lua tonumber can't handle big integer

lua_tobignum.lua

function tobignum(v, f, step)
   -- 自带tonumber遇到大整数溢出
   -- v is a md5 hexdigest value, f is base format like binary or hex, use step to slice v 
   if f == nil then
      f = 16
   end
   if step == nil then
      step = 8
   end
   local len = string.len(v)
   local term = math.ceil(len/step)
   local t = 0
   for i=1, term do
      local start = step*(i-1) + 1
      local seg = string.sub(v, start, start+step-1)
      local m = math.pow(f, len - i*step)
      t = t + tonumber(seg, f) * m
      --ngx.log(ngx.ERR, '---start',start ,'---v', seg, '---m', m, '---t', t)
   end
   return t
end