Adding IEncryptionServices

Implements symetric encoding/decoding services based on a per-tenant key generated randomly during setup in the ShellSettings.
Replaces MachineKey.Encode/Decode usages.
Adding ComputedField to wrap get/set calls from parts, making the Smtp password encrypted in the db automatically.

--HG--
branch : dev

src/Orchard.Web/Modules/Orchard.Setup/Services/SetupService.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Globalization;
 using System.Linq;
+using System.Security.Cryptography;
 using System.Web;
 using Orchard.ContentManagement;
 using Orchard.ContentManagement.MetaData;
@@ -29,6 +30,7 @@ using Orchard.Settings;
 using Orchard.Environment.State;
 using Orchard.Data.Migration;
 using Orchard.Themes.Services;
+using Orchard.Utility.Extensions;
 using Orchard.Widgets.Models;
 using Orchard.Widgets;
 
@@ -118,6 +120,21 @@ namespace Orchard.Setup.Services {
                 shellSettings.DataTablePrefix = context.DatabaseTablePrefix;
             }
 
+            #region Encryption Settings
+
+            // generate random keys for encryption
+            var key = new byte[32];
+            var iv = new byte[16];
+            using ( var random = new RNGCryptoServiceProvider() ) {
+                random.GetBytes(key);
+                random.GetBytes(iv);
+            }
+            
+            shellSettings.EncryptionAlgorithm = "AES";
+            shellSettings.EncryptionKey = key.ToHexString();
+            shellSettings.EncryptionIV = iv.ToHexString();
+            #endregion
+
             var shellDescriptor = new ShellDescriptor {
                 Features = context.EnabledFeatures.Select(name => new ShellFeature { Name = name })
             };