Use of multiple keyfiles for TrueCrypt

Question

As mentioned in documentation, TrueCrypt would use first 1024 kilobytes of any large file to create SHA-512 Hash which will be combined with password to get the final hash.

E.g. SHA-512(SHA-512(keyfile max. size 1MB), password)

Now, If I use mulltiple keyfiles, will it process first 1MB of data from each file? In other words, Does the strength of encryption increase with number of keyfiles? Note that the order in which keyfiles are selected is irrelevant.

Answer 1

I believe that the sought answer is no, it would not increase the strength of the encryption.

If a 1MB keyfile is sufficient to utilize the power of the algorithm given its other limitations, then you have already maxed out the "strength" of the encryption. That is, if we are already harnessing the power available, then a brute-force attack would be no more successful with a triple-keyfile encrypted secret than with a single-keyfile encrypted secret. Notice the big IF in there, however.

Now I know there is a very technical answer, and I don;t have it. So I am just forwarding a proposed answer based on what I know about the principle, not the specific technical merits of the product.

On the other hand, it certainly would be of value in making key recovery more difficult through methods other than brute-force, and this is where the smart money is anyway. If I am trying random files I find on your machine as keyfiles, then we take a large number and increase exponentially. If there are 100 files and only one is a keyfile, then I have to try 100 files, but 10,000 combinations of two keyfiles (assuming re-use) and 1,000,000 combinations of three candidate keyfiles.

My problem with keyfiles is that I never know when the operating system is going to change something that the algorithm is counting on. I consider anything on the system volatile.