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How to create keys by using Visual Basic .NET for use in Forms authentication

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This article describes how to create keys to use for encryption, decryption, and validation of Forms authentication cookie data. You can use the keys that you create in this article for the validationKey and the decryptionKey attributes of the <machineKey> section in the <system.web> element in the Machine.config and the Web.config files.


The following list outlines the recommended hardware, software, network infrastructure, and service packs that you need:
  • Microsoft Windows 2000 or Microsoft Windows XP
  • Microsoft .NET Framework
  • Microsoft Internet Information Services (IIS)

Create the project

  1. Start Microsoft Visual Studio .NET.
  2. On File menu, point to New, and then click Project.
  3. In the Project Types area, click Visual Basic Projects.
  4. In the Templates area, click Console Application.
  5. In the Name text box, type HashConfigVb, and then click OK.

Write the code to hash a password

The code in this section reads two arguments that are passed from the command line:
  • The first argument is the number of bytes that is used to create the decryptionKey attribute.
  • The second argument is the number of bytes that is used to create the validationKey attribute.
The code uses a random number generator to create a random number of bytes based on the command-line arguments. After the random bytes are created, the bytes are formatted into a hexadecimal string that is suitable for use in the .config files.

Note The hexadecimal string that is created is twice the size of the value that is passed on the command line. For example, if you specify 24 bytes for a key, the resulting string is 48 bytes in length after the conversion. The valid values for decryptionKey is 8 or 24. This creates a 16 byte key for Data Encryption Standard (DES) or a 48 byte key for Triple DES, respectively. Valid values for validationKey are 20 to 64. This creates keys from 40 to 128 bytes in length after the conversion. The output from the code is an entire <machineKey> element that you can copy and paste into a Machine.config or a Web.config file.

  1. Add a new class file named KeyCreator to your Visual Basic project.
  2. Replace the existing code in the KeyCreator.vb file with the following code:
    Imports System
    Imports System.Text
    Imports System.Security.Cryptography
    Namespace Crypto
      Public Class KeyCreator
        Public Shared Sub CreateMachineKey()
          Dim commandLineArgs As String()
          commandLineArgs = System.Environment.GetCommandLineArgs()
          Dim decryptionKey As String
          decryptionKey = CreateKey(System.Convert.ToInt32(commandLineArgs(1)))
          Dim validationKey As String
          validationKey = CreateKey(System.Convert.ToInt32(commandLineArgs(2)))
          Console.WriteLine("<machineKey validationKey=""{0}"" decryptionKey=""{1}"" validation=""SHA1""/>", _
          validationKey, decryptionKey)
         End Sub
         Public Shared Function CreateKey(numBytes As Integer) As String
           Dim rng As RNGCryptoServiceProvider = New RNGCryptoServiceProvider()
           Dim buff(numBytes -1) As Byte
           Return BytesToHexString(buff)
         End Function
         Public Shared Function BytesToHexString(bytes As Byte()) As String
           Dim hexString As StringBuilder = New StringBuilder(64)
           Dim counter as Integer
           For counter = 0 To bytes.Length - 1
             hexString.Append(String.Format("{0:X2}", bytes(counter)))
           Return hexString.ToString()
        End Function
      End Class
    End Namespace
  3. Open the Module1.vb file that is created by default, and then add the following code in the Main sub routine:
        Dim MyKeyCreator As New Crypto.KeyCreator()
  4. Build the application.

Generate the hashes

Run the application from a command prompt, and then pass in two integer values that are the size of the decryption and the validation keys. If you named the console application HashConfigVb.exe, type the following syntax at the command prompt in the Bin directory of the application:
HashConfigVb.exe 24 64
The application should return output that is similar to the following output:
<machineKey validationKey="08CE6B478DCE73..........E566D8AC5D1C045BA60"
Note Because the code uses a random number generator, the output is different each time.

Update the configuration file

  1. Locate the Machine.config or the Web.config file.
  2. Locate the <system.web> section in the configuration file.
  3. Replace the <machineKey> section with the output from the console application. If the <machineKey> section does not exist, create it.
  4. Save the configuration file.
  5. Restart IIS on all servers in the Web farm for the Machine.config changes to take effect.


Make sure that the <machineKey> section has identical, explicit keys (that is, do not use the AutoGenerate option for attributes in the <machineKey> section) across the Web farm in the following scenarios:
  • When you use Forms authentication.
  • When you run session state in StateServer mode.
  • When you want ViewState to be available across a Web farm because the enableViewStateMAC attribute is set to True by default.

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For additional information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
308157 How to implement forms-based authentication in your ASP.NET application by using Visual Basic .NET
306238 How to implement role-based security with Forms-based authentication in your ASP.NET application by using Visual Basic .NET
306590 INFO: ASP.NET security overview
307626 INFO: ASP.NET configuration overview

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Keywords: KB313091, kbstate, kbsecurity, kbhowtomaster, kbconfig, kbproductlink

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Article Info
Article ID : 313091
Revision : 13
Created on : 10/29/2007
Published on : 10/29/2007
Exists online : False
Views : 604