Calculate the Yubikey serial number from the OTP

yubikey_otp_to_serial.py

# adapted from Java:
# https://github.com/Yubico/yubikey-salesforce-client/blob/
# e38e46ee90296a852374a8b744555e99d16b6ca7/src/classes/Modhex.cls

ALPHABET = 'cbdefghijklnrtuv'


def yubikey_otp_to_serial(otp):
    """ Takes a Yubikey OTP and calculates the serial number of the key.

    The serial key is printed on the yubikey, in decimal and as a QR code.

    Example:

        >>> yubikey_otp_to_serial(
            'ccccccdefghdefghdefghdefghdefghdefghdefghklv')
        2311522

    """

    token = 'cccc' + otp[:12]

    toggle = False
    keep = 0

    bytesarray = []

    for char in token:
        n = ALPHABET.index(char)
        toggle = not toggle

        if toggle:
            keep = n
        else:
            bytesarray.append((keep << 4) | n)

    value = 0

    # in Java, shifts on integers are masked with 0x1f using AND
    # https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.19
    mask_value = 0x1f

    for i in range(0, 8):
        shift = (4 - 1 - i) * 8
        value += (bytesarray[i] & 255) << (shift & mask_value)

    return value


if __name__ == '__main__':
    assert yubikey_otp_to_serial(
        'ccccccdefghdefghdefghdefghdefghdefghdefghklv') == 2311522