tests: add test for KeyPair

Reuse existing bip340 test vectors.

Author: josibake

src/test/key_tests.cpp

@@ -327,6 +327,16 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
         // Verify those signatures for good measure.
         BOOST_CHECK(pubkey.VerifySchnorr(msg256, sig64));
 
+        // Repeat the same check, but use the KeyPair directly without any merkle tweak
+        KeyPair keypair = key.ComputeKeyPair(/*merkle_root=*/nullptr);
+        CKey keypair_seckey;
+        BOOST_CHECK(keypair.GetKey(keypair_seckey));
+        BOOST_CHECK(key == keypair_seckey);
+        bool kp_ok = keypair.SignSchnorr(msg256, sig64, aux256);
+        BOOST_CHECK(kp_ok);
+        XOnlyPubKey keypair_xonly{keypair_seckey.GetPubKey()};
+        BOOST_CHECK(keypair_xonly.VerifySchnorr(msg256, sig64));
+
         // Do 10 iterations where we sign with a random Merkle root to tweak,
         // and compare against the resulting tweaked keys, with random aux.
         // In iteration i=0 we tweak with empty Merkle tree.
@@ -340,6 +350,16 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
             bool ok = key.SignSchnorr(msg256, sig64, &merkle_root, aux256);
             BOOST_CHECK(ok);
             BOOST_CHECK(tweaked_key.VerifySchnorr(msg256, sig64));
+
+            // Repeat the same check, but use the KeyPair class directly
+            KeyPair keypair = key.ComputeKeyPair(&merkle_root);
+            CKey keypair_seckey;
+            BOOST_CHECK(keypair.GetKey(keypair_seckey));
+            XOnlyPubKey keypair_xonly{keypair_seckey.GetPubKey()};
+            BOOST_CHECK(tweaked_key == keypair_xonly);
+            bool kp_ok = keypair.SignSchnorr(msg256, sig64, aux256);
+            BOOST_CHECK(kp_ok);
+            BOOST_CHECK(keypair_xonly.VerifySchnorr(msg256, sig64));
         }
     }
 }