authserver.lua

local chan = 15432

local k = require("k")
local secret = "secretword"
local mon = peripheral.wrap("right")
local mod = peripheral.wrap("top")

mod.open(chan)

local function authSuccess(dist)
    mon.setBackgroundColor(colors.black)
    mon.clear()
    mon.setBackgroundColor(colors.green)
    mon.setTextColor(colors.white)
    mon.setCursorPos(1,1)
    mon.write("succ")
    mon.setCursorPos(1,2)
    mon.write("dist:")
    mon.setCursorPos(1,3)
    mon.write(tostring(dist))
    rs.setOutput("left", true)
end

local function authFail()
    mon.setBackgroundColor(colors.black)
    mon.clear()
    mon.setBackgroundColor(colors.red)
    mon.setTextColor(colors.white)
    mon.setCursorPos(1,1)
    mon.write("fail")
    rs.setOutput("left", false)
end

local timer = os.startTimer(1)
local lastChall = ""
local lastChallTime = os.epoch("utc")
local gotResponse = false
while true do
    local e, p1, p2, p3, p4, p5 = os.pullEvent()
    if e == "timer" and p1 == timer then
        if not gotResponse then
            print("auth timeout/too far away")
            authFail()
        end
        gotResponse = false

        lastChallTime = os.epoch("utc")
        lastChall = "authchall:" .. tostring(os.getComputerID()) .. ":" .. tostring(os.epoch("utc"))
        mod.transmit(chan, chan, lastChall)
        timer = os.startTimer(1)
    elseif e == "modem_message" and p2 == chan and type(p4) == "string" and p5 and
        p4:sub(1, 9) == "authresp:" and p5 < 5 and (os.epoch("utc") - lastChallTime < 100) then
        local resp = p4:sub(10)
        local expected = k.sha256(lastChall .. ":" .. secret .. ":" .. tostring(p5))
        if resp == expected then
            print("received valid response")
            gotResponse = true
            authSuccess(p5)
        else
            print("received invalid response")
            authFail()
        end
    end
end