Link

--[[
link is a secure device-pairing system that can be used in a multitude of ways
]]--

local links

local function genNew()
    local t = ""
    for i= 1, 15 do
        t = t .. string.char( math.random( 0, 255 ) )
    end
    return t
end

local function increment( secret, current )
    local new = ""
    for i = 1, 15 do
        local n = tostring( secret:sub( i, i ):byte()^math.min( current:sub( i, i ):byte(), 128 ) )
        print( n )
        if #n < 5 then
            new = new .. string.char( tonumber( n:sub( 1, 2 ) ) )
        else
            local result = math.floor( tonumber( n:sub( 1, 2 ) ) / 99/math.ceil( #n - 2 ) )
            new = new .. n:sub( result * 2 + 1, result * 2 + 2 )
        end
    end
    return new
end

local function save() --saves the links
    local file = fs.open( ".links", "w" )
    file.write( textutils.serialize( links ) )
    file.close()
end

--loading linkages
do
    local file = fs.open( ".links", "r" )
    if file then
        local data = file.readAll()
        file.close()
        links = textutils.unserialize( data )
    else
        links = {}
        links.__secret = ""
        for i = 1, 15 do --generate a secret key
            links.__secret = links.__secret .. string.char( math.random( 31, 127 ) )
        end
        save()
    end
end

function addLink( devicename, secret )
    links[ devicename ] = {secret = secret, current=genNew()}
end

function getSecret()
    return links.__secret
end

function setDeviceName( name )
    links.__deviceName = name
end

local mt = {
    send = function( self, message )
        local auth = increment( links[ self.deviceName ].secret, links[ self.deviceName ].current )
        rednet.broadcast( textutils.serialize( {msg=message, auth=auth} ) )
        local id = os.startTimer( 0.05 )
        while true do
            local event = {os.pullEvent()}
            if event[ 1 ] == "rednet_message" and event[ 2 ] == "_confirm_" then
                links[ self.deviceName ].current = auth
                save()
                break
            elseif event[ 1 ] == "timer" and event[ 2 ] == id then
                break
            end
        end
    end,

    event = function( self, tEventData )
        local e = tEventData or {os.pullEvent()}
        if e[ 1 ] == "rednet_message" then
            local msg = textutils.unserialize( e[ 3 ] )
            local new = increment( links.__secret, links[ self.deviceName ].current )
            if type( msg ) == "table" and msg.auth == new then
                links[ self.deviceName ].current = new
                save()
                rednet.send( e[2], "_confirm_" )
                return "link_message", self.deviceName, msg
            end
        end
        return e
    end,
}

function establishLink( deviceName )
    return setmetatable( {deviceName = deviceName}, {__index = mt} )
end