d

local oldpullevent = os.pullevent
local os.pullevent = os.pullEventRaw
local auth_channel = 234 --#Ensure that the auth channel is the same!
local myChannel = 10 --#Any channel the user wishes
local modem = peripheral.wrap("top")
modem.open(myChannel) --#We don't need to open the auth_channel to send messages, only if we need to receive on that channel

while true do
  term.clear() term.setCursorPos(1,1)
  term.write("Username: ")
  local username = read() --#Gets the user's input
  local password = read("*") --#The asterisk is the masking character

  local user_table = {["username"] = username, ["password"] = password}
  modem.transmit(auth_channel, myChannel, textutils.serialize(user_table)) --#Go ahead and send off the info to the server
  while true do --Now we wait for a response
        local event, modem_side, sender_channel, reply_channel, message = os.pullEvent("modem_message")
        if reply_channel == auth_channel then --#If it is indeed the server that sent the message
          if message == "valid" then --#It was indeed a valid username/password combo
                printError("Valid username/password!") --#Do whatever you want here + sets the exit error.
            os.pullEvent = oldpullevent
            error() -- Exit the program

          else --#Nope, invalid.
                print("Invalid username/password!")
            sleep(3)
          end
        end
  end
end