`DefiniteDescriptorKey` does not expose a way to get the key out if it's internally a single key

[apoelstra]

Right now if you have a DescriptorPublicKey and you convert it to a DefiniteDescriptorKey with at_derivation_index, you lose any good way to determine whether you have a single key or not, and you lose access to the key.

With DescriptorPublicKey you can just match on it since it's a public enum and is either Single or XPub. But DefiniteDescriptorKey is an opaque struct.

Consider the case where you have a descriptor public key, potentially with hardened wildcards, but it's user-provided so it could be anything. To get a specific public key out of this you need to talk to a hww. With a DescriptorPublicKey you'd match on the key; for Single you directly pull the key out, and for Xpub you pass the derivation path over USB to your hww and use the result. But now with DefiniteDescriptorKey you're forced to call derive_public_key, which requires you have a secp context available, and only after this fails do you know that you need to switch to the hww.

I'm not sure what the best solution here is. Probably just a single_key -> Option<SinglePub> method on DefiniteDescriptorKey.

[apoelstra]

Ah, if we have an owned DefiniteDescriptorKey you can convert it back to a DescriptorPublicKey with From. We should add a similar conversion method for a borrowed DefiniteDescriptorKey and also a dedicated method.

[evd0kim]

Hi, @apoelstra,
do I understand it right that there are no convenient ways to use DescriptorPublicKey and DescriptorSecretKey for signing PSBT? This issue leaves such impression since I already going through KeyMap and trying to match DescriptorSecretKey, extract key and sign with it and obtaining MiniscriptError(CouldNotSatisfy) in the end.

[apoelstra]

@engenegr no, this issue doesn't really have anything to do with PSBT or with signing ergonomics. It is also fixed by #492 and should be closed.

Having said that, if you are going trying to sign with in-memory keys, you will need to have a map (of some sort) from public keys to private keys. This library doesn't really provide facilities for that map, or any operations with secret keys, really, other than to support serializing or deserializing private descriptors.