Powershell OTP function

otp.ps1

function otp($SECRET="ABCDEFGHIJKLMNOP", $LENGTH=6, $WINDOW=30){
    $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))
    $timeBytes = [BitConverter]::GetBytes($unixTime)
    [array]::Reverse($timeBytes)

    $enc = [System.Text.Encoding]::UTF8
    $hmac = New-Object -TypeName System.Security.Cryptography.HMACSHA1
    $hmac.key = Convert-Base32ToBytes($SECRET.ToUpper())
    $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)
    set-clipboard $otp
    return $otp
}

function Convert-Base32ToBytes($base32) {
    $base32chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
    $bits = "";
    $hex = "";

    for ($i = 0; $i -lt $base32.Length; $i++) {
        $val = $base32chars.IndexOf($base32.Chars($i));
        $binary = [Convert]::ToString($val, 2)
        $bits += $binary.ToString().PadLeft(5, '0')
    }

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

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