Make AES work on Windows 7

Creating a key from an AES algorithm then changing the cipher mode on the key is a Windows 8 feature; Windows 7 require that the algorithm be set correctly then the key built from the correct one.

Author: bartonjs

src/Common/src/Interop/Windows/BCrypt/Cng.cs

@@ -164,27 +164,14 @@ private struct BCRYPT_KEY_DATA_BLOB_HEADER
             public const UInt32 BCRYPT_KEY_DATA_BLOB_VERSION1 = 0x1;
         }
 
-        public static void SetCipherMode(this SafeKeyHandle hKey, CipherMode cipherMode)
+        public static void SetCipherMode(this SafeAlgorithmHandle hAlg, string cipherMode)
         {
-            String cipherModePropertyName;
-            switch (cipherMode)
-            {
-                case CipherMode.CBC: 
-                    cipherModePropertyName = "ChainingModeCBC"; 
-                    break;
-                case CipherMode.CTS: 
-                    throw new NotSupportedException();
-                case CipherMode.ECB:
-                    cipherModePropertyName = "ChainingModeECB";
-                    break;
-
-                default:
-                    throw new NotSupportedException();
-            }
-            NTSTATUS ntStatus = Interop.BCryptSetProperty(hKey, "ChainingMode", cipherModePropertyName, (cipherModePropertyName.Length + 1) * 2, 0);
+            NTSTATUS ntStatus = Interop.BCryptSetProperty(hAlg, "ChainingMode", cipherMode, (cipherMode.Length + 1) * 2, 0);
+
             if (ntStatus != NTSTATUS.STATUS_SUCCESS)
+            {
                 throw CreateCryptographicException(ntStatus);
-            return;
+            }
         }
 
         // Note: input and output are allowed to be the same buffer. BCryptEncrypt will correctly do the encryption in place according to CNG documentation.
@@ -300,7 +287,7 @@ private static class Interop
             public static extern unsafe NTSTATUS BCryptGetProperty(SafeBCryptHandle hObject, String pszProperty, byte* pbOutput, int cbOutput, out int pcbResult, int dwFlags);
 
             [DllImport(CngDll, CharSet = CharSet.Unicode)]
-            public static extern unsafe NTSTATUS BCryptSetProperty(SafeBCryptHandle hObject, String pszProperty, String pbInput, int cbInput, int dwFlags);
+            public static extern unsafe NTSTATUS BCryptSetProperty(SafeAlgorithmHandle hObject, String pszProperty, String pbInput, int cbInput, int dwFlags);
 
             [DllImport(CngDll, CharSet = CharSet.Unicode)]
             public static extern NTSTATUS BCryptImportKey(SafeAlgorithmHandle hAlgorithm, IntPtr hImportKey, String pszBlobType, out SafeKeyHandle hKey, IntPtr pbKeyObject, int cbKeyObject, byte[] pbInput, int cbInput, int dwFlags);

src/System.Security.Cryptography.Algorithms/src/Internal/Cryptography/AesCngCryptoTransform.cs

@@ -20,8 +20,9 @@ protected AesCngCryptoTransform(CipherMode cipherMode, PaddingMode paddingMode,
                 _iv = cipherIv.CloneByteArray();
                 _currentIv = new byte[cipherIv.Length];
             }
-            _hKey = s_hAlg.BCryptImportKey(key);
-            _hKey.SetCipherMode(cipherMode);
+
+            SafeAlgorithmHandle hAlg = GetCipherAlgorithm(cipherMode);
+            _hKey = hAlg.BCryptImportKey(key);
             Reset();
         }
 
@@ -72,6 +73,29 @@ protected override void Reset()
             }
         }
 
+        private static SafeAlgorithmHandle GetCipherAlgorithm(CipherMode cipherMode)
+        {
+            // Windows 8 added support to set the CipherMode value on a key,
+            // but Windows 7 requires that it be set on the algorithm before key creation.
+            switch (cipherMode)
+            {
+                case CipherMode.CBC:
+                    return s_hAlgCbc;
+                case CipherMode.ECB:
+                    return s_hAlgEcb;
+                default:
+                    throw new NotSupportedException();
+            }
+        }
+
+        private static SafeAlgorithmHandle OpenAesAlgorithm(string cipherMode)
+        {
+            SafeAlgorithmHandle hAlg = Cng.BCryptOpenAlgorithmProvider(Cng.BCRYPT_AES_ALGORITHM, null, Cng.OpenAlgorithmProviderFlags.NONE);
+            hAlg.SetCipherMode(cipherMode);
+
+            return hAlg;
+        }
+
         protected abstract int UncheckedTransformBlock(SafeKeyHandle hKey, byte[] currentIv, byte[] inputBuffer, int inputOffset, int inputCount, byte[] outputBuffer, int outputOffset);
 
         protected abstract byte[] UncheckedTransformFinalBlock(SafeKeyHandle hKey, byte[] currentIv, byte[] inputBuffer, int inputOffset, int inputCount);
@@ -80,6 +104,7 @@ protected override void Reset()
         private byte[] _iv;         // _iv holds a copy of the original IV for Reset(), until it is cleared by Dispose().
         private byte[] _currentIv;  // CNG mutates this with the updated IV for the next stage on each Encrypt/Decrypt call.
 
-        private static readonly SafeAlgorithmHandle s_hAlg = Cng.BCryptOpenAlgorithmProvider(Cng.BCRYPT_AES_ALGORITHM, null, Cng.OpenAlgorithmProviderFlags.NONE);
+        private static readonly SafeAlgorithmHandle s_hAlgCbc = OpenAesAlgorithm("ChainingModeCBC");
+        private static readonly SafeAlgorithmHandle s_hAlgEcb = OpenAesAlgorithm("ChainingModeECB");
     }
 }