Make signing follow BIP340 exactly w.r.t. aux randomness

libsecp256k1's secp256k1_schnorrsig_sign only follows BIP340 exactly
if an aux_rand32 argument is passed. When no randomness is used
(as is the case in the current codebase here), there is no impact
on security between not providing aux_rand32 at all, or providing
an empty one. Yet, for repeatability/testability it is simpler
to always use an all-zero one.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 4cb2125747..b282f39e6d 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -81,7 +81,8 @@ bool MutableTransactionSignatureCreator::CreateSchnorrSig(const SigningProvider&
     uint256 hash;
     if (!SignatureHashSchnorr(hash, execdata, *txTo, nIn, nHashType, sigversion, *m_txdata, MissingDataBehavior::FAIL)) return false;
     sig.resize(64);
-    if (!key.SignSchnorr(hash, sig, merkle_root, nullptr)) return false;
+    // Use uint256{} as aux_rnd for now.
+    if (!key.SignSchnorr(hash, sig, merkle_root, {})) return false;
     if (nHashType) sig.push_back(nHashType);
     return true;
 }