AES Encryption

Xceed's implementation of AES encryption lets you incorporate industry standard strong encryption in your .NET applications.

Encryption method

The EncryptionMethod enumeration provides the following values, representing the kind of encryption used:

Value	Description
Compatible	The traditional ZIP encryption. This is a weak encryption method.
WinZipAES	The WinZip AES encryption method. This is a strong encryption method. Note: This encryption method is not currently available in Xceed's .NET Compact Framework products.

You can specify the implementation to be used for AES encryption. See Using alternative AES implementations under Advanced Concepts.

The length of the password is not limited. However, to preserve compatibility with other Zip compression utilities, it is recommended that you limit passwords to no greater than 80 characters. It is also advisable to only use ASCII characters because there is no accepted standard on how to encode non-ASCII characters in passwords.

Encrypting

The following code demonstrates how to use Xceed's AES encryption. WinZipAes is used as the encryption method, and the encryption strength is set to 256 bits:

C#
VB.NET

public void EncryptExample()

{

  AbstractFile zipFile = new DiskFile( @"ZipFile.zip" );



  // If the zip file already exists

  if( zipFile.Exists )

  {

    // Delete it

    zipFile.Delete();

  }



  ZipArchive zip = new ZipArchive( zipFile );



  /* The length of the password is not limited. However, to preserve compatibility

  with other Zip compression utilities, it is recommended that you limit passwords

  to no greater than 80 characters.

  

  It is also advisable to only use ASCII characters because there is no

  accepted standard on how to encode non-ASCII characters in passwords. */

  string password = "MyPassword";



  zip.DefaultEncryptionMethod = EncryptionMethod.WinZipAes;

  zip.DefaultEncryptionStrength = 256;

  zip.DefaultEncryptionPassword = password;



  using( AutoBatchUpdate batch = new AutoBatchUpdate( zip ) )

  {

    AbstractFolder sourceFolder = new DiskFolder( @"D:\Temp\foo" );



    // Zip the contents of the source folder

    sourceFolder.CopyFilesTo( zip, true, true );

  }

}

    Public Sub EncryptExample()

      Dim zipFile As AbstractFile = New DiskFile("ZipFile.zip")



      ' If the zip file already exists

      If zipFile.Exists Then

        ' Delete it

        zipFile.Delete()

      End If



      Dim zip As New ZipArchive(zipFile)



'       The length of the password is not limited. However, to preserve compatibility

'      with other Zip compression utilities, it is recommended that you limit passwords

'      to no greater than 80 characters.

'      

'      It is also advisable to only use ASCII characters because there is no

'      accepted standard on how to encode non-ASCII characters in passwords. 

      Dim password As String = "MyPassword"



      zip.DefaultEncryptionMethod = EncryptionMethod.WinZipAes

      zip.DefaultEncryptionStrength = 256

      zip.DefaultEncryptionPassword = password



      Using batch As New AutoBatchUpdate(zip)

        Dim sourceFolder As AbstractFolder = New DiskFolder("D:\Temp\foo")



        ' Zip the contents of the source folder

        sourceFolder.CopyFilesTo(zip, True, True)

      End Using

    End Sub

Decrypting

Decryption is demonstrated in the following code. Note that it is not necessary to specify the encryption method or strength, as these are detected automatically.

C#
VB.NET

public void DecryptExample()

{

  // Setup a zip file

  AbstractFile zipFile = new DiskFile( @"ZipFile.zip" );



  // If the zip file doesn't exist

  if( !zipFile.Exists )

  {

    // Fail

    return;

  }



  // Setup a destination folder

  AbstractFolder destinationFolder = new DiskFolder( @"D:\Temp\Unzip" );



  ZipArchive zip = new ZipArchive( zipFile );



  /* The length of the password is not limited. However, to preserve compatibility

  with other Zip compression utilities, it is recommended that you limit passwords

  to no greater than 80 characters.

  

  It is also advisable to only use ASCII characters because there is no

  accepted standard on how to encode non-ASCII characters in passwords. */



  // Set up the list of password possible for the items in this archive

  this.m_passwords = new string[]

  {

    "MyPassword",

    "wrong password",

    "fkE%I-969%=6kei$[BbZ \"6Iq- =[",

    "}8{)zM#$k//O?t~=iG'Si{AF\"S~\'/8@1n",

  };



  // Use our most likely password as the default

  zip.DefaultDecryptionPassword = this.m_passwords[ 0 ];



  // Subscribe to the events that will allow us to handle invalid passwords

  ZipEvents events = new ZipEvents();

  events.ItemProgression += new ItemProgressionEventHandler( OnItemProgression );

  events.ItemException += new ItemExceptionEventHandler( OnItemException );



  // Unzip the contents of the zip archive to the destination folder

  zip.CopyFilesTo( events, zip, destinationFolder, true, true );

}



private void OnItemProgression( object sender, ItemProgressionEventArgs e )

{

  /* We're about to process a new item */



  // Reset the password index

  this.m_passwordIndex = 0;

}



private void OnItemException( object sender, ItemExceptionEventArgs e )

{

  bool badPassword = false;



  // Get the archive object we passed through the user data

  ZipArchive zip = ( ZipArchive ) e.UserData;



  /* We will try to ascertain if we have an invalid password */



  InvalidDecryptionPasswordException invalidDecryptionPasswordException;

  FileSystemIOException fileSystemIOException;



  invalidDecryptionPasswordException = e.Exception as InvalidDecryptionPasswordException;

  fileSystemIOException = e.Exception as FileSystemIOException;



  // If the exception says that a bad password was supplied

  if( invalidDecryptionPasswordException != null )

  {

    badPassword = true;

  }

  // If we had an I/O error during decryption

  else if( fileSystemIOException != null )

  {

    ZippedFile zippedFile = e.CurrentItem as ZippedFile;



    // If we were reading from a zipped file encrypted in the 'compatible' method

    if( zippedFile != null && zippedFile.Encrypted &&

        zippedFile.EncryptionMethod.Equals( EncryptionMethod.Compatible ) )

    {

      /* It's possible the I/O error occurred because the password is invalid.

