diff --git a/lib/livebook/utils.ex b/lib/livebook/utils.ex
index 16d1714da..7d41cd9a8 100644
--- a/lib/livebook/utils.ex
+++ b/lib/livebook/utils.ex
@@ -24,7 +24,7 @@ defmodule Livebook.Utils do
   """
   @spec random_cookie() :: atom()
   def random_cookie() do
-    :crypto.strong_rand_bytes(35) |> Base.encode32(case: :lower) |> String.to_atom()
+    :"c_#{Base.url_encode64(:crypto.strong_rand_bytes(39))}"
   end
 
   @doc """