BIP 0384

BIP: 384
 Layer: Applications
 Title: combo() Output Script Descriptors
 Author: Pieter Wuille <pieter@wuille.net>
 Ava Chow <me@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
==Abstract==
This document specifies combo() 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: combo(KEY).
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 combo() expression always produces a P2PK and P2PKH script, the same as putting the key in both a pk() and a pkh() 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 wpkh() and sh(wpkh()) 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.
* combo(L4rK1yDtCWekvXuE6oXD9jCYfFNV2cWRpVuPLBcCU2z8TrisoyY1)
** 2103a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bdac
** 76a9149a1c78a507689f6f54b847ad1cef1e614ee23f1e88ac
** 00149a1c78a507689f6f54b847ad1cef1e614ee23f1e
** a91484ab21b1b2fd065d4504ff693d832434b6108d7b87
* combo(04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235)
** 4104a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235ac
** 76a914b5bd079c4d57cc7fc28ecf8213a6b791625b818388ac
* combo([01234567]xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL)
** 2102d2b36900396c9282fa14628566582f206a5dd0bcc8d5e892611806cafb0301f0ac
** 76a91431a507b815593dfc51ffc7245ae7e5aee304246e88ac
** 001431a507b815593dfc51ffc7245ae7e5aee304246e
** a9142aafb926eb247cb18240a7f4c07983ad1f37922687
* combo(xprvA2JDeKCSNNZky6uBCviVfJSKyQ1mDYahRjijr5idH2WwLsEd4Hsb2Tyh8RfQMuPh7f7RtyzTtdrbdqqsunu5Mm3wDvUAKRHSC34sJ7in334/*)
** Child 0
*** 2102df12b7035bdac8e3bab862a3a83d06ea6b17b6753d52edecba9be46f5d09e076ac
*** 76a914f90e3178ca25f2c808dc76624032d352fdbdfaf288ac
*** 0014f90e3178ca25f2c808dc76624032d352fdbdfaf2
*** a91408f3ea8c68d4a7585bf9e8bda226723f70e445f087
** Child 1
*** 21032869a233c9adff9a994e4966e5b821fd5bac066da6c3112488dc52383b4a98ecac
*** 76a914a8409d1b6dfb1ed2a3e8aa5e0ef2ff26b15b75b788ac
*** 0014a8409d1b6dfb1ed2a3e8aa5e0ef2ff26b15b75b7
*** a91473e39884cb71ae4e5ac9739e9225026c99763e6687
Invalid descriptors
* combo() in sh : sh(combo(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))
* combo() in wsh : wsh(combo(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))
* Script in combo(): combo(pkh(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd))
==Backwards Compatibility==
combo() descriptors use the format and general operation specified in 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==
combo() descriptors have been implemented in Bitcoin Core since version 0.17.