Support for custom paths

[junderw]

Given an xpub, there should be a way to override the path. Perhaps using some keywords.

If not specified the default is %CHANGE%/%INDEX%

ie

CUSTOM_PATH="123/%CHANGE%/1/8/23/%INDEX%/2/4"

will result in the first receive address paths of:

• 123/0/1/8/23/0/2/4
• 123/0/1/8/23/1/2/4
• 123/0/1/8/23/2/2/4

and the first change address paths of:

• 123/1/1/8/23/0/2/4
• 123/1/1/8/23/1/2/4
• 123/1/1/8/23/2/2/4

Some things to consider as design choices:

1. Should it be possible to have %CHANGE% come AFTER %INDEX%
2. Should it be possible to omit %CHANGE% (if ie. an exchange would rather explicitly specify change address(es) for whatever reason)
3. Should it be further abstracted where variables like %CHANGE% and %INDEX% can be created/added and given parameters (ie. Creating a non-hardened "BIP44 Account"-like system where you only check the next number if there is tx history in any receive address of the previous number)

I would say 1=no, 2=yes, 3="someday... maybe"

[shesek]

This will be made possible with script descriptors.

In the meanwhile, bwt does support specifying a --bare-xpub to derive child keys directly without separate internal/external chains (just xpub.../i instead of both xpub.../0/i and xpub.../1/i). But this assumes that the index is the last derivation, and so couldn't be used with some of your examples.