Handle platform not supported during test discovery.

src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/DSA/DSAKeyGeneration.cs

@@ -122,9 +122,16 @@ private static int GetSecondMin(KeySizes[] keySizes)
 
         private static bool GetHasSecondMinSize()
         {
-            using (DSA dsa = DSAFactory.Create())
+            try
+            {
+                using (DSA dsa = DSAFactory.Create())
+                {
+                    return GetSecondMin(dsa.LegalKeySizes) != int.MaxValue;
+                }
+            }
+            catch (PlatformNotSupportedException)
             {
-                return GetSecondMin(dsa.LegalKeySizes) != int.MaxValue;
+                return false;
             }
         }
     }

src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/RSA/ImportExport.cs

@@ -454,9 +454,9 @@ private static bool TestRsa16384()
 
                 return true;
             }
-            catch (CryptographicException)
+            catch (Exception e) when (e is CryptographicException or PlatformNotSupportedException)
             {
-                // The key is too big for this platform.
+                // The key is too big for this platform or the platform is not supported.
                 return false;
             }
         }

src/libraries/System.Security.Cryptography/tests/DefaultECDiffieHellmanProvider.Browser.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+
+namespace System.Security.Cryptography.EcDiffieHellman.Tests
+{
+    public partial class ECDiffieHellmanProvider : IECDiffieHellmanProvider
+    {
+        public bool IsCurveValid(Oid oid) => false;
+        public bool ExplicitCurvesSupported => false;
+        public bool CanDeriveNewPublicKey => false;
+        public bool SupportsRawDerivation => false;
+        public bool SupportsSha3 =>  false;
+    }
+}

src/libraries/System.Security.Cryptography/tests/DefaultECDsaProvider.Browser.cs

@@ -0,0 +1,14 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+
+namespace System.Security.Cryptography.EcDsa.Tests
+{
+    public partial class ECDsaProvider : IECDsaProvider
+    {
+        public bool IsCurveValid(Oid oid) => false;
+        public bool ExplicitCurvesSupported => false;
+        private static bool IsValueOrFriendlyNameValid(string friendlyNameOrValue) => false;
+    }
+}

src/libraries/System.Security.Cryptography/tests/System.Security.Cryptography.Tests.csproj

@@ -388,14 +388,18 @@
     <Compile Include="DefaultECDsaProvider.Windows.cs" />
     <Compile Include="DefaultECDiffieHellmanProvider.Windows.cs" />
   </ItemGroup>
-  <ItemGroup Condition="'$(TargetPlatformIdentifier)' != 'windows' and '$(UseAndroidCrypto)' != 'true'">
+  <ItemGroup Condition="'$(TargetPlatformIdentifier)' != 'windows' and '$(UseAndroidCrypto)' != 'true' and '$(TargetPlatformIdentifier)' != 'browser'">
     <Compile Include="DefaultECDsaProvider.Unix.cs" />
     <Compile Include="DefaultECDiffieHellmanProvider.Unix.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.Libraries.cs"
              Link="Common\Interop\Unix\Interop.Libraries.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\System.Security.Cryptography.Native\Interop.Initialization.cs"
              Link="Common\Interop\Unix\System.Security.Cryptography.Native\Interop.Initialization.cs" />
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetPlatformIdentifier)' == 'browser'">
+    <Compile Include="DefaultECDsaProvider.Browser.cs" />
+    <Compile Include="DefaultECDiffieHellmanProvider.Browser.cs" />
+  </ItemGroup>
   <ItemGroup Condition="'$(UseAndroidCrypto)' == 'true' or '$(UseAppleCrypto)' == 'true'">
     <Compile Include="X509Certificates\X509StoreMutableTests.cs" />
   </ItemGroup>

src/libraries/System.Security.Cryptography/tests/X509Certificates/CertificateCreation/CertificateRequestChainTests.cs

@@ -534,6 +534,12 @@ private static bool DetectPssSupport()
                 return false;
             }
 
+            if (PlatformDetection.IsBrowser)
+            {
+                // Browser doesn't support PSS or RSA at all.
+                return false;
+            }
+
             using (X509Certificate2 cert = new X509Certificate2(TestData.PfxData, TestData.PfxDataPassword))
             using (RSA rsa = cert.GetRSAPrivateKey())
             {

src/libraries/System.Security.Cryptography/tests/X509Certificates/SignatureSupport.cs

@@ -13,7 +13,23 @@ public class SignatureSupport
         //
         // If there's ever a platform that blocks RSASSA+SHA-1 but doesn't block ECDSA or DSA with SHA-1,
         // the logic here will need to get more complicated.
-        public static bool SupportsX509Sha1Signatures { get; } =
-            System.Security.Cryptography.Tests.SignatureSupport.CanProduceSha1Signature(RSA.Create());
+        public static bool SupportsX509Sha1Signatures { get; } = GetSupportsX509Sha1Signatures();
+
+
+        private static bool GetSupportsX509Sha1Signatures()
+        {
+            RSA rsa;
+
+            try
+            {
+                rsa = RSA.Create();
+            }
+            catch (PlatformNotSupportedException)
+            {
+                return false;
+            }
+
+            return System.Security.Cryptography.Tests.SignatureSupport.CanProduceSha1Signature(rsa);
+        }
     }
 }