Specify BIP 384: Combo descriptors

Author: achow101

README.mediawiki

@@ -1078,6 +1078,13 @@ Those proposing changes should consider that ultimately consent may rest with th
 | Pieter Wuille, Andrew Chow
 | Informational
 | Draft
+|-
+| [[bip-0384.mediawiki|384]]
+| Applications
+| combo() Output Script Descriptors
+| Pieter Wuille, Andrew Chow
+| Informational
+| Draft
 |}
 
 <!-- IMPORTANT!  See the instructions at the top of this page, do NOT JUST add BIPs here! -->

bip-0380.mediawiki

@@ -258,4 +258,7 @@ This Table lists all available Script expressions and the BIPs specifying them.
 |-
 | <tt>sortedmulti(NUM, KEY, ..., KEY)</tt>
 | [[bip-0383.mediawiki|383]]
+|-
+| <tt>combo(KEY)</tt>
+| [[bip-0384.mediawiki|384]]
 |}

bip-0384.mediawiki

@@ -0,0 +1,48 @@
+<pre>
+  BIP: 384
+  Layer: Applications
+  Title: combo() Output Script Descriptors
+  Author: Pieter Wuille <[email protected]>
+          Andrew Chow <[email protected]>
+  Comments-Summary: No comments yet.
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0384
+  Status: Draft
+  Type: Informational
+  Created: 2021-06-27
+  License: BSD-2-Clause
+</pre>
+
+==Abstract==
+
+This document specifies <tt>combo()</tt> output script descriptors.
+These take a key and produce P2PK, P2PKH, P2WPKH, and P2SH-P2WPKH output scripts if applicable to the key.
+
+==Copyright==
+
+This BIP is licensed under the BSD 2-clause license.
+
+==Motivation==
+
+In order to make the transition from traditional key based wallets to descriptor based wallets easier, it is useful to be able to take a key and produce the scripts which have traditionally been produced by wallet software.
+
+==Specification==
+
+A new top level script expression is defined: <tt>combo(KEY)</tt>.
+This expression can only be used as a top level expression.
+It takes a single key expression as an argument and produces either 2 or 4 output scripts, depending on the key.
+A <tt>combo()</tt> expression always produces a P2PK and P2PKH script, the same as putting the key in both a <tt>pk()</tt> and a <tt>pkh()</tt> expression.
+If the key is/has a compressed public key, then P2WPKH and P2SH-P2WPKH scripts are also produced, the same as putting the key in both a <tt>wpkh()</tt> and <tt>sh(wpkh())</tt> expression.
+
+==Test Vectors==
+
+TBD
+
+==Backwards Compatibility==
+
+<tt>combo()</tt> descriptors use the format and general operation specified in [[bip-0380.mediawiki|380]].
+As this is a wholly new descriptor, it is 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>combo</tt> descriptors have been implemented in Bitcoin Core since version 0.17.