I am about to go study for my masters in machine learning, data mining and high

Question

I am about to go study for my masters in machine learning, data mining and high performance computing, but have recently become very interested in cryptography after taking Dan Boneh's Cryptography course on coursera.com.

I was wondering if there are any practical applications for machine learning in cryptography? A quick google search didn't seem to reveal much for me so i was wondering if someone here knows something.

My first impression would be that machine learning techniques could be used to learn the inverse function for some cipher and passphrase given enough plaintext/ciphertext pairs, but i am not sure how capable things like neural networks are at learning functions which are not linearly separable.

Explanation / Answer

I would personally be very surprised if machine learning was of any use in a known plaintext attack.

We design our ciphers to look a lot like random functions; you give the black box an input, and an output spits out. You give it a second input (possibly the same input in the case of nondetermanistic encryption), and a second output spits out. What we try to achieve is that no one can determine whether the black box was our cipher (with an unknown key), or whether it's just spitting out random outputs.

Now, we assume that the attacker has the complete design of our input (apart from the 'key'); in a successful cipher, he still cannot determine it. In fact, we design things so that the attacker can submit inputs of his own choosing; he still cannot determine whether he's giving inputs to the cipher or a random function.

Now, what machine learning would be trying to do is essentially this, except that you would be ignoring the design (because there's no way to give the design to the machine learning process), and you are limited to a known plaintext attack (because your machine learner has no way to generate the chosen inputs). In addition, your machine learner would also not have the best asset a cryptanalyst has (which is his cleverness; programs aren't good at 'clever').

Now, I don't know enough about machine learning to say that it can't be used elsewhere (or that a clever cryptanalysis couldn't use a machine learning program to help with his cryptanalysis); however it certainly sounds to me that machine learning would not be enough to do the entire cryptanalysis job.

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