BIP Draft: unspendable() Descriptor Key Expression

[Andrew Toth]

This is a BIP Draft for an unspendable key expression that can be used as the taproot internal key. The expression creates a provably unspendable key that is deterministically created with only the descriptor. This allows all participants to verify that the keypath is unspendable, while also hiding that fact from outside observers.

Previous discussion on delving https://delvingbitcoin.org/t/unspendable-keys-in-descriptors/304.

https://github.com/bitcoin/bips/pull/1746