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

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

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

object source = "This is the data to encrypt";

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

  encrypt.EncryptionMethod = rijndael; 

  encrypt.WriteFile( ref source, XceedEncryptionLib.EXEFileProcessing.efpEncrypt, true, 
                     @"c:\test\file.enc", false );      
}
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;
  object decryptedData = encrypt.ReadFile( @"c:\test\file.enc", 0, 0,
                                           XceedEncryptionLib.EXEFileProcessing.efpDecrypt,
                                           true, ref bytesRead );

  MessageBox.Show( System.Text.Encoding.Unicode.GetString( ( byte[] )decryptedData ) );        
}
catch( System.Runtime.InteropServices.COMException except )
{
  MessageBox.Show( except.ToString() );
}