Create a new Duo-compatible HOTP secret for Yubikeys

New-HOTPSecret.ps1

<#
.SYNOPSIS
Generate an HOTP secret compatible with Yubikey and Duo.

.DESCRIPTION
Thanks to Yubico for the example:
https://support.yubico.com/hc/en-us/articles/360015668699-Generating-Base32-string-examples
#>
Function New-HOTPSecret {
    [CmdletBinding()]
    param(
        # The length to generate.
        [Parameter(Position = 0, Mandatory = $false, ValueFromPipeline = $false)]
        [ValidateScript({ $_ % 2 -eq 0 }, ErrorMessage = "The HOTP secret's length must be an even number.")]
        [int] $Length = 32
    )

    $rng = [Security.Cryptography.RNGCryptoServiceProvider]::Create()
    [Byte[]] $buffer = 1

    for ($secret = ""; $secret.length -lt $Length) {
        $rng.GetBytes($buffer)

        # DUO Only supports A-F
        if ([char]$buffer[0] -clike "[2-7A-F]") {
            $secret += [char]$buffer[0]
        }
    }

    $secret
}