How can I encrypt user content on my site so that not even I can access the content?

Question

I need to encrypt content in my web application on a per-user basis.

I, the root user, do not want to have access to users' content, period.

How can I make it so users are the only ones with access to their content? Perhaps I can make it so a hash of their login password acts as an encryption and decryption key (then their password is stored one-way hashed in my database, and the encryption/decryption hash is generated from their raw password on login and stored in a local cookie)? But what if they change their password? Then I have to update all their content which could take a lot of processing power.

Is there an encryption method that would provide this, without having to re-encrypt their content if their password changes? Something similar to ecryptfs on Linux, perhaps? Is researching ecryptfs a good place to start?

Is making it so only the user can access their content on my servers (and not even me) even feasible?

Answer 1

Process:

1. Generate a random secret to encrypt their content.
2. Using their provided password encrypt the random secret from #1.
3. Store their password as a one-way hash (with salt, maybe multi-hash).

Upon Password change:

1. Re-generate the value from step #2.
2. Re-generate the hash-cache from step #3.

Upon Login:

1. Hash password and check against hash generated in step #3.
2. If password matches - use actual provided password to decrypt random secret from #2.
3. Use random secret from #2 to unlock data encrypted in #1.

Notes:

• No one can decode the data without knowing the random secret (#1). Random secret can only be unlocked with user's actual password (#2) (short of brute-force). User's actual password is only known in one-way hashed form (#3) so you can confirm it's the same, but cannot decode it and recover #2.
• A forgotten password process is not possible (you can regenerate #3, but random key in #2 is now lost as is everything locked in their vault).
• You don't have to re-encrypt everything in step #1 every time they change their password, only the (simple/quick) random secret from #2.
• If you cache their provided password, or the random secret generated at step 1, or their (decrypted) content anywhere you could cause data leaks.