Diffie-Hellman API implemented by me.

function connect(modem,AUTH_CHANNEL,BigNum)
    local prime = BigNum.new("625210769")
    local base = BigNum.new("11")
    local secret = BigNum.new(math.random(1,1000))
    local event = {}
    local PING_PORT = 1
    local KEY_EXCHANGE_PORT = 2
    local order
    local _, mySecret = (base^secret)/prime
    local hisSecret

    modem.open(AUTH_CHANNEL)
    modem.transmit(AUTH_CHANNEL,PING_PORT,"ping")

    while true do
        event = {os.pullEvent()}
        --event, side, frequency, replyFrequency, message, distance
        if event[1] == "modem_message" and event[4] == PING_PORT then
            if event[5] == "ping" then
            order = 2
            modem.transmit(AUTH_CHANNEL,1,"pong")
            break
            elseif event[5] == "pong" then
            order = 1
            break
            end
        end
    end

    if order == 1 then
        modem.transmit(AUTH_CHANNEL,KEY_EXCHANGE_PORT,mySecret)
        repeat
            event = {os.pullEvent()}
        until event[1] == "modem_message" and event[4] == KEY_EXCHANGE_PORT
        hisSecret = event[5]
        elseif order == 2 then
        repeat
            event = {os.pullEvent()}
        until event[1] == "modem_message" and event[4] == KEY_EXCHANGE_PORT
        hisSecret = event[5]
        modem.transmit(AUTH_CHANNEL,KEY_EXCHANGE_PORT,mySecret)
    end
    local _,v = (hisSecret^secret)/prime
    return v, order
end