A Reversible XOR Encryption Using a Rotating Key

Category:

String Manipulation

Type:

Snippets

Difficulty:

Intermediate

Author:

Stefan

Version Compatibility: Visual Basic 6

More information:

This is a fully commented reversible XOR encryption demonstration. It uses a rotating key to achieve a reasonable level of protection. It also provides a good introduction to encryption concepts.

Instructions: Copy the declarations and code below and paste directly into your VB project.

Declarations:
'Controls you will need, in order they should appear on the form (top to bottom) '2 textboxes & 1 button (txtInput, txtKey, cmdEncrypt) '1 multiline textbox & 1 button (txtEncrypted, cmdDecrypt) '1 textbox & 1 button (txtDecrypted, cmdReset) 'all propeties not stated are default

Code:

'This is a demonstration of simple XOR hashing using a variable rotating key.
'Produced by Stefan Miles (Stefan_MCP@live.co.uk)
'Feel free to reuse, modify or distribute this code, but please don't remove
'my name and contact details if it is published at source level

'Each character in the plaintext string is converted to an ASCII code and
'then XOR'ed with the ASCII code of the next character from the key string.
'The resulting value is then converted back into a text character and stored
'to make up the Hashed Text(encrypted version). If the text to be encrypted
'is longer that the Key, the Key cycles as many times as necessary.

'To decrypt the Hashed Text we just need to XOR the ASCII value of the
'hashed character with the ASCII code from the same character of the Key
'that was used to encrypt it.

'Note that to keep the code short and simple i havent included code
'to force the user to perform the operations in the correct order.

Dim vPlainTextString As String  'Unencrypted Data
Dim vHashedString As String     'Encrypted Data
Dim vDecryptedString As String  'Decrypted Data

Dim vKey As String              'SecretKey Data

Private Sub cmdEncrypt_Click()
Dim charbuf As String   'buffer for current PlainText character
Dim vKeybuf As String   'buffer for current Key character
Dim i As Integer        'Counter for current PlainText Character
Dim Ki As Integer       'Counter for current Key Character

Dim vKeyLoopCounter As Integer 'counts complete key cycles, for demo purposes only
vKeyLoopCounter = 0     'initialise the counter

'get the string to encrypt
vPlainTextString = txtInput.Text

'Get the key string
vKey = txtKey.Text

For i = 1 To Len(vPlainTextString)      'for each char in our plain text string

Ki = Ki + 1     'increment the Key counter
    If Ki = Len(vKey) + 1 Then  'if on last key character go back to first
        Ki = 1
        vKeyLoopCounter = vKeyLoopCounter + 1   'note that the key has been looped, only for demo purposes
    End If
    
    'get the ASCII code of the next Key character
    vKeybuf = Asc(Mid(vKey, Ki, 1))
    
    'Get the ASCII code of the next Plaintext character
    charbuf = Asc(Mid(vPlainTextString, i, 1))
    
    'XOR the Plaintext and Key ASCII codes
    charbuf = charbuf Xor vKeybuf
    
    'Convert the result from an ASCII code to a text Character
    charbuf = Chr(charbuf)
    
    'Commit the new hashed character to memory
    vHashedString = vHashedString & charbuf
Next

txtEncrypted.Text = vHashedString   'Show the user the resulting Hashed string

Me.Caption = "Stefs XOR Encryptor - Data Encrypted" 'Let the user know that we're done encrypting

'Because we are working with ASCII codes we cant be sure that the
'XOR operation will generate a value that has an equivalent printable text
'character. In these cases the character cant be printed into a textbox
'and so appears blank to the user. However in the vHashedString our
'value is still stored and works fine as this can hold non printables characters.
'The below code is just to make sure that the user is aware of this.
If Len(txtEncrypted.Text) < Len(vHashedString) Then
        txtEncrypted.Text = txtEncrypted.Text & vbCrLf & "***Note some characters dont have printable equivalents and couldnt be shown here***" & vbCrLf & "Encrypted data length =" & Len(vHashedString) & vbCrLf & "Key was cycled " & vKeyLoopCounter & " time(s)"
    Else
        txtEncrypted.Text = txtEncrypted.Text & vbCrLf & "Encrypted data length =" & Len(vHashedString) & vbCrLf & "Key was cycled " & vKeyLoopCounter & " time(s)"
End If
End Sub

Private Sub cmdDecrypt_Click()
Dim charbuf As String   'buffer for current PlainText character
Dim vKeybuf As String   'buffer for current Key character

Dim i As Integer        'Counter for current PlainText Character
Dim Ki As Integer       'Counter for current Key Character

'Get the key string
vKey = txtKey...

Instructions: Click the link below to download the code. Select 'Save' from the IE popup dialog. Once downloaded, open the .zip file from your local drive using WinZip or a comparable program to view the contents.

Download sampleencryption.zip