From 761ef12782dc02af48b3e3b5a5238faeec6870fe Mon Sep 17 00:00:00 2001
From: Andrew Chow <achow101-github@achow101.com>
Date: Tue, 29 Jun 2021 16:33:02 -0400
Subject: Specify BIP 386: Taproot descriptors

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+<pre>
+  BIP: 386
+  Layer: Applications
+  Title: tr() Output Script Descriptors
+  Author: Pieter Wuille <pieter@wuille.net>
+          Andrew Chow <andrew@achow101.com>
+  Comments-Summary: No comments yet.
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0386
+  Status: Draft
+  Type: Informational
+  Created: 2021-06-27
+  License: BSD-2-Clause
+</pre>
+
+==Abstract==
+
+This document specifies <tt>tr()</tt> output script descriptors.
+<tt>tr()</tt> descriptors take a key and optionally a tree of scripts and produces a P2TR output script.
+
+==Copyright==
+
+This BIP is licensed under the BSD 2-clause license.
+
+==Motivation==
+
+Taproot added one additional standard output script format: P2TR.
+These expressions allow specifying those formats as a descriptor.
+
+==Specification==
+
+A new script expression is defined: <tt>tr()</tt>.
+A new expression is defined: Tree Expressions
+
+===Tree Expression===
+
+A Tree Expression (denoted <tt>TREE</tt>) is an expression which represents a tree of scripts.
+The way the tree is represented in an output script is dependent on the higher level expressions.
+
+A Tree Expression is:
+* Any Script Expression that is allowed at the level this Tree Expression is in.
+* A pair of Tree Expressions consisting of:
+** An open brace <tt>{</tt>
+** A Tree Expression
+** A comma <tt>,</tt>
+** A Tree Expression
+** A closing brace <tt>}</tt>
+
+===<tt>tr()</tt>===
+
+The <tt>tr(KEY)</tt> or <tt>tr(KEY, TREE)</tt> expression can only be used as a top level expression.
+All key expressions under any <tt>tr()</tt> expression must create x-only public keys.
+
+<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.
+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]].
+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.
+This can be achieved by computing the output key point as ''Q = P + int(hash<sub>TapTweak</sub>(bytes(P)))G''."
+
+<pre>
+internal_key:       lift_x(KEY)
+32_byte_output_key: internal_key + int(HashTapTweak(bytes(internal_key)))G
+scriptPubKey:       OP_1 <32_byte_output_key>
+</pre>
+
+<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.
+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]].
+The Tree expression becomes the Taproot script tree as described in BIP 341.
+A merkle root is computed from this tree and combined with the internal key to create the Taproot output key.
+
+<pre>
+internal_key:       lift_x(KEY)
+merkle_root:        HashTapBranch(TREE)
+32_byte_output_key: internal_key + int(HashTapTweak(bytes(internal_key) || merkle_root))G
+scriptPubKey:       OP_1 <32_byte_output_key>
+</pre>
+
+===Modified Key Expression===
+
+Key Expressions within a <tt>tr()</tt> expression must only create x-only public keys.
+Uncompressed public keys are not allowed, but compressed public keys would be implicitly converted to x-only public keys.
+The keys derived from extended keys must be serialized as x-only public keys.
+An additional key expression is defined only for use within a <tt>tr()</tt> descriptor:
+
+* A 64 hex character string representing an x-only public key
+
+==Test Vectors==
+
+TBD
+
+==Backwards Compatibility==
+
+<tt>tr()</tt> descriptors use the format and general operation specified in [[bip-0380.mediawiki|380]].
+As these are a set of 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.
+
+Tree Expressions are largely incompatible with existing script expressions due to the restrictions in those expressions.
+As of 2021-06-27, the only allowed script expression that can be used in a tree expression is <tt>pk()</tt>.
+However there will be future BIPs that specify script expressions that can be used in tree expressions.
+
+==Reference Implementation==
+
+<tt>tr()</tt> descriptors have been implemented in Bitcoin Core since version 22.0.
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