Specify bip-multipath-descs

Author: achow101

bip-multipath-descs.mediawiki

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+<pre>
+  BIP: multipath-descs
+  Layer: Applications
+  Title: Multipath Descriptor Key Expressions
+  Author: Andrew Chow <[email protected]>
+  Comments-Summary: No comments yet.
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-multipath-descs
+  Status: Draft
+  Type: Informational
+  Created: 2022-07-26
+  License: BSD-2-Clause
+</pre>
+
+==Abstract==
+
+This document specifies a modification to Key Expressions of Descriptors that are described in BIP 380.
+This modification allows Key Expressions to indicate BIP 32 derivation path steps that can have multiple values.
+
+==Copyright==
+
+This BIP is licensed under the BSD 2-clause license.
+
+==Motivation==
+
+Descriptors can describe the scripts that are used in a wallet, but wallets often require at least two descriptors for all of the scripts that they watch for.
+Wallets typically have one descriptor for producing receiving addresses, and the other for change addresses.
+These descriptors are often extremely similar - they produce the same types of scripts, derive keys from the same master key, and use derivation paths that are almost identical.
+The only differences are in the derivation path where one of the steps will be different between the descriptors.
+Thus it is useful to have a notation to represent both descriptors as a single descriptor where one of the derivation steps is a pair of values.
+
+==Specification==
+
+For extended keys and their derivations paths in a Key Expression, BIP 380 states:
+
+* <tt>xpub</tt> encoded extended public key or <tt>xprv</tt> encoded extended private key (as defined in BIP 32)
+** Followed by zero or more <tt>/NUM</tt> or <tt>/NUMh</tt> path elements indicating BIP 32 derivation steps to be taken after the given extended key.
+** Optionally followed by a single <tt>/*</tt> or <tt>/*h</tt> final step to denote all direct unhardened or hardened children.
+
+This is modifed to state:
+
+* <tt>xpub</tt> encoded extended public key or <tt>xprv</tt> encoded extended private key (as defined in BIP 32)
+** Followed by zero or more <tt>/NUM</tt> or <tt>/NUMh</tt> path elements indicating BIP 32 derivation steps to be taken after the given extended key.
+** Followed by zero or one <tt>/<NUM;NUM</tt> (<tt>NUM</tt> may be followed by <tt>h</tt> to indicated a hardened step) path element indicating a tuple of BIP 32 derivation steps to be taken after the given extended key.
+*** Followed by zero or more <tt>;NUM</tt> or <tt>;NUMh</tt> additional tuple values of BIP 32 derivation steps
+*** Followed by a single <tt>></tt>
+** Followed by zero or more <tt>/NUM</tt> or <tt>/NUMh</tt> path elements indicating BIP 32 derivation steps to be taken after the given extended key.
+** Optionally followed by a single <tt>/*</tt> or <tt>/*h</tt> final step to denote all direct unhardened or hardened children.
+
+When a <tt>/<NUM;NUM;...;NUM></tt> is encountered, parsers should produce multiple descriptors where the first descriptor uses the first <tt>NUM</tt>, and a second descriptor uses the second <tt>NUM</tt>, and so on, until each <tt>NUM</tt> is accounted for.
+Descriptors that contain multiple Key Expressions that each have a <tt>/<NUM;NUM;...;NUM></tt> must have tuples of exactly the same length.
+
+The common use case for this is to represent descriptors for producing receiving and change addresses.
+When interpreting for this use case, wallets should use the first descriptor for producing receiving addresses, and the second descriptor for producing change addresses.
+For this use case, the element will commonly be the value <tt>/<0;1></tt>
+
+==Test Vectors==
+
+TBD
+
+==Backwards Compatibility==
+
+This is an addition to the Key Expressions defined in BIP 380.
+Key Expressions using the format described in BIP 380 are compatible with this modification and parsers that implement this will still be able to parse such descriptors.
+However as this is an addition to Key Expressions, older parsers will not be able to understand such descriptors.
+
+This modification to Key Expressions uses two new characters: <tt><</tt> and <tt>;</tt>.
+These are part of the descriptor character set and so are covered by the checksum algorithm.
+As these are previously unused characters, old parsers will not accidentally mistake them for indicating something else.
+
+==Reference Implementation==
+
+https://github.com/bitcoin/bitcoin/pull/22838