<pre>
  BIP: 384
  Layer: Applications
  Title: combo() 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-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==

Valid descriptors followed by the scripts they produce. Descriptors involving derived child keys will have the 0th, and 1st scripts in additional sub-bullets.

* <tt>combo(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1)</tt>
** <tt>2103a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bdac</tt>
** <tt>76a9149a1c78a507689f6f54b847ad1cef1e614ee23f1e88ac</tt>
** <tt>00149a1c78a507689f6f54b847ad1cef1e614ee23f1e</tt>
** <tt>a91484ab21b1b2fd065d4504ff693d832434b6108d7b87</tt>
* <tt>combo(04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235)</tt>
** <tt>4104a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235ac</tt>
** <tt>76a914b5bd079c4d57cc7fc28ecf8213a6b791625b818388ac</tt>
* <tt>combo([01234567]xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL)</tt>
** <tt>2102d2b36900396c9282fa14628566582f206a5dd0bcc8d5e892611806cafb0301f0ac</tt>
** <tt>76a91431a507b815593dfc51ffc7245ae7e5aee304246e88ac</tt>
** <tt>001431a507b815593dfc51ffc7245ae7e5aee304246e</tt>
** <tt>a9142aafb926eb247cb18240a7f4c07983ad1f37922687</tt>
* <tt>combo(xprvA2JDeKCSNNZky6uBCviVfJSKyQ1mDYahRjijr5idH2WwLsEd4Hsb2Tyh8RfQMuPh7f7RtyzTtdrbdqqsunu5Mm3wDvUAKRHSC34sJ7in334/*)</tt>
** Child 0
*** <tt>2102df12b7035bdac8e3bab862a3a83d06ea6b17b6753d52edecba9be46f5d09e076ac</tt>
*** <tt>76a914f90e3178ca25f2c808dc76624032d352fdbdfaf288ac</tt>
*** <tt>0014f90e3178ca25f2c808dc76624032d352fdbdfaf2</tt>
*** <tt>a91473e39884cb71ae4e5ac9739e9225026c99763e6687</tt>
** Child 1
*** <tt>21032869a233c9adff9a994e4966e5b821fd5bac066da6c3112488dc52383b4a98ecac</tt>
*** <tt>76a914a8409d1b6dfb1ed2a3e8aa5e0ef2ff26b15b75b788ac</tt>
*** <tt>0014a8409d1b6dfb1ed2a3e8aa5e0ef2ff26b15b75b7</tt>
*** <tt>a91473e39884cb71ae4e5ac9739e9225026c99763e6687</tt>

Invalid descriptors

* <tt>combo()</tt> in <tt>sh</tt> : <tt>sh(combo(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>
* <tt>combo()</tt> in <tt>wsh</tt> : <tt>wsh(combo(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>
* Script in <tt>combo()</tt>: <tt>combo(pkh(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))</tt>

==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 Implementation==

<tt>combo()</tt> descriptors have been implemented in Bitcoin Core since version 0.17.
