Helper method for invoking native methods with a CER.

Contexts/Context.cs

@@ -178,7 +178,7 @@ namespace NSspi
             SecureBuffer paddingBuffer;
             SecureBufferAdapter adapter;
 
-            SecurityStatus status = SecurityStatus.InvalidHandle;
+            SecurityStatus status;
             byte[] result = null;
             int remaining;
             int position;
@@ -242,37 +242,12 @@ namespace NSspi
 
             using( adapter = new SecureBufferAdapter( new [] { trailerBuffer, dataBuffer, paddingBuffer } ) )
             {
-                bool gotRef = false;
-
-                RuntimeHelpers.PrepareConstrainedRegions();
-                try
-                {
-                    this.ContextHandle.DangerousAddRef( ref gotRef );
-                }
-                catch( Exception )
-                {
-                    if( gotRef )
-                    {
-                        this.ContextHandle.DangerousRelease();
-                        gotRef = false;
-                    }
-
-                    throw;
-                }
-                finally
-                {
-                    if( gotRef )
-                    {
-                        status = ContextNativeMethods.DecryptMessage(
-                            ref this.ContextHandle.rawHandle,
-                            adapter.Handle,
-                            0,
-                            0
-                        );
-
-                        this.ContextHandle.DangerousRelease();
-                    }
-                }
+                status = ContextNativeMethods.SafeDecryptMessage(
+                    this.ContextHandle,
+                    0,
+                    adapter.Handle,
+                    0
+                );
             }
 
             if( status != SecurityStatus.OK )

Contexts/ContextNativeMethods.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using System.Runtime.ConstrainedExecution;
 using System.Runtime.InteropServices;
 using System.Text;
@@ -141,5 +142,47 @@ namespace NSspi
 
         [DllImport( "Secur32.dll", EntryPoint = "FreeContextBuffer", CharSet = CharSet.Unicode )]
         public static extern SecurityStatus FreeContextBuffer( IntPtr handle );
+
+        public static SecurityStatus SafeDecryptMessage( 
+            SafeContextHandle handle, 
+            int qualityOfProtection, 
+            IntPtr bufferDescriptor, 
+            int sequenceNumber )
+        {
+            SecurityStatus status = SecurityStatus.InvalidHandle;
+            bool gotRef = false;
+
+            RuntimeHelpers.PrepareConstrainedRegions();
+            try
+            {
+                handle.DangerousAddRef( ref gotRef );
+            }
+            catch( Exception )
+            {
+                if( gotRef )
+                {
+                    handle.DangerousRelease();
+                    gotRef = false;
+                }
+
+                throw;
+            }
+            finally
+            {
+                if( gotRef )
+                {
+                    status = ContextNativeMethods.DecryptMessage(
+                        ref handle.rawHandle,
+                        bufferDescriptor,
+                        0,
+                        0
+                    );
+
+                    handle.DangerousRelease();
+                }
+            }
+
+            return status;
+        }
     }
 }