Fix MLDsaImplementation on Windows to throw a more clear exception and add a test

Author: vcsjones

src/libraries/Common/src/System/Security/Cryptography/MLDsaImplementation.Windows.cs

@@ -34,8 +34,15 @@ private MLDsaImplementation(
         [MemberNotNullWhen(true, nameof(s_algHandle))]
         internal static partial bool SupportsAny() => s_algHandle is not null;
 
-        protected override void SignDataCore(ReadOnlySpan<byte> data, ReadOnlySpan<byte> context, Span<byte> destination) =>
+        protected override void SignDataCore(ReadOnlySpan<byte> data, ReadOnlySpan<byte> context, Span<byte> destination)
+        {
+            if (!_hasSecretKey)
+            {
+                throw new CryptographicException(SR.Cryptography_MLDsaNoSecretKey);
+            }
+
             Interop.BCrypt.BCryptSignHashPqcPure(_key, data, context, destination);
+        }
 
         protected override bool VerifyDataCore(ReadOnlySpan<byte> data, ReadOnlySpan<byte> context, ReadOnlySpan<byte> signature) =>
             Interop.BCrypt.BCryptVerifySignaturePqcPure(_key, data, context, signature);

src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/MLDsa/MLDsaTestsBase.cs

@@ -279,6 +279,18 @@ public void ImportPrivateSeed_Export(MLDsaKeyInfo info)
                 AssertExtensions.SequenceEqual(info.PrivateSeed, export(mldsa)));
         }
 
+        [Theory]
+        [MemberData(nameof(MLDsaTestsData.IetfMLDsaAlgorithms), MemberType = typeof(MLDsaTestsData))]
+        public void SignData_PublicKeyOnlyThrows(MLDsaKeyInfo info)
+        {
+            using MLDsa mldsa = ImportPublicKey(info.Algorithm, info.PublicKey);
+            byte[] destination = new byte[info.Algorithm.SignatureSizeInBytes];
+            CryptographicException ce =
+                Assert.ThrowsAny<CryptographicException>(() => mldsa.SignData("hello"u8, destination));
+
+            Assert.DoesNotContain("unknown", ce.Message, StringComparison.OrdinalIgnoreCase);
+        }
+
         protected static void ExerciseSuccessfulVerify(MLDsa mldsa, byte[] data, byte[] signature, byte[] context)
         {
             ReadOnlySpan<byte> buffer = [0, 1, 2, 3];