Musig: Minor comment corrections

[siv2r]

Changes:

• PartialSigVerify mistakenly refers to pubnonce as aggnonce.
  • Edit: Just noticed that this correction was already pointed out here: Add module "musig" that implements MuSig2 multi-signatures (BIP 327) bitcoin-core/secp256k1#1479 (comment)
• KeyAgg comments inconsistently use both x and pk to refer to the public key argument. Updated to use only pk.

[siv2r]

secp256k1-zkp/src/modules/musig/session_impl.h

Lines 579 to 587 in 6152622

	secp256k1_fe_normalize_var(&pk.y);
	/* Negate sk if secp256k1_fe_is_odd(&cache_i.pk.y)) XOR cache_i.parity_acc.
	* This corresponds to the line "Let d = g⋅gacc⋅d' mod n" in the
	* specification. */
	if ((secp256k1_fe_is_odd(&cache_i.pk.y)
	!= cache_i.parity_acc)) {
	secp256k1_scalar_negate(&sk, &sk);
	}

I'm not sure why pk.y is being normalized here. Shouldn't cache_i.pk.y be normalized instead? That's the one passed to the fe_is_odd function, which requires the field element to be normalized.

Just a side note: all tests still pass even after removing this function call (line 579).

[jonasnick]

Thanks, I think all the corrections were either addressed or pointed out in the PR to upstream libsecp256k1.

[jonasnick]

I think it's best if we merge the PR in upstream libsecp first and then merge the changes to this repo.

[real-or-random]

I think it's best if we merge the PR in upstream libsecp first and then merge the changes to this repo.

Yes, this can be closed then. Updating to the upstream musig module is tracked in #301.