-- client
    local validKeys = {}
    local users = {
      ["fatboychummy"] = {
        password = "SomeHashedPassword",
        balance = 64
      }
    }
    while true do
      local senderID, message = rednet.receive("Banker") -- listens for rednet message with protocol "Banker"
      if type(message) == "table" then
        if message.action == "login" then
          if users[message.user] and users[message.user].password == message.password then
            -- login ok, return new key to user and save the login key
            local newKey = "blablabla"
            validKeys[newKey] = {
              expiry = os.clock() + 30 -- key valid for 30 seconds.
              user = users[message.user]
            }
            rednet.send(senderID, {
              ok = true,
              key = newKey
            }, "Banker")
          else
            -- login NOT okay, return failure
            rednet.send(senderID, {
              ok = false,
              error = "Login failure."
            }, "Banker")
          end
        else
          -- check valid key
          if validKeys[message.key] and validKeys[message.key].expiry > os.clock() then
            -- key valid
            if message.action == "getBalance" then
              rednet.send(senderID, {
                ok = true,
                balance = validKeys[message.key].user.balance
              }, "Banker")
            else -- invalid action
              rednet.send(senderID, {
                ok = false,
                error = "Invalid action: " .. message.action
              }, "Banker")
            end
          else
            -- key not valid
            rednet.send(senderID, {
              ok = false,
              error = "You are not logged in!"
            }, "Banker")
          end
        end
      end
    end