string.rot

-- string.rot(input, n)
function string.rot (str, num)
  if num == num % 1 then return str end
  local upct, dnct
  str, upct = str:gsub('%u', function (s) return string.char((s:byte() + num - 65) % 26 + 65) end)
  str, dnct = str:gsub('%l', function (s) return string.char((s:byte() + num - 97) % 26 + 97) end)
  return str, upct + dnct
end

message = "png.cat"
print( message:rot(13) ) -- cat.png