#! /usr/bin/perl

# a1-totp - Time-based One-time Password algorithm, as described in RFC 6283

# Author: Wessel Dankers
# LICENSE: WTFPLv2

#
# DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#                   Version 2, December 2004
# Copyright (C) 2004 Sam Hocevar <sam@hocevar.net>
#
# Everyone is permitted to copy and distribute verbatim or modified
# copies of this license document, and changing it is allowed as long
# as the name is changed.
#
#           DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#  TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
#
# 0. You just DO WHAT THE FUCK YOU WANT TO.
#


# https://tools.ietf.org/html/rfc6238
# https://en.wikipedia.org/wiki/Time-based_One-time_Password_algorithm

# Date: Wed, 14 Oct 2020 11:07:07 +0200
# From: Wessel Dankers
# Subject: perl bijna-oneliner
# Message-ID: <20201014090707.GU2003855@homsar.uvt.nl>

# installation: sudo apt install libconvert-base32-perl

# Usage: totp <token> [<interval>]
#
# Interval is optioneel en andere intervallen dan de default worden toch
# vrijwel nooit ondersteund.

# сре 14 11:13 < Fruit> je krijgt van [de andere kant] een code
# сре 14 11:13 < Fruit> die code voer je aan dat scriptje
# сре 14 11:53 <Fruit> zo'n code ziet eruit als iets van icunwencwuinsaduiakdj
# сре 14 11:54 <Fruit> oh eventueel met cijfertjes ook
# сре 14 11:54 <Fruit> base32
#
# сре 14 11:18 < joostvb> en t scriptje draai je op een air-gapped super secure
#    apparaat


use strict;
use warnings FATAL => 'all';

use Digest::SHA qw(hmac_sha1);
use Convert::Base32;

sub totp {
   my ($key, $interval) = @_;
   $interval ||= 30;

   my $time = pack('Q>', time / $interval);
   my $hmac = hmac_sha1($time, decode_base32($key));

   my $offset = vec($hmac, length($hmac) * 2 - 2, 4);
   my ($value) = unpack('L>', substr($hmac, $offset, 4));

   return sprintf("%u", ($value & 0x7FFFFFFF) % 1000000);
}

print totp(@ARGV), "\n";