.NET Questions and Solutions

Symptoms
When you try to use the RijndaelManaged class to decrypt data, a System.Security.Cryptography.CryptographicException exception may occur. Additionally, you may receive the following error message:

An unhandled exception of type ‘System.Security.Cryptography.CryptographicException’ occurred in mscorlib.dll
Additional information: PKCS7 padding is invalid and cannot be removed.
Resolution
This problem occurs because the initialization vector that you are using to try to decrypt the data is different from the initialization vector that you used to encrypt the data.
The RijndaelManaged class does not zero out an initialization vector. Therefore, each instance of the RijndaelManaged class has an initialization vector that is different from other instances of the RijndaelManaged class. Even if you use the same secret key, you cannot decrypt data that was encrypted by using a different initialization vector.
Note In rare cases, two instances of the RijndaelManaged class may have the same initialization vector.

Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that are required:

• Microsoft Visual Studio

This article assumes that you are familiar with the following topics:

• General familiarity with XML
• General familiarity with Visual C#

XML Serialization

Serialization is the process of taking the state of an object and persisting it in some fashion. The Microsoft .NET Framework includes powerful objects that can serialize any object to XML. The System.Xml.Serialization namespace provides this capability.
Follow these steps to create a console application that creates an object, and then serializes its state to XML:

1. In Visual C#, create a new Console Application project.
2. On the Project menu, click Add Class to add a new class to the project.
3. In the Add New Item dialog box, change the name of the class to clsPerson.
4. Click Add. A new class is created.
   Note In Visual Studio .NET 2003, click Open.
5. Add the following code after the Public Class clsPerson statement

    public   string FirstName;
    public   string MI;
    public   string LastName;

6. Switch to the code window for Program.cs in Visual Studio or for Class1.cs in Visual Studio .NET 2003.
7. In the void Main method, declare and create an instance of the clsPerson class:

   clsPerson p = new clsPerson();

8. Set the properties of the clsPerson object:

   p.FirstName = "Jeff";
   p.MI = "A";
   p.LastName = "Price";

9. The Xml.Serialization namespace contains an XmlSerializer class that serializes an object to XML. When you create an instance of XmlSerializer, you pass the type of the class that you want to serialize into its constructor:

   System.Xml.Serialization.XmlSerializer x = new System.Xml.Serialization.XmlSerializer(p.GetType());

10. The Serialize method is used to serialize an object to XML. Serialize is overloaded and can send output to a TextWriter, Stream, or XMLWriter object. In this example, you send the output to the console:

    x.Serialize(Console.Out,p);
    Console.WriteLine();
    Console.ReadLine();