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123 lines
6.6 KiB
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123 lines
6.6 KiB
Plaintext
<pre>
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BIP: 386
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Layer: Applications
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Title: tr() 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-0386
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Status: Final
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Type: Informational
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Created: 2021-06-27
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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>tr()</tt> output script descriptors.
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<tt>tr()</tt> descriptors take a key and optionally a tree of scripts and produces a P2TR output script.
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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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Taproot added one additional standard output script format: P2TR.
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These expressions allow specifying those formats as a descriptor.
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==Specification==
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A new script expression is defined: <tt>tr()</tt>.
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A new expression is defined: Tree Expressions
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===Tree Expression===
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A Tree Expression (denoted <tt>TREE</tt>) is an expression which represents a tree of scripts.
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The way the tree is represented in an output script is dependent on the higher level expressions.
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A Tree Expression is:
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* Any Script Expression that is allowed at the level this Tree Expression is in.
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* A pair of Tree Expressions consisting of:
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** An open brace <tt>{</tt>
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** A Tree Expression
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** A comma <tt>,</tt>
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** A Tree Expression
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** A closing brace <tt>}</tt>
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===<tt>tr()</tt>===
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The <tt>tr(KEY)</tt> or <tt>tr(KEY, TREE)</tt> expression can only be used as a top level expression.
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All key expressions under any <tt>tr()</tt> expression must create x-only public keys.
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<tt>tr(KEY)</tt> takes a single key expression as an argument and produces a P2TR output script which does not have a script path.
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Each key produced by the key expression is used as the internal key of a P2TR output as specified by [[bip-0341.mediawiki#cite_ref-22-0|BIP 341]].
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Specifically, "If the spending conditions do not require a script path, the output key should commit to an unspendable script path instead of having no script path.
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This can be achieved by computing the output key point as ''Q = P + int(hash<sub>TapTweak</sub>(bytes(P)))G''."
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<pre>
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internal_key: lift_x(KEY)
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32_byte_output_key: internal_key + int(HashTapTweak(bytes(internal_key)))G
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scriptPubKey: OP_1 <32_byte_output_key>
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</pre>
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<tt>tr(KEY, TREE)</tt> takes a key expression as the first argument, and a tree expression as the second argument and produces a P2TR output script which has a script path.
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The keys produced by the first key expression are used as the internal key as specified by [[bip-0341.mediawiki#Constructing_and_spending_Taproot_outputs|BIP 341]].
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The Tree expression becomes the Taproot script tree as described in BIP 341.
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A merkle root is computed from this tree and combined with the internal key to create the Taproot output key.
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<pre>
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internal_key: lift_x(KEY)
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merkle_root: HashTapBranch(TREE)
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32_byte_output_key: internal_key + int(HashTapTweak(bytes(internal_key) || merkle_root))G
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scriptPubKey: OP_1 <32_byte_output_key>
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</pre>
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===Modified Key Expression===
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Key Expressions within a <tt>tr()</tt> expression must only create x-only public keys.
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Uncompressed public keys are not allowed, but compressed public keys would be implicitly converted to x-only public keys.
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The keys derived from extended keys must be serialized as x-only public keys.
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An additional key expression is defined only for use within a <tt>tr()</tt> descriptor:
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* A 64 hex character string representing an x-only public key
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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(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd)</tt>
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** <tt>512077aab6e066f8a7419c5ab714c12c67d25007ed55a43cadcacb4d7a970a093f11</tt>
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* <tt>tr(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1)</tt>
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** <tt>512077aab6e066f8a7419c5ab714c12c67d25007ed55a43cadcacb4d7a970a093f11</tt>
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* <tt>tr(xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc/0/*,pk(xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc/1/*))</tt>
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** <tt>512078bc707124daa551b65af74de2ec128b7525e10f374dc67b64e00ce0ab8b3e12</tt>
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** <tt>512001f0a02a17808c20134b78faab80ef93ffba82261ccef0a2314f5d62b6438f11</tt>
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** <tt>512021024954fcec88237a9386fce80ef2ced5f1e91b422b26c59ccfc174c8d1ad25</tt>
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* <tt>tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,pk(669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))</tt>
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** <tt>512017cf18db381d836d8923b1bdb246cfcd818da1a9f0e6e7907f187f0b2f937754</tt>
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* <tt>tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,{pk(xprvA2JDeKCSNNZky6uBCviVfJSKyQ1mDYahRjijr5idH2WwLsEd4Hsb2Tyh8RfQMuPh7f7RtyzTtdrbdqqsunu5Mm3wDvUAKRHSC34sJ7in334/0),{{pk(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL),pk(02df12b7035bdac8e3bab862a3a83d06ea6b17b6753d52edecba9be46f5d09e076)},pk(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1)}})</tt>
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** <tt>512071fff39599a7b78bc02623cbe814efebf1a404f5d8ad34ea80f213bd8943f574</tt>
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Invalid Descriptors
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* Uncompressed private key: <tt>tr(5KYZdUEo39z3FPrtuX2QbbwGnNP5zTd7yyr2SC1j299sBCnWjss)</tt>
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* Uncompressed public key: <tt>tr(04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235)</tt>
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* <tt>tr()</tt> nested in <tt>wsh</tt>: <tt>wsh(tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>
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* <tt>tr()</tt> nested in <tt>sh</tt>: <tt>sh(tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>
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* <tt>pkh()</tt> nested in <tt>tr</tt>: <tt>tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd, pkh(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1))</tt>
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==Backwards Compatibility==
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<tt>tr()</tt> descriptors use the format and general operation specified in [[bip-0380.mediawiki|380]].
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As these are a set of 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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Tree Expressions are largely incompatible with existing script expressions due to the restrictions in those expressions.
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As of 2021-06-27, the only allowed script expression that can be used in a tree expression is <tt>pk()</tt>.
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However there will be future BIPs that specify script expressions that can be used in tree expressions.
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==Reference Implementation==
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<tt>tr()</tt> descriptors have been implemented in Bitcoin Core since version 22.0.
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