Password Verification with PBKDF2 in Java

Question

Hi everybody, im doing password based file encryption in Java. Im using AES as the underlying encryption algorithm and PBKDF2WithHmacSHA1 to derive a key from a salt and password combination using the following code(which i got from another generous poster on this site).

SecretKeyFactory f = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
KeySpec ks = new PBEKeySpec(password,salt,1024,128);
SecretKey s = f.generateSecret(ks);
Key k = new SecretKeySpec(s.getEncoded(),"AES");

I share the salt, the user enters their password at each end and encrytion and decryption work fine :-) My problem is that i would like to be able to verify that the password the user enters is correct before embarking on the (potentialy long) decrytpion process. I know the PBKD spec includes an optional 2 byte verification value but im not sure how to generate this value using the above approach. Does java provide support for this or if not what would be a secure alternative?

Thanks for your time.

Answer 1

There is no "quick check" mechanism that is secure, by definition. The whole point of using PBKDF2 or related techniques is to make password checking slow, to foil password cracking programs. If you added a quick check system, password crackers would be able to guess passwords in bulk very quickly.

Answer 2

Compute some sort of password verification tag and store that alongside the encrypted file data so that you can check it first. This might be something like the PBMAC of a fixed (short) string. Of course, this needs to be a non-reversible function so a cracker could not determine the password, and not be too quick to compute so as to confound the brute force attack.

Have you considered whether (and how) you will detect if the whole file has been decrypted correctly? You should probably look into some combination of PBES2 and PBMAC rather than using AES directly.

Answer 3

Hey, thanks to crazy scot and Chris for there help. After doing some digging i decided to use the methods described on Dr Gladmans file encryption page for doing both password verification and message authentication. I believe this method, based on the PBKDF2 and a MAC, makes deriving the verfication value for m the password sufficiently expensive as to make it secure. Thanks again, and i hope this solution aids others.