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bip-raw-as-subscript.md

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  BIP: ???
  Layer: Applications
  Title: raw() as subscript in Output Script Descriptors
  Author: Matias Furszyfer 
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-???
  Status: Draft
  Type: Standards Track
  Created: ?
  License: BSD-2-Clause

==Abstract==

This document specifies raw() as subscript for output script descriptors. raw() encapsulates a raw hex script. This BIP allows raw() to be used in the context of other descriptors.

==Copyright==

This BIP is licensed under the BSD 2-clause license.

==Motivation==

Allowing arbitrary hex data to be wrapped in sh(), wsh(), or even within the TREE argument of a tr(KEY, TREE) descriptor enables the representation of currently inexpressible information in the descriptors' language.

Specifically, the absence of this feature limits the representation of non-standard redeem and witness scripts. This occurs because they can currently only be represented as top-level raw(HEX) descriptors, which retain only the output script information and lack the ability to preserve the actual script.

Additionally, as noted here, there are other useful scenarios for this feature. For example, it allows representing in a descriptor that we lack complete knowledge of all solvability paths but can still solve the output. This includes cases like a taproot tree where we know only one of its paths. Or, participating in signing a script without knowing all participants' keys, relying solely on the script structure.

==Specification==

For sh() and wsh() descriptors:

raw(HEX) must represent the arbitrary script data within the provided context. This for example means that a P2SH output script (OP_HASH160 <hash160(HEX_script)> OP_EQUAL) must be created from the provided hex data when a sh(raw(HEX)) descriptor is provided.

Parallelly, a P2WSH output script (OP_0 <hash160(HEX_script)>) must be created from the provided hex data when a wsh(raw(HEX)) descriptor is provided.

For tr(KEY, TREE) descriptors:

Two new fragments are allowed within the taproot TREE context: rawnode(HEX) and raw(HEX).

rawnode(HEX):

Indicating a tree node with specified 32-bit hash, but no specified subtree. This can serve as either a tree branch or the root of the Merkle tree.

raw(HEX):

Defines a tree leaf containing the specified script in hex. Note: The leaf version must be internally fixed at the existing 0xC0 to prevent introducing unsupported or undefined functionality. If a different version is required for any use case, a new BIP could introduce raw(HEX, VERSION) in the future.

==Test Vectors==

Valid descriptors followed by the scripts they produce.

  • sh(raw(5387)) ** a9149e02f205612b4d7fe9466a789764b0eafe7eb07287
  • sh(wsh(raw(5387))) ** a9140d1a6a9fd7e20b6e4091e2c10284fb1130afd46787
  • wsh(raw(5387)) ** 00205c5fc1afc3d712a8e8602cee8590234ab2213be58943fca65436439f08017a64

TODO: Complete examples:

  1. tr(key, rawnode())
  2. tr(key, {pk(), raw()})
  3. tr(key, {pk(), rawnode()})
  4. tr(key, {pk(), sortedmulti_a(2,key1,key2,raw())})
  5. tr(key, {pk(raw())})

Invalid descriptors

  • Non-hex script: sh(raw(asdf))
  • Non 32-bit hash in rawnode: tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd, rawnode(kjke))
  • Non-hex in rawnode: tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd, rawnode(<complete me with a 32-bit non-hex hash>))
  • raw in the key path tr: tr(raw(asdf), pk(key))

==Backwards Compatibility==

raw() as subscript 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.

==Reference Implementation==

TODO: add Bitcoin-Core PR..

==Acknowledgements==

Thanks to Pieter Wuille who came up with the original idea (bitcoin/bitcoin#24114) and brainstorming support.