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.

src/key.cpp

@@ -275,7 +275,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
     return true;
 }
 
-bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256* aux) const
+bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256& aux) const
 {
     assert(sig.size() == 64);
     secp256k1_keypair keypair;
@@ -288,7 +288,7 @@ bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint2
         uint256 tweak = XOnlyPubKey(pubkey_bytes).ComputeTapTweakHash(merkle_root->IsNull() ? nullptr : merkle_root);
         if (!secp256k1_keypair_xonly_tweak_add(GetVerifyContext(), &keypair, tweak.data())) return false;
     }
-    bool ret = secp256k1_schnorrsig_sign(secp256k1_context_sign, sig.data(), hash.data(), &keypair, aux ? (unsigned char*)aux->data() : nullptr);
+    bool ret = secp256k1_schnorrsig_sign(secp256k1_context_sign, sig.data(), hash.data(), &keypair, (unsigned char*)aux.data());
     if (ret) {
         // Additional verification step to prevent using a potentially corrupted signature
         secp256k1_xonly_pubkey pubkey_verify;

src/key.h

@@ -130,7 +130,7 @@ class CKey
 
     /**
      * Create a BIP-340 Schnorr signature, for the xonly-pubkey corresponding to *this,
-     * optionally tweaked by *merkle_root. Additional nonce entropy can be provided through
+     * optionally tweaked by *merkle_root. Additional nonce entropy is provided through
      * aux.
      *
      * merkle_root is used to optionally perform tweaking of the private key, as specified
@@ -143,7 +143,7 @@ class CKey
      *                              (this is used for key path spending, with specific
      *                              Merkle root of the script tree).
      */
-    bool SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root = nullptr, const uint256* aux = nullptr) const;
+    bool SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256& aux) const;
 
     //! Derive BIP32 child key.
     bool Derive(CKey& keyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc) const;

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;
 }

src/test/key_tests.cpp

@@ -321,7 +321,7 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
         key.Set(sec.begin(), sec.end(), true);
         XOnlyPubKey pubkey(key.GetPubKey());
         BOOST_CHECK(std::equal(pubkey.begin(), pubkey.end(), pub.begin(), pub.end()));
-        bool ok = key.SignSchnorr(msg256, sig64, nullptr, &aux256);
+        bool ok = key.SignSchnorr(msg256, sig64, nullptr, aux256);
         BOOST_CHECK(ok);
         BOOST_CHECK(std::vector<unsigned char>(sig64, sig64 + 64) == sig);
         // Verify those signatures for good measure.
@@ -337,7 +337,7 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
             BOOST_CHECK(tweaked);
             XOnlyPubKey tweaked_key = tweaked->first;
             aux256 = InsecureRand256();
-            bool ok = key.SignSchnorr(msg256, sig64, &merkle_root, &aux256);
+            bool ok = key.SignSchnorr(msg256, sig64, &merkle_root, aux256);
             BOOST_CHECK(ok);
             BOOST_CHECK(tweaked_key.VerifySchnorr(msg256, sig64));
         }