The WinZip AES encryption method is implemented using a core AES encryption algorithm implementation provided by the .NET framework, along with data formatting that follows a specification first introduced by the WinZip company, hence the name WinZip AES.

Several implementations of the AES encryption algorithm have been added to the .NET framework, all based on the System.Security.Cryptography.SymmetricAlgorithm abstract class.

By default, Xceed Zip for .NET uses the System.Security.Cryptography.RijndaelManaged class. But other implementations are available and can be used by the developer. All implementations will perform the same task and provide the same result. The only difference might be speed in some very specific scenarios.

In most if not all cases, RijndaelManaged is adequate. Our tests have shown that with the average modern CPU, the default RijndaelManaged is as fast as the other implementations. It is therefore recommended that applications should use the default implementation unless they have a very specific speed requirement, as well as very limited installation needs, as the alternative AES implementations require specific versions of Windows. 

To use an alternative AES implementation with Xceed Zip for .NET, you must create a class that implements the Xceed.Zip.IAesAlgorithmManager interface. An instance of that class must then be set to the ZipArchive.AesAlgorithmManager property, or to the ZipWriter.AesAlgorithmManager or ZipReader.AesAlgorithmManager.

The Xceed.Zip.IAesAlgorithmManager interface exists only to allow the component to create instances of the SymmetricAlgorithm class as needed. The following example shows how an implementation of IAesAlgorithmManager would use .NET's AesCryptoServiceProvider.

public class AesManagedManager : IAesAlgorithmManager
{
  public SymmetricAlgorithm CreateAesAlgorithm()
  {
    return new System.Security.Cryptography.AesCryptoServiceProvider();
  }
}

Public Class AesManagedManager
  Implements IAesAlgorithmManager
  Public Function CreateAesAlgorithm() As SymmetricAlgorithm
    Return New System.Security.Cryptography.AesCryptoServiceProvider()
  End Function
End Class

Using a custom implementation of SymmetricAlgorithm is just as simple. The next example uses the implementation from the "CLR Security" project. After adding the project's Security.Cryptography.dll to your application's references, the IAesAlgorithmManager implementation would be as follows:

public class AesCngManager : IAesAlgorithmManager
{
  public SymmetricAlgorithm CreateAesAlgorithm()
  {
    return new Security.Cryptography.AesCng();
  }
}

Public Class AesCngManager
  Implements IAesAlgorithmManager
  Public Function CreateAesAlgorithm() As SymmetricAlgorithm
    Return New Security.Cryptography.AesCng()
  End Function
End Class

Then, an instance of your class is given to the component for use when zipping/unzipping circumstances require it:

IAesAlgorithmManager aesManager = new AesManagedManager();
ZipArchive.AesAlgorithmManager = aesManager;
ZipReader.AesAlgorithmManager = aesManager;
ZipWriter.AesAlgorithmManager = aesManager;

Dim aesManager As IAesAlgorithmManager = New AesManagedManager()
ZipArchive.AesAlgorithmManager = aesManager
ZipReader.AesAlgorithmManager = aesManager
ZipWriter.AesAlgorithmManager = aesManager

Setting the value of AesAlgorithmManager to null (Nothing in Visual Basic), the default value, causes the component to use the RijndaelManaged implementation of AES.

The component will call Dispose on the SymmetricAlgorithm instance when it is no longer needed. This includes error conditions.