Using MD5 to generate an encryption key from password?
I'm writing a simple program for file encryption. Mostly as an academic exercise but possibly for future serious use. All of the heavy lifting is done with third-party libraries, but putting the pieces together in a secure manner is still quite a challenge for the non-cryptographer. Basically, I've got just about everything working the way I think it should.
I'm using 128-bit AES for the encryption with a 128-bit key length. I want users to be able to enter in variable-length passwords, so I decided to hash the password with MD5 and then use the hash as the key. I figured this was acceptable--the key is always supposed to be a secret, so there's no reason to worry about collision attacks.
Now that I've implemented this, I ran across a couple articles indicating that this is a bad idea. My question is: why? If a good password is chosen, the cipher is supposed to be strong enough on its own to never reveal the key except via an extraordinary (read: currently infeasible) brute-force effort, right? Should I be using something like PBKDF2 to generate the key or is that just overkill for all but the most extreme cryptographic applications?
Answer
This article on Key strengthening might help you. Basically you want to make the key stronger (more entropy than in a password) and make its derivation from the password reliably time consuming.