Unit test for ellswift creation/decoding roundtrip

Co-authored-by: Pieter Wuille <bitcoin-dev@wuille.net>

src/test/key_tests.cpp

@@ -344,4 +344,24 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
     }
 }
 
+BOOST_AUTO_TEST_CASE(key_ellswift)
+{
+    for (const auto& secret : {strSecret1, strSecret2, strSecret1C, strSecret2C}) {
+        CKey key = DecodeSecret(secret);
+        BOOST_CHECK(key.IsValid());
+
+        uint256 ent32 = InsecureRand256();
+        auto ellswift = key.EllSwiftCreate(AsBytes(Span{ent32}));
+
+        CPubKey decoded_pubkey = ellswift.Decode();
+        if (!key.IsCompressed()) {
+            // The decoding constructor returns a compressed pubkey. If the
+            // original was uncompressed, we must decompress the decoded one
+            // to compare.
+            decoded_pubkey.Decompress();
+        }
+        BOOST_CHECK(key.GetPubKey() == decoded_pubkey);
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()