Extract AAX checksum from Audible Audio Book

audible.ps1

function Read-AudioBookChecksum {
    param(
        [Parameter(Mandatory=$true,
                Position=0,
                ParameterSetName="Path",
                ValueFromPipeline=$true,
                ValueFromPipelineByPropertyName=$true)]
        [ValidateNotNullOrEmpty()]
        [ValidateScript({ Test-Path $_ -Type Leaf })]
        [string]
        $Path
    )
    Process {
        try {
            $buffer = New-Object Byte[] 20
            $fs = [IO.File]::OpenRead($Path)
            $fs.Seek(581, [System.IO.SeekOrigin]::Begin) | Out-Null
            $fs.Read($buffer, 0, 4) | Out-Null
            if([System.Text.Encoding]::UTF8.GetString($buffer) -eq 'adrm') {
                $fs.Seek(653, [System.IO.SeekOrigin]::Begin) | Out-Null
                $fs.Read($buffer, 0, 20) | Out-Null
                return ($buffer | ForEach-Object ToString x2) -join ''
            }
            return $null
        }
        finally {
            $fs.Close();
        }
    }
}


Get-ChildItem "*.aax" | Read-AudioBookChecksum $_