php mcrypt使用会话ID作为密钥

I successfully use mycrypt to encrypt id's in an admin panel as they are often seen in the URL. I have previously added a key manually as I code, but this time I plan to use the users account id which is kept in a session. Is this a good way, or should I keep with a fixed key throughout the site.

Thanks

Never pass SESSION IDs over URLs, even if encrypted (though it's weird and still quite unsafe). Transferring SESSION IDs has been deprecated long time ago by PHP (session.use_trans_id=0). Session ID is transferred automatically by browser, you don't need to take care of it at all.

The only use case might be if you want forcibly allow browsers with disabled cookies, then you would need to append such a parameter to all links (mimicking deprecated bahaviour). However:

• it would be still great security risk, since mostly attackers and malicious webbots disable cookies,
• encrypting in any ways makes no sense coz string can be still hijacked and server will decrypt into original session ID

So you just pull out userID stored in session and don't worry about transferring session ID over URL.