Can I use PBKDF2 to generate an AES256 key to encrypt and implicitly authenticate?

I have 2 devices and I want to set up a secure communication channel between them. The only shared secret is a (7- to 20- character ASCII) passphrase. If I use PBKDF2 (from RFC 2898) with a common salt, iterations, and passphrase to generate an AES256-CBC key and IV on both sides, I think I can authenticate the user and provide an encrypted channel all in one step. Is that true, or is there some reason why I've only seen people use PBKDF2 to verify passwords?

My reasoning is that both sides need to know the passphrase to generate the same key and IV. So if device B can decrypt data from device A, they both have demonstrated that they have the same passphrase.

Answers


PBKDF2 is a fine way to generate a common key from a shared secret (you should not be generating the IV in such a way though - the IV should be random, and sent alongside the ciphertext).

However, CBC is not an authenticating cipher mode. This is because an attacker can take an encrypted message and make predictable modifications to it, without needing to be able to read the message or know the key. Such attacks have broken real world systems in the past.

You can use an authenticating cipher mode, like Galois Counter Mode (GCM) instead of CBC.

An alternative is Encrypt-Then-MAC. Use PBKDF2 with two different salts to generate two different keys - first the data is encrypted using CBC with the first key, and then a HMAC is calculated over the ciphertext using the second key.

You will also need to use single-use-nonces to prevent replay attacks.


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