cryptography - Password Verification with PBKDF2 in Java

Question

I'm doing password based file encryption in Java; I'm 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 encryption 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 (potentially long) decryption process. I know the PBKD spec includes an optional 2 byte verification value but I'm 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.