public static function fromDER(string $der, int $partLength)

    {

        $hex = unpack('H*', $der)[1];

        if ('30' !== mb_substr($hex, 0, 2, '8bit')) { // SEQUENCE

            throw new \RuntimeException();

        }

        if ('81' === mb_substr($hex, 2, 2, '8bit')) { // LENGTH > 128

            $hex = mb_substr($hex, 6, null, '8bit');

        } else {

            $hex = mb_substr($hex, 4, null, '8bit');

        }

        if ('02' !== mb_substr($hex, 0, 2, '8bit')) { // INTEGER

            throw new \RuntimeException();

        }

        $Rl = hexdec(mb_substr($hex, 2, 2, '8bit'));

        $R = self::retrievePositiveInteger(mb_substr($hex, 4, $Rl * 2, '8bit'));

        $R = str_pad($R, $partLength, '0', STR_PAD_LEFT);

        $hex = mb_substr($hex, 4 + $Rl * 2, null, '8bit');

        if ('02' !== mb_substr($hex, 0, 2, '8bit')) { // INTEGER

            throw new \RuntimeException();

        }

        $Sl = hexdec(mb_substr($hex, 2, 2, '8bit'));

        $S = self::retrievePositiveInteger(mb_substr($hex, 4, $Sl * 2, '8bit'));

        $S = str_pad($S, $partLength, '0', STR_PAD_LEFT);

        return pack('H*', $R.$S);

    }

    

    /**

     * @param string $data

     *

     * @return string

     */

    private static function retrievePositiveInteger(string $data)

    {

        while ('00' === mb_substr($data, 0, 2, '8bit') && mb_substr($data, 2, 2, '8bit') > '7f') {

            $data = mb_substr($data, 2, null, '8bit');

        }

        return $data;

    }