The 8 S-boxes of DES were designed to provide resistance against attacks. Small modifications to an S-box can significantly weaken DES.

Can the original input (the result of XOR on the expansion permutation of $R_{i-1}$ with the subkey $K_i$) to S-Boxes be determined?

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    $\begingroup$ When cryptoanalyze, we assume that the structure is known by the attacker but the key. As in your first paragraph, you said that changes weaken so It is expected that the attacks, Linear and Differential, can determine the key better than the brute-force. For better analyze and have an answer one needs the definition of the S-Boxes, But I don't think that nobody is going to analyze them for you. $\endgroup$ – kelalaka Mar 16 at 20:56

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