I recently created an ASP.NET application which used Forms Authentication. In the interests of security I configured the machinekey properties of the web.config file. An explanation of the machinekey including its purpose and how to generate the keys is explained in this past Microsoft article. The article explains to create cryptographically random keys you use the System.Security.Cryptography.RNGCryptoServiceProvider class.

You use different length keys for different types of encryption.

• For SHA1, set the validationKey to 64 bytes (128 hexadecimal characters).
• For AES, set the decryptionKey to 32 bytes (64 hexadecimal characters).
• For 3DES, set the decryptionKey to 24 bytes (48 hexadecimal characters).

Example code for generating keys is shown below;
Example in C#

using System;
using System.Text;
using System.Security;
using System.Security.Cryptography;

class App {
  static void Main(string[] argv) {
    int len = 128;
    if (argv.Length > 0)
      len = int.Parse(argv[0]);
    byte[] buff = new byte[len/2];
    RNGCryptoServiceProvider rng = new
                            RNGCryptoServiceProvider();
    rng.GetBytes(buff);
    StringBuilder sb = new StringBuilder(len);
    for (int i=0; i<buff.Length; i++)
      sb.Append(string.Format("{0:X2}", buff[i]));
    Console.WriteLine(sb);
  }
}

Example in VB.NET

Imports System
Imports System.Text
Imports System.Security
Imports System.Security.Cryptography

Module App
    Sub Main(ByVal argv() As String)
        Dim len As Integer = 128
        If argv.Length > 0 Then
            len = Integer.Parse(argv(0))
        End If
        Dim buff(len / 2) As Byte
        Dim rng As New RNGCryptoServiceProvider()
        rng.GetBytes(buff)
        Dim sb As New StringBuilder(len)
        Dim i As Integer
        For i = 0 To buff.Length - 1
            sb.Append(String.Format("{0:X2}", buff(i)))
        Next i
        Console.WriteLine(sb)
        Console.ReadLine()

    End Sub 'Main
End Module

I have taken this code and created a more user friendly, though still pretty basic windows application.

A copy of the code can be downloaded from MachineKeyGenerator repository on Github.