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.