Unspendable keys in descriptors

[salvatoshi]

@andrewtoth

If we would like an unspendable internal xpub to be standardized between descriptors and wallet policies, then we must remove duplicates. I think sorting and removing duplicates of all xpubs, then concatenating and SHA256 hashing the result would be the simplest to implement.

That sounds good to me; in the context of wallet policies, it just means “sorting the vector of keys” (and dropping key origins), then concatenating the compressed pubkeys, then hashing.

In descriptors, it would need to be decided what to do (or if _ is supported at all) when there are key expressions that are not xpubs.

@andrewtoth

We would also have to restrict all xpubs to xpub/<0;1>/* with no allowance for optional derivation paths . I think that’s ok because that allowance is for legacy wallets, who would not have taproot support anyways.

This restriction doesn’t quite make sense in the context of wallet policies: it is desired (and expected) that people would use the same xpub in the descriptor with different ‘endings’ for different spending paths (e.g. xpub/<0;1>/* for one spending path, xpub/<2;3>/* for another, etc.). Note that those would indeed be a single xpub in the wallet policy. This is in fact the reason wallet policies keep the /<m;n>/* in the descriptor template rather than the key information vector.

Vendors/wallets, of course, can decide to support a subset of valid descriptors/policies if they want additional properties that the general class of descriptors doesn’t or can’t satisfy.