I'm seeking an example of a cipher construction with two variants differing by key/block/state size yet sharing most of the internal structure. Where the larger varient is easier to break than the smaller version. Should be clearly related to a well known cipher so that claiming strength of smaller varient will have merit. Bonus points for increasing also key size not just block/state. I'm aware of how this could happen, just looking for an example where it does.
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2$\begingroup$ Isn't there a related-key attack against AES-256 that makes it much worse than AES-128 (but only in a highly unrealistic scenario since, well, it's a related-key attack)? $\endgroup$– Gilles 'SO- stop being evil'Jun 2, 2018 at 8:25
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2$\begingroup$ This is the paper Gilles was talking about, where AES-256 has "much less security" than the other two standardized variants. $\endgroup$– SEJPMJun 2, 2018 at 8:40
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