otp.lua

--[[
MIT License

Copyright (c) 2021 Cody Tilkins

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
]]

-- please point these to proper location
local basexx = require("basexx")
local sha1 = require("sha1")
local util = require("util")
local bit32 = require("bit32")

local otp = {
	util = util
}

--[[
	{...} contains:
		otp.type == totp
			> Number interval
			
		otp.type == hotp
			> nil
			
		otp.type == otp
			> nil
--]]
otp.new = function(secret, digits, digest, ...)
	local this = {}
	this.secret = secret
	this.digits = digits or 6
	this.digest = digest or "sha1"
	
	local args = {...}
	if(string.lower(otp.type) == "totp")then
		this.interval = args[1]
	elseif(string.lower(otp.type) == "hotp")then
		
	end
	
	return this
end



otp.generate_otp = function(instance, input)
	if (input < 0) then
		return nil
	end
	
	local hash = sha1.hmac_binary(otp.byte_secret(instance), otp.int_to_bytestring(input))
	local offset = bit32.band(string.byte(hash:sub(-1, -1)), 0xF) + 1
	
	local bhash = util.str_to_byte(hash)
	
	local code = bit32.bor(
		bit32.lshift(bit32.band(bhash[offset], 0x7F), 24),
		bit32.lshift(bit32.band(bhash[offset + 1], 0xFF), 16),
		bit32.lshift(bit32.band(bhash[offset + 2], 0xFF), 8),
		bit32.lshift(bit32.band(bhash[offset + 3], 0xFF), 0)
	)
	
	local str_code = tostring(math.floor(code % (10 ^ instance.digits)))
	while #str_code < instance.digits do
		str_code = '0' .. str_code
	end
	
	return str_code
end

otp.byte_secret = function(instance)
	local missing_padding = #(instance.secret) % 8
	if (missing_padding ~= 0) then
		instance.secret = instance.secret .. string.rep('=', (8 - missing_padding))
	end
	return basexx.from_base32(instance.secret)
end

otp.int_to_bytestring = function(i, padding)
	local bytes = {}
	while (i ~= 0) do
		table.insert(bytes, bit32.band(i, 0xFF))
		i = bit32.rshift(i, 8)
	end
	return string.rep('\0', math.max(0, (padding or 8) - #bytes)) .. util.byte_arr_tostring(util.arr_reverse(bytes))
end

return otp