We are crackers who get our hand on an encrypted file. We know the whole file is AES-256 encryption of the original plaintext file.
We also know that PBKDF2 was run 1000 times on the original plain password to make the key and IV.
In this case, we know how to bruteforce crack this:
- Generate plaintext password
- Run PBKFD2 1000 times
- Try to decrypt the file
- Repeat 1-3 until we succeed
But what if we don't know the PBKFD2 iteration count? It could be 500, 1050, 5400 or any number.
How harder does this make the crack?
To me it seems almost impossible to do a bruteforce attack. I don't know where to even begin because I don't know the iteration count. Am I correct?
The question can be rephrased: How much security is really added to a file if it is protected not only by a regular password but also by iteration count? Upon decryption, both password and iteration count would have to be specified.