refactor: move SignSchnorr logic to KeyPair

Move `SignSchnorr` to `KeyPair`. This makes `CKey::SignSchnorr` now
compute a `KeyPair` object and then call `KeyPair::SignSchorr`. The
signing logic is move-only with the exception of changing
`keypair.data()` to `my_keypair->data()`, since we now have access to
the private member `m_keypair`.

Instead of calling memory_cleanse on the `secp256k1_keypair`,
invalidate `KeyPair` by creating an empty one.

src/key.cpp

@@ -271,19 +271,11 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
 
 bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256& aux) const
 {
-    assert(sig.size() == 64);
-    bool ret = true;
     KeyPair keypair = ComputeKeyPair(merkle_root);
     if (!keypair.IsValid()) return false;
-    ret &= secp256k1_schnorrsig_sign32(secp256k1_context_sign, sig.data(), hash.data(), reinterpret_cast<const secp256k1_keypair*>(keypair.data()), aux.data());
-    if (ret) {
-        // Additional verification step to prevent using a potentially corrupted signature
-        secp256k1_xonly_pubkey pubkey_verify;
-        ret &= secp256k1_keypair_xonly_pub(secp256k1_context_static, &pubkey_verify, nullptr, reinterpret_cast<const secp256k1_keypair*>(keypair.data()));
-        ret &= secp256k1_schnorrsig_verify(secp256k1_context_static, sig.data(), hash.begin(), 32, &pubkey_verify);
-    }
-    if (!ret) memory_cleanse(sig.data(), sig.size());
-    memory_cleanse(&keypair, sizeof(keypair));
+    bool ret = keypair.SignSchnorr(hash, sig, aux);
+    /* Clear the keypair */
+    keypair = KeyPair();
     return ret;
 }
 
@@ -435,6 +427,21 @@ KeyPair::KeyPair(const CKey& key, const uint256* merkle_root)
     if (!ret) ClearKeyPairData();
 }
 
+bool KeyPair::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256& aux) const
+{
+    assert(sig.size() == 64);
+    bool ret = 1;
+    ret &= secp256k1_schnorrsig_sign32(secp256k1_context_sign, sig.data(), hash.data(), reinterpret_cast<const secp256k1_keypair*>(m_keypair->data()), aux.data());
+    if (ret) {
+        // Additional verification step to prevent using a potentially corrupted signature
+        secp256k1_xonly_pubkey pubkey_verify;
+        ret &= secp256k1_keypair_xonly_pub(secp256k1_context_static, &pubkey_verify, nullptr, reinterpret_cast<const secp256k1_keypair*>(m_keypair->data()));
+        ret &= secp256k1_schnorrsig_verify(secp256k1_context_static, sig.data(), hash.begin(), 32, &pubkey_verify);
+    }
+    if (!ret) memory_cleanse(sig.data(), sig.size());
+    return ret;
+}
+
 bool ECC_InitSanityCheck() {
     CKey key = GenerateRandomKey();
     CPubKey pubkey = key.GetPubKey();