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What will happen if we change the DES encryrption algorithm to use the same key in each round ?

How will it affect the encryption - will it only make the encryption same as decryption or are the more effects ? I couldn't find any other possible effects in the DES standard description.

Thanks

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  • $\begingroup$ Don’t do it. It’s a bad idea. $\endgroup$ – Yuval Filmus Nov 13 '18 at 16:18
  • $\begingroup$ @YuvalFilmus - But what will be the effects ? besides making the decryption\encryption the same, what are the additional effects ? It's a theoretical question i have and can't figure out what else will happen $\endgroup$ – caffein Nov 13 '18 at 18:04
  • $\begingroup$ For one, it will make it easier to "guess" they key – all you need to do now is to guess one round key. Also, there might be a way to use a Pollard rho-type idea to somehow get input/output pairs for a single round. $\endgroup$ – Yuval Filmus Nov 13 '18 at 18:17
  • $\begingroup$ @YuvalFilmus - In the first case, did you mean we now have to guess only 2^48 combinations ? But in this case you need to be able to have access to the encryption machine so you'll be able to force the 48-bit value into the K- variable in each round instead of feeding the key from outside. Do you assume such an access to the machine ? how likely is this scenario ? What can you really do with the K value itself in reality - usually all we can do is feed the key itself from outside the machine $\endgroup$ – caffein Nov 13 '18 at 18:24
  • $\begingroup$ Because knowing the K value doesn't mean we know how the permutation of K really works. If we see a "1" somewhere in K we can't know which position in the key it came from $\endgroup$ – caffein Nov 13 '18 at 18:29
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It will make the algorithm insecure.

It will be become vulnerable to slide attacks.

It will make encryption become equivalent to decryption, which is a weakness. This is sometimes called a "weak key"; with your scheme, every key will be "weak".

It will also reduce the keyspace to 48 bits, which is too small to be secure.

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