独自のモデルバインダーを実装できます
public class DecObjModelBinder : IModelBinder
{   
  public object BindModel(ControllerContext controllerContext, 
      ModelBindingContext bindingContext)
   {
    //make a instance of your object
    var myObj = new DecObj()
    //bind the properties from my obj
    myObj.Title= bindingContext
        .ValueProvider
        .GetValue("Title") // The Property name sent from the browser
        .ToString();
    /* then the property you want to decrypt */
    var encBody = bindingContext
        .ValueProvider
        .GetValue("EncBody") // The Property name sent from the browser
        .ToString();
    /* decryption logic here to the encBody then after assign the decrypted value to myObj*/
    return myObj;
   }
次にModelBinder、Global.asxのApplication_Startに次の方法で登録します。ModelBinders.Binders.Add(typeof(DecObj), new  DecObjModelBinder());