Scenario

• Master key is entered upon login then encrypted with $server_key
• Master key is now stored as $_COOKIE['encrypted_key'] variable for persistence (so user doesn't have to enter it every page load)
• $server_key is stored inside a config file in the app server
• Data is decrypted by first decrypting $_COOKIE['encrypted_key'] (using $server_key), therefore revealing Master key
• $_COOKIE['encrypted_key'] is destroyed upon browser exit

Threat

Attacker gaining access to user device/cookie and encrypted data (but not the app server config files).

Question

Does it make sense to encrypt the Master key with $server_key and then store $server_key inside a config file in the app server?

Reasoning

Gaining just the $server_key or just the $_COOKIE['encrypted_key'] will not compromise anything. The attacker must get both.

Update

• This is under HTTPS
• Session hijacking would be hard as we check session fingerprint per request