Two way DB encryption secure even from the Admin

Question

I have an interesting encryption problem at hand. I do not know if it can be solved but here goes:

A database is to contain sensitive user information. As such, the user

Answer 1

What you want is a recovery agent. Encrypt all data twice: once with the user key, once with the recovery agent (public) key; atleast the latter one needs to be asymmetric. Keep the recovery agent key in a pyhsical safe, with a formal access protocol (e.g. four eyes principle). Usually, the administrator cannot access the encrypted data, but if the user loses the key, and recovery is authorized, then the recovery key is obtained.

There are also ways to encrypt the recovery agent's key so that m-out-of-n people have to agree to use it.

Edit: One implementation strategy is to encrypt everything twice. Alternatively, for each data set that needs to be recoverable independently, create a fresh symmetric key, and encrypt only that key twice; the original data get encrypted only with the session key. That approach can extend to multiple independent readers; it requires asymmetric keys per reader (so that you can encrypt the session key with the public keys of all readers - one being the recovery agent).

I copied the terminology from Microsoft's Encrypting File System, which has that scheme implemented.

Answer 2

Can't be done.

In all cases, someone has to be able to recreate the key to decrypt it. Let's consider the options:

1. Key stored on server. Fails: administrator has access.
2. Key encrypted with user's password. Fails: user might forget it.

The solution is to relax the administrator having access restriction, and instead of impossible, you make it just very difficult. For example, if the data were encrypted with a key stored encrypted with the user's password, but that key were escrowed in some other system which can't be accessed in the normal course of events by the administrator (perhaps only another admin has access?) then you can still recover from a user forgetting their password (with intervention of whoever has access to escrowed keys), but the admin can't just download your database and read all the data.