Raw() as subscript for descriptors

bip-0391.mediawiki

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+<pre>
+  BIP: ???
+  Layer: Applications
+  Title: raw() as subscript in Output Script Descriptors
+  Author: Matias Furszyfer [email protected]
+  Comments-Summary: No comments yet.
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-???
+  Status: Draft
+  Type: Informational
+  Created: 2024-12-16
+  License: BSD-2-Clause
+</pre>
+
+==Abstract==
+
+This document specifies <tt>raw()</tt> as subscript for output script descriptors.
+<tt>raw()</tt> encapsulates a raw hex script. This BIP allows <tt>raw()</tt> 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 [https://github.com/bitcoin/bitcoin/issues/24114#issuecomment-1127978154 here],
+there are other useful scenarios for this feature. For example, it allows representing
+in a descriptor that we lack full knowledge of all solvability paths but can still solve
+the output, such as a taproot tree where we know only one of its paths, or even also
+participating in signing a script without knowing all participants' keys, relying solely
+on the script structure.
+
+==Specification==
+
+Allow raw(HEX) to be accepted within the context of `sh()`, `wsh()` and `tr()` descriptors.
+The output script produced by this descriptor must represent the arbitrary data within
+the provided context. For example, a P2SH output script (OP_HASH160 <hash160(script)> OP_EQUAL)
+must be created from the provided hex data when a `sh(raw(HEX))` descriptor is provided.
+
+==Test Vectors==
+
+Valid descriptors followed by the scripts they produce.
+
+* <tt>sh(raw(5387))</tt>
+** <tt>a9149e02f205612b4d7fe9466a789764b0eafe7eb07287</tt>
+* <tt>sh(wsh(raw(5387)))</tt>
+** <tt>a9140d1a6a9fd7e20b6e4091e2c10284fb1130afd46787</tt>
+* <tt>wsh(raw(5387))</tt>
+** <tt>00205c5fc1afc3d712a8e8602cee8590234ab2213be58943fca65436439f08017a64</tt>
+* <tt>tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,raw(5387))</tt>
+** <tt>51206f108fa13b6f1cdd89d8de731712e2357e3b833f1959c0a93c26127c3a6264bb</tt>
+* <tt>tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,{pk(03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5),raw(%s)})</tt>
+** <tt>51206f4954bd11741938d74e744be6b6fa87076660696ddfa3c41c1f834906e17c8e</tt>
+
+Invalid descriptors
+
+* Non-hex script: <tt>sh(raw(asdf))</tt>
+* <tt>raw</tt> in the key path <tt>tr</tt>: <tt>tr(raw(5387), pk(key))</tt>
+
+==Backwards Compatibility==
+
+<tt>raw()</tt> as a 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..