TOTP Client for PowerShell

totp.ps1

#requires -version 2
<#
.SYNOPSIS
  Time-base One-Time Password Algorithm (RFC 6238)

.DESCRIPTION
  This is an implementation of the RFC 6238 Time-Based One-Time Password Algorithm draft based upon the HMAC-based One-Time Password (HOTP) algorithm (RFC 4226). This is a time based variant of the HOTP algorithm providing short-lived OTP values. 

.NOTES
  Version:        1.0
  Author:         Jon Friesen
  Creation Date:  May 7, 2015
  Purpose/Change: Provide an easy way of generating OTPs
  
#>
 
function Get-Otp($SECRET, $LENGTH, $WINDOW){
    $enc = [System.Text.Encoding]::UTF8
    $hmac = New-Object -TypeName System.Security.Cryptography.HMACSHA1
    $hmac.key = Convert-HexToByteArray(Convert-Base32ToHex(($SECRET.ToUpper())))
    $timeBytes = Get-TimeByteArray $WINDOW
    $randHash = $hmac.ComputeHash($timeBytes)
    
    $offset = $randhash[($randHash.Length-1)] -band 0xf
    $fullOTP = ($randhash[$offset] -band 0x7f) * [math]::pow(2, 24)
    $fullOTP += ($randHash[$offset + 1] -band 0xff) * [math]::pow(2, 16)
    $fullOTP += ($randHash[$offset + 2] -band 0xff) * [math]::pow(2, 8)
    $fullOTP += ($randHash[$offset + 3] -band 0xff)

    $modNumber = [math]::pow(10, $LENGTH)
    $otp = $fullOTP % $modNumber
    $otp = $otp.ToString("0" * $LENGTH)
    return $otp
}

function Get-TimeByteArray($WINDOW) {
    $span = (New-TimeSpan -Start (Get-Date -Year 1970 -Month 1 -Day 1 -Hour 0 -Minute 0 -Second 0) -End (Get-Date).ToUniversalTime()).TotalSeconds
    $unixTime = [Convert]::ToInt64([Math]::Floor($span/$WINDOW))
    $byteArray = [BitConverter]::GetBytes($unixTime)
    [array]::Reverse($byteArray)
    return $byteArray
}

function Convert-HexToByteArray($hexString) {
    $byteArray = $hexString -replace '^0x', '' -split "(?<=\G\w{2})(?=\w{2})" | %{ [Convert]::ToByte( $_, 16 ) }
    return $byteArray
}

function Convert-Base32ToHex($base32) {
    $base32chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
    $bits = "";
    $hex = "";

    for ($i = 0; $i -lt $base32.Length; $i++) {
        $val = $base32chars.IndexOf($base32.Chars($i));
        $binary = [Convert]::ToString($val, 2)
        $staticLen = 5
        $padder = '0'
            # Write-Host $binary
        $bits += Add-LeftPad $binary.ToString()  $staticLen  $padder
    }


    for ($i = 0; $i+4 -le $bits.Length; $i+=4) {
        $chunk = $bits.Substring($i, 4)
        # Write-Host $chunk
        $intChunk = [Convert]::ToInt32($chunk, 2)
        $hexChunk = Convert-IntToHex($intChunk)
        # Write-Host $hexChunk
        $hex = $hex + $hexChunk
    }
    return $hex;

}

function Convert-IntToHex([int]$num) {
    return ('{0:x}' -f $num)
}

function Add-LeftPad($str, $len, $pad) {
    if(($len + 1) -ge $str.Length) {
        while (($len - 1) -ge $str.Length) {
            $str = ($pad + $str)
        }
    }
    return $str;
}

@jamez2128 👍

Magnificient mind @jonfriesen, you've blown me away with this writing.
Would you allow me if i follow your logic and will implement it in C#? Credits still be yours. ❤️

@rey021 of course, glad it helps. Though, I'd expect there to be some C# TOTP libraries out in the world already :P

doesn't the secret need to be in base 32? Hoping to generate a QR code to use with authenticator, but that requires a base 32 secret

NVM, Yes if you use base 32 string, and use it to make a QR code it will match the script output <3

Some minor improvements:

diff --git a/tmp/totp-old.ps1 b/tmp/totp.ps1
index 0a50e76..dbc31de 100644
--- a/tmp/totp-old.ps1
+++ b/tmp/totp.ps1
@@ -11,16 +11,20 @@
   Author:         Jon Friesen
   Creation Date:  May 7, 2015
   Purpose/Change: Provide an easy way of generating OTPs
-  
 #>
- 
-function Get-Otp($SECRET, $LENGTH, $WINDOW){
-    $enc = [System.Text.Encoding]::UTF8
+
+function Get-Otp(){
+    param(
+        [Parameter(Mandatory=$true)]$SECRET,
+        $LENGTH = 6,
+        $WINDOW = 30
+    )
+    $SECRET = $SECRET -replace '\s',''
     $hmac = New-Object -TypeName System.Security.Cryptography.HMACSHA1
     $hmac.key = Convert-HexToByteArray(Convert-Base32ToHex(($SECRET.ToUpper())))
     $timeBytes = Get-TimeByteArray $WINDOW
     $randHash = $hmac.ComputeHash($timeBytes)
-    
+
     $offset = $randhash[($randHash.Length-1)] -band 0xf
     $fullOTP = ($randhash[$offset] -band 0x7f) * [math]::pow(2, 24)
     $fullOTP += ($randHash[$offset + 1] -band 0xff) * [math]::pow(2, 16)
@@ -42,7 +46,7 @@ function Get-TimeByteArray($WINDOW) {
 }
 
 function Convert-HexToByteArray($hexString) {
-    $byteArray = $hexString -replace '^0x', '' -split "(?<=\G\w{2})(?=\w{2})" | %{ [Convert]::ToByte( $_, 16 ) }
+    $byteArray = $hexString -replace '^0x', '' -split "(?<=\G\w{2})(?=\w{1,2})" | %{ [Convert]::ToByte( $_, 16 ) }
     return $byteArray
 }

• enc was unused
• removed trailing whitespace
• set default for length/window like @ecspresso
• automatically remove whitespace from secret
• accept uneven length hexString in Convert-HexToByteArray
  • This was an issue with an amazon totp for me. It would throw an error otherwise as there would be a 3 character string into [Convert]::ToByte

Enjoy!