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I was wondering, why DES is secure against Differential Attacks. Some of the S-Boxes have a very bad Differential Uniformity, only one has a DU of 4, 3 have a DU of 6 and the others are much worse.

So is it because of the few good S-Boxes or because of DES uses 8 different S-Boxes?

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It's mainly due to the use of multiple rounds and the presence of the various permutation and expansion mappings that bring the overall maximal differential probability to a very low level for key recovery.

This then leads to unrealistic requirements for chosen plaintext/ciphertext pairs encrypted under the same key, in ECB mode.

And the presence of different sboxes complicates things a bit further. Under current technology, or even for the last 20 years, a simple brute force key search with a small amount of plaintext ciphertext pairs is much simpler and easier.

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