Hi Andrew,

I'm confused why PSBT_IN_TAP_BIP32_DERIVATION and PSBT_OUT_TAP_BIP32_DERIVATION
contain not just the derivation path for the xonlypubkey, but also the tapleaf merkle path.

First I thought it was perhaps necessary in order for a signer to guess which
script leaves it can sign with its own keys. But you can't really know that without
actually seeing the script. When a signer looks at a script, it presumably already
knows the leaf path.

- Sjors

> Op 22 jun. 2021, om 23:22 heeft Andrew Chow via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> het volgende geschreven:
> 
> Hi All,
> 
> I would like to propose a BIP which defines new fields for Taproot
> support in PSBT.
> 
> The full text is below, and the rendered file can be found at

Now at: https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki <https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki>

> -
> | Taproot Key BIP 32 Derivation Path
> | <tt>PSBT_IN_TAP_BIP32_DERIVATION = 0x16</tt>
> | <tt><xonlypubkey></tt>
> | The 32 byte X-only public key
> | <tt><hashes len> <leaf hash>* <32-bit uint> <32-bit uint>*</tt>
> | A compact size unsigned integer representing the number of leaf
> hashes, followed by a list of leaf hashes, followed by the master key
> fingerprint concatenated with the derivation path of the public key. The
> derivation path is represented as 32-bit little endian unsigned integer
> indexes concatenated with each other. Public keys are those needed to
> spend this output. The leaf hashes are of the leaves which involve this
> public key.

> |-
> | Taproot Key BIP 32 Derivation Path
> | <tt>PSBT_OUT_TAP_BIP32_DERIVATION = 0x07</tt>
> | <tt><xonlypubkey></tt>
> | The 32 byte X-only public key
> | <tt><hashes len> <leaf hash>* <32-bit uint> <32-bit uint>*</tt>
>