Subject: Re: [bitcoin-dev] BIP Proposal: Receiving and Change Derivation Paths in a Single Descriptor

From: Andrew Chow <[email-masked]>
Date: Tue, 26 Jul 2022 22:27:58 +0000

I went with just tuples of two values because that's easier to implement and targets exactly what people were asking for. However I don't see why this couldn't generalize to any sized tuples. As long as the tuples are all the same length, and the limit is one tuple per key expression, then we don't get any combinatorial blowup issues.

Are there any use cases for variable length tuples?

Andrew

On 07/26/2022 05:56 PM, Pavol Rusnak wrote:

> Thanks Andrew for this BIP. We've been already using this for quite some time for Trezor in production.
>
> Just one clarification: Should <NUM;NUM;NUM>, <NUM;NUM;NUM;NUM>, ... also work or we only aim to support only tuples of exactly two values?
>
> On Tue, 26 Jul 2022 at 23:51, Andrew Chow via bitcoin-dev <[email-masked]> wrote:
>
>> Hi All,
>>
>> I would like to propose a BIP that de-duplicates and simplifies how we
>> represent descriptors for receiving and change addresses. Under the
>> existing BIPs, this requires two descriptors, where the vast majority of
>> the descriptors are the same, except for a single derivation path
>> element. This proposal allows descriptors to have a single derivation
>> path element that can specify a pair of indexes. Parsers would then
>> expand these into two almost identical descriptors with the difference
>> being that the first uses the first of the pair of indexes, and the
>> second uses the second.
>>
>> The proposed notation is `<a;b>`. As an example,
>> `wpkh(xpub.../0/<0;1>/*)` would be expanded into `wpkh(xpub.../0/0/*)`
>> and `wpkh(xpub.../0/1/*)`.
>>
>> This also works for descriptors involving multiple keys - the first
>> element in every pair is used for the first descriptor, and the second
>> element of each pair in the second descriptor.
>>
>> The full text of the BIP can be found at
>> https://github.com/achow101/bips/blob/bip-multipath-descs/bip-multipath-descs.mediawiki
>> and also copied below. An implementation of it to Bitcoin Core is
>> available at https://github.com/bitcoin/bitcoin/pull/22838.
>>
>> Any feedback on this would be appreciated.
>>
>> Thanks,
>> Andrew Chow
>>
>> ---
>>
>> <pre>
>> BIP: multipath-descs
>> Layer: Applications
>> Title: Multipath Descriptor Key Expressions
>> Author: Andrew Chow <[email-masked]>
>> 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 pair 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> 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></tt> is encountered, parsers should produce two
>> descriptors where the first descriptor uses the first <tt>NUM</tt>, and
>> a second descriptor uses the second <tt>NUM</tt>.
>>
>> 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
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> [email-masked]
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
> --
>
> Best Regards / S pozdravom,
>
> Pavol "stick" Rusnak
> Co-Founder, SatoshiLabs