Specify BIP 383: Multisig descriptors

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+<pre>
+  BIP: 383
+  Layer: Applications
+  Title: Multisig 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-0383
+  Status: Draft
+  Type: Informational
+  Created: 2021-06-27
+  License: BSD-2-Clause
+</pre>
+
+==Abstract==
+
+This document specifies <tt>multi()</tt>, and <tt>sortedmulti()</tt> output script descriptors.
+Both functions take a threshold and one or more public keys and produce a multisig output script.
+<tt>multi()</tt> specifies the public keys in the output script in the order given in the descriptor while <tt>sortedmulti()</tt> sorts the public keys lexicographically when the output script is produced.
+
+==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: <tt>multi()</tt>, and <tt>sortedmulti()</tt>.
+Both expressions produce the scripts of the same template and take the same arguments.
+They are written as <tt>multi(k,KEY_1,KEY_2,...,KEY_n)</tt>.
+<tt>k</tt> is the threshold - the number of keys that must sign the input for the script to be valid.
+<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>.
+
+<tt>multi()</tt> and <tt>sortedmulti()</tt> expressions can be used as a top level expression, or inside of either a <tt>sh()</tt> or <tt>wsh()</tt> descriptor.
+Depending on the higher level descriptors, there may be restrictions on the type of public keys that can be included.
+
+Depending on the higher level descriptors, there are also restrictions on the number of keys that can be present, i.e. the maximum value of <tt>n</tt>.
+When used at the top level, there can only be at most 3 keys.
+When used inside of a <tt>sh()</tt> expression, there can only be most 15 compressed public keys (this is limited by the P2SH script limit).
+Otherwise the maximum number of keys is 20.
+
+The output script produced also depends on the value of <tt>k</tt>. If <tt>k</tt> is less than or equal to 16:
+<pre>
+OP_k KEY_1 KEY_2 ... KEY_n OP_CHECKMULTISIG
+</pre>
+
+if <tt>k</tt> is greater than 16:
+<pre>
+k KEY_1 KEY_2 ... KEY_n OP_CHECKMULTISIG
+</pre>
+
+===<tt>sortedmulti()</tt>===
+
+The only change for <tt>sortedmulti()</tt> is that the 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 the key expressions in a <tt>multi()</tt> or <tt>sortedmulti()</tt> 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==
+
+TBD
+
+==Backwards Compatibility==
+
+<tt>multi()</tt>, and <tt>sortedmulti()</tt> descriptors use the format and general operation specified in [[bip-0380.mediawiki|380]].
+As these are a 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 Implemntation==
+
+<tt>multi()</tt>, and <tt>multi()</tt> descriptors have been implemented in Bitcoin Core since version 0.17.