Merge branch 'review-fixes'

include/secp256k1_frost.h

@@ -520,6 +520,25 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_frost_sign(
  *  through NonceAgg as a pubnonce contribution, and a pubnonce's components
  *  are never the point at infinity); if it does, this function fails.
  *
+ *  WARNING: the derivation above is the whole of what the nonce depends on. It
+ *  does NOT commit to the pubshares, to the untweaked threshold public key, or
+ *  to which tweaks the cache accumulated -- only to the x-only encoding of the
+ *  _tweaked_ threshold public key (this is BIP 445's det_nonce_hash, not a
+ *  deviation). Two tweak caches can therefore agree on that x-only key and
+ *  still disagree on the sign g*gacc that multiplies the secret share, because
+ *  Q and -Q have the same x-coordinate: a cache initialized from the threshold
+ *  public key and one initialized from its negation are the simplest example.
+ *  Two calls that differ only in that way emit the SAME pubnonce and two
+ *  partial signatures that differ only in the sign of the secret-share term,
+ *  which is two equations in the nonce and the secret share -- the secret
+ *  share falls out of the pair.
+ *
+ *  The caller must therefore treat the tweak cache and the pubshares as fixed
+ *  key material belonging to the group, established once at key generation,
+ *  and never as per-session parameters accepted from a coordinator or any
+ *  other peer. Given that, repeating a call reproduces a byte-identical result
+ *  and is harmless, which is the point of a deterministic nonce.
+ *
  *  Returns: 0 if the arguments are invalid or signing fails, 1 otherwise
  *  Args:             ctx: pointer to a context object
  *  Out:      partial_sig: pointer to a partial_sig object