      The was the 'compatible' encryption is designed doesn't provide for 100%

      accurate bad password detection, unfortunately.

        

      Note that AES encryption has no such design flaw. */

      badPassword = true;

    }

  }



  // If we had a bad password

  if( badPassword )

  {

    // If we haven't gone through our password list

    if( this.m_passwordIndex < this.m_passwords.Length )

    {

      // Set the current password and move the index to the next password

      zip.DefaultDecryptionPassword = this.m_passwords[ this.m_passwordIndex++ ];



      // Retry unzipping the file

      e.Action = ItemExceptionAction.Retry;

    }

    else

    {

      // Skip the file

      e.Action = ItemExceptionAction.Ignore;

    }

  }

}



private string[] m_passwords;

private int m_passwordIndex;

    Public Sub DecryptExample()

      ' Setup a zip file

      Dim zipFile As AbstractFile = New DiskFile("ZipFile.zip")



      ' If the zip file doesn't exist

      If (Not zipFile.Exists) Then

        ' Fail

        Return

      End If



      ' Setup a destination folder

      Dim destinationFolder As AbstractFolder = New DiskFolder("D:\Temp\Unzip")



      Dim zip As New ZipArchive(zipFile)



'       The length of the password is not limited. However, to preserve compatibility

'      with other Zip compression utilities, it is recommended that you limit passwords

'      to no greater than 80 characters.

'      

'      It is also advisable to only use ASCII characters because there is no

'      accepted standard on how to encode non-ASCII characters in passwords. 



      ' Set up the list of password possible for the items in this archive

      Me.m_passwords = New String() { "MyPassword", "wrong password", "fkE%I-969%=6kei$[BbZ ""6Iq- =[", "}8{)zM#$k//O?t~=iG'Si{AF""S~'/8@1n" }



      ' Use our most likely password as the default

      zip.DefaultDecryptionPassword = Me.m_passwords(0)



      ' Subscribe to the events that will allow us to handle invalid passwords

      Dim events As New ZipEvents()

      AddHandler events.ItemProgression, AddressOf OnItemProgression

      AddHandler events.ItemException, AddressOf OnItemException



      ' Unzip the contents of the zip archive to the destination folder

      zip.CopyFilesTo(events, zip, destinationFolder, True, True)

    End Sub



    Private Sub OnItemProgression(ByVal sender As Object, ByVal e As ItemProgressionEventArgs)

      ' We're about to process a new item 



      ' Reset the password index

      Me.m_passwordIndex = 0

    End Sub



    Private Sub OnItemException(ByVal sender As Object, ByVal e As ItemExceptionEventArgs)

      Dim badPassword As Boolean = False



      ' Get the archive object we passed through the user data

      Dim zip As ZipArchive = CType(e.UserData, ZipArchive)



      ' We will try to ascertain if we have an invalid password 



      Dim invalidDecryptionPasswordException As InvalidDecryptionPasswordException

      Dim fileSystemIOException As FileSystemIOException



      invalidDecryptionPasswordException = TryCast(e.Exception, InvalidDecryptionPasswordException)

      fileSystemIOException = TryCast(e.Exception, FileSystemIOException)



      ' If the exception says that a bad password was supplied

      If invalidDecryptionPasswordException IsNot Nothing Then

        badPassword = True

      ' If we had an I/O error during decryption

      ElseIf fileSystemIOException IsNot Nothing Then

        Dim zippedFile As ZippedFile = TryCast(e.CurrentItem, ZippedFile)



        ' If we were reading from a zipped file encrypted in the 'compatible' method

        If zippedFile IsNot Nothing AndAlso zippedFile.Encrypted AndAlso zippedFile.EncryptionMethod.Equals(EncryptionMethod.Compatible) Then

'           It's possible the I/O error occurred because the password is invalid.

'          The was the 'compatible' encryption is designed doesn't provide for 100%

'          accurate bad password detection, unfortunately.

'            

'          Note that AES encryption has no such design flaw. 

          badPassword = True

        End If

      End If



      ' If we had a bad password

      If badPassword Then

        ' If we haven't gone through our password list

        If Me.m_passwordIndex < Me.m_passwords.Length Then

          ' Set the current password and move the index to the next password

          zip.DefaultDecryptionPassword = Me.m_passwords(Me.m_passwordIndex)

          Me.m_passwordIndex += 1



          ' Retry unzipping the file

          e.Action = ItemExceptionAction.Retry

        Else

          ' Skip the file

          e.Action = ItemExceptionAction.Ignore

        End If

      End If

    End Sub



    Private m_passwords() As String

    Private m_passwordIndex As Integer