XceedEncryptionLib.XceedEncryption encrypt = new XceedEncryptionLib.XceedEncryptionClass(); 

encrypt.License( @"Your license key" ); 

XceedEncryptionLib.XceedRijndaelEncryptionMethod rijndael = new XceedEncryptionLib.XceedRijndaelEncryptionMethodClass();         

try
{
  rijndael.SetSecretKeyFromPassPhrase( "This is a weak pass phrase", 128 );
  encrypt.EncryptionMethod = rijndael;

  object bytesRead = null;      

  encrypt.ProcessFile( @"c:\test\file.txt", 0, 0, 
                       XceedEncryptionLib.EXEFileProcessing.efpEncrypt, true, 
                       @"c:\test\file.enc", false, ref bytesRead );
}
catch( System.Runtime.InteropServices.COMException except )
{
  MessageBox.Show( except.ToString() );
}

XceedEncryptionLib.XceedEncryption encrypt = new XceedEncryptionLib.XceedEncryptionClass(); 

encrypt.License( @"Your license key" ); 

XceedEncryptionLib.XceedRijndaelEncryptionMethod rijndael = new XceedEncryptionLib.XceedRijndaelEncryptionMethodClass();           

try
{
  rijndael.SetSecretKeyFromPassPhrase( "This is a weak pass phrase", 128 );
  encrypt.EncryptionMethod = rijndael; 

  object bytesRead = null;      

  encrypt.ProcessFile( @"c:\test\file.enc", 0, 0,
                       XceedEncryptionLib.EXEFileProcessing.efpDecrypt, true,
                       @"c:\test\file2.txt", false, ref bytesRead );
}
catch( System.Runtime.InteropServices.COMException except )
{
  MessageBox.Show( except.ToString() );
}