Encrypt/decrypt same file with two different passwords

Question

Hi everyone:
I'm quite new to Java and cryptography in general and have a theoretical question. Is the following scenario possible and how would it be implemented:
Two users with two passwords (say, a regular user and a superuser) encrypt, decrypt, read from and write to the same file. The secret key for encryption and decryption should be based on their passwords (generated from their passwords), i.e. not stored anywhere on the system.
I've been racking my brains but can't think of a way. Am I missing an obvious solution?
Can it be done?
Thanks,
Michael

Answer 1

I don't think you can avoid having more than just a password hash stored on the system. Using a combination of my approach and Jeff's approach I can implement this as long as you allow a password protected key store to be stored on each system. A given user's key store would contain his RSA private key and associated public key together with the admin user's RSA certificate (thought the admin user's public key could be stored in the program since it does not have to be kept private). The admin user's key store would contain only his RSA private and public keys.
Assume that the data file is to be create by a standard non-admin user. His code performs the following actions -
1) Generates a random symmetric algorithm key. Say a 128 bit AES key.
2) He write a digest of this to the output file.
3) He writes the random key encrypted with his public key to the file.
4) He writes his public key (or certificate) to the file.
5) He writes the random key encrypted with the admin users public key to the file.
6) He encrypts the data using the random key writes the result to the file.
This user can then update the file by
1) reading from the file the digest of the random key.
2) reading the random key encrypted with his public key.
3) Decrypting this encrypted random key using his private key extracted from his keystore.
4) Check the digest of this key to make sure he has the correct random key.
5) skipping his certificate and the random key encrypted using the admin user's public key.
5) Decrypting the data using the random key.
6) Update the data.
7) Re-encrypt the file as described in the first part using a new random key.
The admin user can
1) read from the file the digest of the random key.
2) skip the random key encrypted using the user's public key.
3) reading the user's public key from the file (for use later if the file needs to be updated).
4) read the random key encrypted using the admin's public key.
5) decrypting the random key using the admin's private key obtained from his key store.
6) check the digest of the random key to make sure it is correct.
7) decrypt the the data.
The admin can edit the data since he can re-encrypt the data in a similar manner to the way it was created in the first place.