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While working on a homework assignment, I came across an interesting dilemma that I want to delve more into. The question essentially said to flip the first bit in the key string and then see what S boxes it will effect. Then it says to do the same for decryption.

I went through the process of doing this encryption and then decryption and found that each process is the exact opposite. This makes sense logically but I feel like that is not as... secure? Would the S boxes be the exact same in the decryption phase as in the encryption phase just in reverse? Wouldn't there be a way to sense the pattern here? I know DES is not the most secure system anyway but this seems even more unsecured? I'm just a bit confused on how the math from encryption to decryption works. Clearly it isn't linear but it seems like it?

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marked as duplicate by e-sushi Oct 24 '18 at 22:29

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  • $\begingroup$ try describing the truth table of an s-box with a linear equation if you think it looks linear, it will then be clear it is not (for DES anyway) $\endgroup$ – Richie Frame Oct 16 '18 at 0:31
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The $S$-boxes are used in the round function of a Feistel cipher, so they will be exactly the same for encryption and decryption.

We're not using a SP-network here like AES, where we need the inverses of $S$-boxes for decryption.

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