Native sessions or CI sessions.

[eluser]WanWizard[/eluser]
Sigh... Native sessions are NOT secure! They're quite easy to hijack, and if you're on a shared server, other sites can read your session files unless you're with a good hoster how has taken precautions.

CI's sessions use a session cookie that's encrypted, uses automatic session id rotation, and contains extra fields to prevent session hijacking.
Session data is stored server side in a database table (do not use cookie based sessions unless you store a very small amount of data), no data is ever sent to the client.