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I am a beginner in this field, and I was thinking about brute force strategies to break symmetric key encryption.

Let's say we have a block cipher in CTR mode and the key is 56 bits in size. What would a process of finding the key look like (brute forced strategy)?

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  • $\begingroup$ It seems you want to work on DES, see the historical machines on DES here and note that some of today's supercomputers can deal with less than an hour. $\endgroup$
    – kelalaka
    Feb 13 '21 at 17:47
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    $\begingroup$ The RSA DES challenges are nice examples. You need at least one known-plaintext to execute the attack, possibly more than one. Now, considering that you tagged CTR, are you going to execute crib-dragging on the CTR mode that requires that the $(key, IV)$ pair is used again? $\endgroup$
    – kelalaka
    Feb 13 '21 at 18:00
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Brute forcing the key simply means iterating over all the possible keys, assuming you know the mode of operation (in this case CTR) and the IV. Then if a likely plaintext is found then you've found your key.

It's always possible you find the wrong key, but the chances of you doing so diminishes with the amount of bits known of the plaintext. Usually you know enough about the plaintext to do so.

For instance, you can use the fact that the highest order bit of ASCII is always zero, most files have specific headers, you can do frequency analysis, etc. etc. If you perform an attack you might as well assume you know, say, 4 blocks of plaintext. In that case you can be next to certain that you found the right key to decrypt the rest.

Note that brute force attacks are very easy to parallelize; just split the key space and perform the decrypt / validation phase in separate threads.

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