Enable cross-platform AES-GCM support for .NET 6+ (#3396)

* Initial plan

* Add System.Security.Cryptography.AesGcm support for .NET 6+

Co-authored-by: westin-m <127992899+westin-m@users.noreply.github.com>

* Update tests to support AesGcm on .NET 6+

Co-authored-by: westin-m <127992899+westin-m@users.noreply.github.com>

* Address code review feedback - use consistent null checking

Co-authored-by: westin-m <127992899+westin-m@users.noreply.github.com>

* move conditions for readability

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: westin-m <127992899+westin-m@users.noreply.github.com>

Author: Copilot

src/Microsoft.IdentityModel.Tokens/opensource/AesGcm/AesGcm.cs

@@ -10,8 +10,12 @@ internal class AesGcm : IDisposable
         public const int NonceSize = 12;
         public const int TagSize = 16;
 
+#if NET6_0_OR_GREATER
+        private System.Security.Cryptography.AesGcm _aesGcm;
+#else
         private static readonly SafeAlgorithmHandle s_aesGcm = AesBCryptModes.OpenAesAlgorithm(Cng.BCRYPT_CHAIN_MODE_GCM).Value;
         private SafeKeyHandle _keyHandle;
+#endif
         private bool _disposed;
 
         public AesGcm(byte[] key)
@@ -24,7 +28,17 @@ public AesGcm(byte[] key)
 
         private void ImportKey(byte[] key)
         {
+
+#if NET8_0_OR_GREATER
+            _aesGcm = new System.Security.Cryptography.AesGcm(key, TagSize);
+#elif NET6_0_OR_GREATER
+            // .NET 6 and .NET 7 use the obsolete constructor without tagSizeInBytes parameter
+#pragma warning disable SYSLIB0053 // Type or member is obsolete
+            _aesGcm = new System.Security.Cryptography.AesGcm(key);
+#pragma warning restore SYSLIB0053 // Type or member is obsolete
+#else
             _keyHandle = Interop.BCrypt.BCryptImportKey(s_aesGcm, key);
+#endif
         }
 
         public void Dispose()
@@ -38,21 +52,33 @@ protected virtual void Dispose(bool disposing)
             if (!_disposed && disposing)
             {
                 _disposed = true;
+#if NET6_0_OR_GREATER
+                _aesGcm?.Dispose();
+#else
                 _keyHandle.Dispose();
+#endif
             }
         }
 
         public void Decrypt(byte[] nonce, byte[] ciphertext, byte[] tag, byte[] plaintext, byte[] associatedData = null)
         {
             AesAead.CheckArgumentsForNull(nonce, plaintext, ciphertext, tag);
+#if NET6_0_OR_GREATER
+            _aesGcm.Decrypt(nonce, ciphertext, tag, plaintext, associatedData);
+#else
             AesAead.Decrypt(_keyHandle, nonce, associatedData, ciphertext, tag, plaintext, clearPlaintextOnFailure: true);
+#endif
         }
 
         #region FOR TESTING ONLY
         internal void Encrypt(byte[] nonce, byte[] plaintext, byte[] ciphertext, byte[] tag, byte[] associatedData = null)
         {
             AesAead.CheckArgumentsForNull(nonce, plaintext, ciphertext, tag);
+#if NET6_0_OR_GREATER
+            _aesGcm.Encrypt(nonce, plaintext, ciphertext, tag, associatedData);
+#else
             AesAead.Encrypt(_keyHandle, nonce, associatedData, plaintext, ciphertext, tag);
+#endif
         }
         #endregion
     }

test/Microsoft.IdentityModel.Tokens.Tests/AuthenticatedEncryptionProviderTests.cs

@@ -67,8 +67,8 @@ public class AuthenticatedEncryptionProviderTests
         [Fact]
         public void AesGcmEncryptionOnWindows()
         {
-#if NET10_0_OR_GREATER
-            // AES-GCM is now supported on all platforms in .NET 10+
+#if NET6_0_OR_GREATER
+            // AES-GCM is now supported on all platforms in .NET 6+
             var context = new CompareContext();
             try
             {
@@ -104,8 +104,8 @@ public void AesGcmEncryptionOnWindows()
         [Fact]
         public void AesGcm_Dispose()
         {
-#if !NET10_0_OR_GREATER
-            // In .NET 9 and below, AES-GCM is only supported on Windows
+#if !NET6_0_OR_GREATER
+            // In .NET versions below .NET 6, AES-GCM is only supported on Windows
             if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
                 Assert.Throws<PlatformNotSupportedException>(() => new AuthenticatedEncryptionProvider(Default.SymmetricEncryptionKey256, SecurityAlgorithms.Aes256Gcm));
 #endif

test/Microsoft.IdentityModel.Tokens.Tests/ReferenceTests.cs

@@ -58,8 +58,8 @@ public void ECDH_ESReferenceTest()
         [Fact]
         public void AesGcmReferenceTest()
         {
-#if NET10_0_OR_GREATER
-            // AES-GCM is now supported on all platforms in .NET 10+
+#if NET6_0_OR_GREATER
+            // AES-GCM is now supported on all platforms in .NET 6+
             var context = new CompareContext();
             var providerForDecryption = CryptoProviderFactory.Default.CreateAuthenticatedEncryptionProvider(new SymmetricSecurityKey(RSAES_OAEP_KeyWrap.CEK), AES_256_GCM.Algorithm);
             var plaintext = providerForDecryption.Decrypt(AES_256_GCM.E, AES_256_GCM.A, AES_256_GCM.IV, AES_256_GCM.T);