Client

local LastUpdateTime = "April 19, 2016";
local Protocol       = "CCCS Login";
local Domain         = "SystematicLoop.net/CCCS Server";

local function open()
    local bOpen, sFreeSide = false, nil
    for n,sSide in pairs(rs.getSides()) do
        if peripheral.getType( sSide ) == "modem" then
            sFreeSide = sSide
            if rednet.isOpen( sSide ) then
                bOpen = true
                break
            end
        end
    end

    if not bOpen then
        if sFreeSide then
            print( "No modem active. Opening "..sFreeSide.." modem" )
            rednet.open( sFreeSide )
            return true
        else
            print( "No modem attached" )
            return false
        end
    end
    return true
end

term.clear ();
term.setCursorPos (1, 1);
print ("CCCS Client Version 1.1 Loading...");
print ("Last updated "..LastUpdateTime);

open ();
local ServerIP = rednet.lookup (Protocol, Domain);

if (ServerIP) then
    print ("Found Server with address: "..tostring (ServerIP));
    write ("Username: ");
    local Username = read ();
    print ();
    write ("Password: ");
    local Password = read ("*");
    print ("Validating access...");
    rednet.send (ServerIP, Username, Protocol);
    local SenderID, Message, _ = rednet.receive (Protocol);
    if (SenderID == ServerIP) then
        if (Message == Password) then
            print ("Access Granted!");
        else
            print ("Access Denied!");
            rednet.close ();
            shell.run ("startup");
        end
    end
else
    print ("A connection to the server could not be established.");
    print ("Check modem connections, weather conditions, address, and try again");
    print ("Program will automatically restart in 10 seconds...");
    sleep (10);
    shell.run ("startup");
end