Two new script expressions are defined: <tt>multi_a()</tt> and <tt>sortedmulti_a()</tt>.
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Both expressions produce the scripts of the same template and take the same arguments.
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They are written as <tt>multi_a(k,KEY_1,KEY_2,...,KEY_n)</tt>.
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<tt>k</tt> is the threshold - the number of keys that must sign the input for the script to be valid.
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<tt>KEY_1,KEY_2,...,KEY_n</tt> are the key expressions for the multisig. <tt>k</tt> must be less than or equal to <tt>n</tt>.
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<tt>multi_a()</tt> and <tt>sortedmulti_a()</tt> expressions can only be used inside of a <tt>tr()</tt> descriptor.
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The maximum number of keys is 999.
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The output script produced also depends on the value of <tt>k</tt>. If <tt>k</tt> is less than or equal to 16:
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<pre>
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KEY_1 OP_CHECKSIG KEY_2 OP_CHECKSIGADD ... KEY_n OP_CHECKSIGADD OP_k OP_NUMEQUAL
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</pre>
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if <tt>k</tt> is greater than 16:
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<pre>
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KEY_1 OP_CHECKSIG KEY_2 OP_CHECKSIGADD ... KEY_n OP_CHECKSIGADD k OP_NUMEQUAL
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</pre>
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===<tt>sortedmulti_a()</tt>===
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The only change for <tt>sortedmulti_a()</tt> is that the x-only public keys are sorted lexicographically prior to the creation of the output script.
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This sorting is on the keys that are to be put into the output script, i.e. after all extended keys are derived.
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===Multiple Extended Keys</tt>===
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When one or more of the key expressions in a <tt>multi_a()</tt> or <tt>sortedmulti_a()</tt> expression are extended keys, the derived keys use the same child index.
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This changes the keys in lockstep and allows for output scripts to be indexed in the same way that the derived keys are indexed.
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==Test Vectors==
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Valid descriptors followed by the scripts they produce. Descriptors involving derived child keys will have the 0th, 1st, and 2nd scripts listed.
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* <tt>tr(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1,multi_a(1,KzoAz5CanayRKex3fSLQ2BwJpN7U52gZvxMyk78nDMHuqrUxuSJy))</tt>
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** <tt>5120eb5bd3894327d75093891cc3a62506df7d58ec137fcd104cdd285d67816074f3</tt>
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* <tt>tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,multi_a(1,669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))</tt>
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** <tt>5120eb5bd3894327d75093891cc3a62506df7d58ec137fcd104cdd285d67816074f3</tt>
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* <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(2,[00000000/111'/222]xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0))</tt>
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** <tt>51202eea93581594a43c0c8423b70dc112e5651df63984d108d4fc8ccd3b63b4eafa</tt>
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* <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,sortedmulti_a(2,[00000000/111'/222]xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0))</tt>
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** <tt>512016fa6a6ba7e98c54b5bf43b3144912b78a61b60b02f6a74172b8dcb35b12bc30</tt>
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* <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,sortedmulti_a(2,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0/0/*))</tt>
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** <tt>5120abd47468515223f58a1a18edfde709a7a2aab2b696d59ecf8c34f0ba274ef772</tt>
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** <tt>5120fe62e7ed20705bd1d3678e072bc999acb014f07795fa02cb8f25a7aa787e8cbd</tt>
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** <tt>51201311093750f459039adaa2a5ed23b0f7a8ae2c2ffb07c5390ea37e2fb1050b41</tt>
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* <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(2,xprv9s21ZrQH143K31xYSDQpPDxsXRTUcvj2iNHm5NUtrGiGG5e2DtALGdso3pGz6ssrdK4PFmM8NSpSBHNqPqm55Qn3LqFtT2emdEXVYsCzC2U/2147483647'/0,xprv9vHkqa6EV4sPZHYqZznhT2NPtPCjKuDKGY38FBWLvgaDx45zo9WQRUT3dKYnjwih2yJD9mkrocEZXo1ex8G81dwSM1fwqWpWkeS3v86pgKt/1/2/*,xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi/10/20/30/40/*'))</tt>
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** <tt>5120e4c8f2b0a7d3a688ac131cb03248c0d4b0a59bbd4f37211c848cfbd22a981192</tt>
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** <tt>5120827faedaa21e52fca2ac83b53afd1ab7d4d1e6ce67ff42b19f2723d48b5a19ab</tt>
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** <tt>5120647495ed09de61a3a324704f9203c130d655bf3141f9b748df8f7be7e9af55a4</tt>
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Invalid descriptors
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* Unsupported top level: <tt>multi_a(1,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0)</tt>
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* Unsupported <tt>sh()</tt> context: <tt>sh(multi_a(1,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))</tt>
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* Unsupported <tt>wsh()</tt> context: <tt>wsh(multi_a(1,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))</tt>
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* Invalid threshold: <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(a,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>
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* Threshold of 0: <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(0,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>
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* Uncompressed pubkey: <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(1,04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235))</tt>
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* Threshold larger than keys: <tt>tr(50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0,multi_a(3,L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1,5KYZdUEo39z3FPrtuX2QbbwGnNP5zTd7yyr2SC1j299sBCnWjss))</tt>
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==Backwards Compatibility==
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<tt>multi_a()</tt> and <tt>sortedmulti_a()</tt> descriptors use the format and general operation specified in [[bip-0380.mediawiki|380]].
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As these are wholly new descriptors, they are not compatible with any implementation.
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However, the scripts produced are standard scripts, so existing software are likely to be familiar with them.
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==Reference Implementation==
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<tt>multi_a()</tt> and <tt>sortedmulti_a()</tt> descriptors were implemented in Bitcoin Core in https://github.com/bitcoin/bitcoin/pull/24043 and have been available since version 24.0.
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