A cookie should store only a randomly-generated unique ID
Are you suggesting that I should rely on the randomness of the ID to prevent spoofing. Surely storing a serial ID and also a secret unique to the session would be better.
There are also issues with your approach such as the complexity of checking that the ID is unique and also generating unique numbers when the available pool is largely used. Admittedly these would not be issues for low traffic but they do exist.
--tidiness is the memory loss of environmental mnemonics