Saving work.. working on implementing EncryptMessage. The buffer types and allocation that get passed to the call is really crazy in the sample.

Contexts/Context.cs

@@ -49,5 +49,9 @@ namespace NSspi
 
             this.disposed = true;
         }
+
+        public byte[] Encrypt( byte[] input )
+        {
+        }
     }
 }

Contexts/ContextNativeMethods.cs

@@ -148,5 +148,19 @@ namespace NSspi.Contexts
             SetLastError = true
         )]
         public static extern SecurityStatus DeleteSecurityContext( ref long contextHandle );
+
+        [DllImport(
+            "Secur32.dll",
+            EntryPoint = "EncryptMessag",
+            CallingConvention = CallingConvention.Winapi,
+            CharSet = CharSet.Unicode,
+            SetLastError = true
+        )]
+        public static extern SecurityStatus EncryptMessage(
+            ref long contextHandle,
+            int qualityOfProtection,
+            IntPtr bufferDescriptor,
+            int sequenceNumber
+        );
     }
 }