Enhancing Security by Splitting Key in Different Places

Enhancing Security by Splitting Key in Different Places

Essentially what you’re attempting to do is backdoor each user’s account in a way that allows only the user and a single administrative user to access their data. This can be achieved as follows:

us  = User salt  = Random unique value (not the same salt as used for authentication)
as  = Admin salt = Random unique value (not the same salt as used for authentication)

uk  = User key    = pbkdf2(user_pass, salt1, 256, rounds)
dk  = Data key    = random 256-bit key
ak  = Admin key   = pbkdf2(admin_pass, salt2, 256, rounds)
P   = RSA private key
p   = RSA public key

uk' = Encrypted user key    = dk ^ uk
dk' = Encrypted data key    = RSA_Encrypt(dk, p)
P'  = Encrypted private key = AES_Encrypt(P, ak)

m = message
iv = initialisation vector (random unique value)
c = ciphertext = AES_Encrypt(m, iv, dk)

We then store uk', dk' and iv with the data record we’re encrypting. We store us with the user account record. We also store P' and as with our admin account record. The public key P can be stored in the code.

The user decryption works as follows:

1. Compute uk from the user password and us.
2. Xor uk' with uk to retrieve dk.
3. Decrypt c using iv and dk, giving us m.

The admin decryption works as follows:

1. Compute ak from the admin password and as.
2. Decrypt P using AES_Decrypt(P', ak)
3. Decrypt dk using RSA_Decrypt(dk', P)
4. Decrypt c using iv and dk, giving us m.

Note that when I say “encrypt” and “decrypt” in reference to RSA, it’s not really encryption, but it is more or less semantically equal.