random_keys.lua

-- If you need pure Lua *and* don't need crypto strength. 

local function random_keys(n)
    local t = {}
    return function()
        for i = 1, n do t[i] = string.char(math.random(0, 255)) end
        return table.concat(t)
    end
end

local next_key = random_keys(16)
local key1 = next_key()
local key2 = next_key()
--- ... etc

seq_keys.lua

local function seq_keys(n)
    local t = {}
    for i = 1, n - 1 do t[i] = 0 end
    t[n] = -1
    return function()
        for i = n, 1, -1 do
            if t[i] == 255 then
                t[i] = 0
            else
                t[i] = t[i] + 1
                break
            end
        end
        return string.char(table.unpack(t))
    end
end

local next_key = seq_keys(16)
local key1 = next_key()
local key2 = next_key()
--- ... etc