Monday, February 4, 2013

Properly Encrypting With AES With CommonCrypto

Rob Napier (via Jonathan Rentzsch):

First, you need to salt your key. That means adding random data to it so that if the same data is encrypted with the same password, the ciphertext will still be different. The key should then be hashed, so that the final result is the correct length. The correct way to do this is with PKCS #5 (PBKDF2).

[…]

OK, now you have a salt. What do you do with it? Save it with the cipertext. You’ll need it later to decrypt. The salt is considered public information so you don’t need to protect it.

And now the mystical initialization vector (IV) that confuses everyone. In CBC-mode, each 16-byte encryption influences the next 16-byte encryption. This is a good thing. It makes the encryption much stronger. It’s also the default. The problem is, what about block 0? The answer is you make up a random block -1. That’s the IV.

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