bitcoindev@googlegroups.com

Hi all,

Pursuant to the discussions about standardizing an unspendable NUMS key for an script path only taproot policy https://delvingbitcoin.org/t/unspendable-keys-in-descriptors/304 and https://github.com/bitcoin/bips/pull/1746 ; we undertook the issue with these guiding principles:

- We don't police adherence to other bips except when they directly affect the issue at hand.
- We use the already in place mechanisms in other bips.
- We don't claim semantic canonicalization of the script and only normalize the policy to some opinionated extent.

The gist is, we define _​ as a placeholder for the unspendable NUMS key. We sort keys inside musig(...) and sortedmulti_a. We sort sibling pairs of the tree. If there is multipath extension, we add the same length tuple to the _​ so that we don't fall to reuse. Then we pull the keys out and us @i​ placeholders and hash the whole thing consisting of the normalized descriptor and the keys. That would be the chain code for the H​ defined in BIP 341.

The details can be found here: https://github.com/bitryonix/bips/blob/bip-xxxx-unspendable-internal-keys/bip-xxxx-unspendable-internal-keys.mediawiki

Best, NTL

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