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Easy String Compression and Encryption

, 1 Jul 2008
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Class module to easily de-/compress and en-/decrypt big strings

Introduction

This article discusses string compression with optional decent encryption with pure VB.NET code, and no external tools required.

It can easily be integrated into existing projects. As the code is kept simple, it's suitable for beginners and a conversion to C# can be done easily.

Background

In need of a routine to quickly and safely deflate and inflate big strings, I searched the net for a solution. A comprehensive set of functionalities didn't show up, so I decided to write this class module, which encapsulates all the functionality needed to complete the task.

Using the Code

Although strings of any length can be applied to the process, the compression of short strings (i.e. 'Hello World!') is counterproductive as it results in even bigger compressed counterparts. The CompressionRatio property of the class tells you how effective the compression was. You can decide then, if you want to use the compressed string and if so, prefix and suffix can be automatically applied to it, to distinguish between compressed and uncompressed content afterwards.

Process overview:

Plain text -> to byte array -> gzip compression -> encryption -> to base64 string = shrinked text

shrinked text -> to byte array -> decryption -> gzip decompression -> to string = plain text

The code is simple to use. Here's the quick way to compress a string:

    Dim CompStr As New clsCompressedString(System.Text.Encoding.UTF8)

    CompStr.UnCompressed = "some large text content..."

    MsgBox "The compressed string is: " & CompStr.Compressed  

... and the way back:

    Dim CompStr As New clsCompressedString(System.Text.Encoding.UTF8)

    CompStr.Compressed = "..." '<- the previously compressed string

    MsgBox "The uncompressed string is: " & CompStr.UnCompressed

Error handling is kept at the minimum. The class returns empty strings when fed with corrupt data or supplied with wrong passphrase.

Optional encryption is performed by utilizing the .NET built-in RijndaelManaged at maximum key length and simplified usage: You just need to provide a single passphrase for encryption and decryption. Encryption key and iv are generated based on the passphrase by using SHA256 and MD5 hash value generation.

The demo project shows all features available.

Points of Interest

With string conversions involved, text encoding has to be addressed properly. Otherwise some or all characters could get messed up in the process of compression/decompression, depending on what content you try to compress/decompress.

Why Not Use ICSharpCode.SharpZipLib?

Well, you can easily alter the compression routines in the class to use ZipLib. I experimented with that and it showed, that ZipLib (0.85.4.369) is only up to 7% more efficient than the built in GZip. To get this slight better performance, you have to set ZipLib to the highest compression level (9). But that comes with a price: ZipLib at highest level is very slow compared to GZip and therefore takes several times longer to compress a huge string. So I prefer GZip for this task as it is fast, reliable and doesn't require to link to additional DLLs and I don't run into licensing and security issues by using comprehensive third party code.

Preferences could possibly change when it comes to binary file compression. Maybe then ZipLib outruns GZip - but binary file compression was not the assigned task in this case.

History

  • 1st July, 2008: This is the first version. Participate and help to optimize and extend the code.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

BTDex

Germany Germany
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Comments and Discussions

 
AnswerAbout the decompressing bug in C# Pinmembershenqing5-Feb-13 17:12 
GeneralMy vote of 2 PinmemberMahdi Nejadsahebi4-Nov-12 0:57 
QuestionCan't decompress at client app after receive via .NET socket TCP PinmemberUmesh Sharma Kota8-Oct-12 4:22 
GeneralMy vote of 5 PinmemberUmesh Sharma Kota7-Oct-12 13:38 
Questionjava text compression PinmemberMember 807347310-Jul-11 19:23 
GeneralNice and useful article Pinmemberbuitrunghieu20-Nov-09 12:04 
Questionis it possible in VB6? Pinmemberbuffysabelle16-Nov-09 9:43 
QuestionMini-Bug? PinmemberFridolin22-Sep-09 3:53 
AnswerRe: Mini-Bug? Pinmemberjessehogan8-Oct-09 6:52 
GeneralRe: Mini-Bug? PinmemberMember 443577521-Mar-10 8:49 
GeneralThanks! PinmemberMember 19516102-Jun-09 20:37 
GeneralParadox PinmemberMr.PoorEnglish17-Jul-08 23:29 
GeneralRe: Paradox PinmemberBTDex20-Jul-08 3:46 
GeneralRe: Paradox [modified] PinmemberMr.PoorEnglish20-Jul-08 10:44 
BTDex wrote:
But when it comes to data exchange via xml you'll have to convert to string in the end.

Yes. But I think, I would do the data-exchange with the compressed streams directly. Not convert them to a base64-string.
That means, the sender writes its data into a encryption-stream, the encryptionstream into a compressStream, the compressStream into a NetWorkStream.
The receiver loads its XmlDocument from a DecryptionStream, which reads from a DecompressStream, which reads from a NetWorkStream.
 
Here a sample, how one can stick streams together, for to get such transformer-behaviour:
   Private Sub btZip_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btZip.Click
      Using ReadStream As New FileStream("Test.doc", FileMode.Open), _
            WriteStream As New FileStream("Test.Zip", FileMode.Create), _
            Zipper As New GZipStream(WriteStream, CompressionMode.Compress)
         ReadStream.WriteTo(Zipper)
      End Using
   End Sub
 
   Private Sub btUnZip_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btUnZip.Click
      Using ReadStream As New FileStream("Test.Zip", FileMode.Open), _
            UnZipper As New GZipStream(ReadStream, CompressionMode.Decompress), _
            WriteStream As New FileStream("Test2.doc", FileMode.Create)
         UnZipper.WriteTo(WriteStream)
      End Using
   End Sub
   <extension()> _
   Public Sub WriteTo( _
         ByVal ReadStream As Stream, _
         ByVal WriteStream As Stream, _
         Optional ByVal BytesToRead As Long = -1, _
         Optional ByVal Bufsize As Integer = Byte.MaxValue)
      Dim Buf(Bufsize - 1) As Byte
      If BytesToRead < 0 Then
         If ReadStream.CanSeek Then
            BytesToRead = ReadStream.Length
         Else
            'ReadStream.Length - Property not available
            Do
               Bufsize = ReadStream.Read(Buf, 0, Bufsize)
               WriteStream.Write(Buf, 0, Bufsize)
            Loop Until Bufsize < Buf.Length
            Return
         End If
      End If
      Dim Sum = 0L
      Do
         Dim Portion = ReadStream.Read(Buf, 0, CInt(Math.Min(BytesToRead, Bufsize)))
         BytesToRead -= Portion
         WriteStream.Write(Buf, 0, Portion)
      Loop Until BytesToRead = 0
   End Sub
I think, one could apply that principle also to EncryptionStreams and NetworkStreams (which may be a little more complicated).
GeneralRe: Paradox PinmemberBTDex22-Jul-08 13:09 
Generalok, I see PinmemberMr.PoorEnglish22-Jul-08 21:20 
GeneralGZip... PinmemberFrankidoze5-Jul-08 4:32 

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