Proposed BIP for MuSig2 PSBT Fields

The email discusses the absence of adaptor signature support in BIP 327. The sender mentions that the feature is missing in both BIP 327 and libsecp256k1-zkp, but provides a link to the implementation in the latter (https://github.com/BlockstreamResearch/secp256k1-zkp/blob/master/include/secp256k1_musig.h).

The sender then goes on to describe the expected changes for adaptor signature support in BIP 327. These changes include an additional field to specify the adaptor (PSBT_IN_MUSIG2_PUB_ADAPTOR), which signers have to take into account, and another field to specify the adaptor secret (PSBT_IN_MUSIG2_PRIV_ADAPTOR), added by a Signer role. The PartialSigAgg should check if PUB_ADAPTOR is present and incorporate the value from PSBT_IN_MUSIG2_PRIV_ADAPTOR, failing if it isn't present.

The sender also notes that when using adaptor signatures, signers who don't know the adaptor secret will want to ensure that the partial signatures provided by signers who do/might know the secret are valid. However, this validation depends on the protocol and cannot be automated at the PSBT level.

The sender expresses the need for these specifications to be provided as soon as possible so that they can be supported by all signers. Additionally, the sender points out a typo in the document where "participant" is misspelled as "particpant" multiple times. They also suggest improving the readability of the tables by considering putting the description as a separate row, providing an example link (https://github.com/ajtowns/bips/blob/202310-table/bip-musig2-psbt.mediawiki).

In conclusion, the email raises the issue of missing adaptor signature support in BIP 327 and suggests the necessary changes and improvements to address this.