Support fingerprints on descriptors

[ChristopherA]

For reference, Sparrow has this descriptor for the Yimn Blue test seed when used as a segwit '84 account.

wpkh([604b93f2/84h/0h/0h]xpub6DVfq9VduocgjGeR69Nyr8CCi9w5gywnU7wXMYGswpHjffjcbLYNzz6G6555VDcSZLDwZPzJHJQabVWWgkpvYntpunL3UjHGrkCJ6VndbQf/<0;1>/*)#8hmvszgp

A) I believe the #8hmvszgp is the unique fingerprint for this account. If it is truly unique, we need to start using it.

B) We should document this and support it when saving account descriptors to text.

[shannona]

That's the checksum:

https://github.com/bitcoin/bitcoin/blob/master/doc/descriptors.md#checksums

I don't think there's any guarantee it's unique. It's just meant to detect errors in transmission.

It can be calculated using the bitcoin-core getdescriptorinfo RPC call.

[shannona]

Here's a function for the checksum: https://github.com/bitcoin/bitcoin/blob/master/src/script/descriptor.cpp#L95

[wolfmcnally]

Fixed in next beta release.

[shannona]

The checksum now shows up on all output descriptors.

[ChristopherA]

@shannona can you verify the results are same in Sparrow?

[shannona]

As far as I've been able to tell, Sparrow doesn't output descriptors.

The gold standard would be to test against getdescriptorinfo in bitcoin-cli. I can spin up an instance to check, it'll just take a bit.

[shannona]

Running through the descriptors on bitcoin-cli:

• Native Segwit Single Key: CHECKS
• Nested Segwit Single Key: CHECKS
• Legacy Single Key: CHECKS
• Taproot: CHECKS

The three Cosigner descriptors are not checkable because Bitcoin Core does not recognize the "cosigner" function in descriptors:

error code: -5
error message:
A function is needed within P2SH

I in fact can't find cosigner documented anywhere as a valid descriptor, not even in more extensive uses such BDK. @wolfmcnally can you reference where it came from and how standard it is?

In any case, this should be closed, but I want to make sure the compatibility question of the cosigner function is addressed by us.