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Kaweichel is a port of Blowfish for 64-bit architectures, its key size accepted is 1920-bytes, instead of 16 rounds it has 32 rounds, the double of Blowfish.

ICE is a feistel block cipher that has a variant called ICE-n that accepts any key size being a multiple of 64, for this variant the number of rounds should be multiplied by the number of 64-bits slices in the key material.

/\ These two ciphers left me doubts.

Can the security margin of any feistel block cipher be multiplied if I just multiply the number of slices of original key size of the cipher by the number of rounds (of course, multiplying the number of random keys too)?

** Sorry about my English, someone correct if necessary.

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  • $\begingroup$ It's possible to construct a Feistel cipher that becomes insecure at a certain high number of rounds due to weakness in key schedule or/and round function, and IIRC there are practical ciphers where that happens to some degree. Which would answer the question in the title by no. That could change is we add "with random independent subkeys". But I'm unsure and out of my comfort zone, thus I let others answer. Note: "security margin" would need to be measured as a (dimensionless) factor of attack effort rather than on a base-2 log scale of that (in bit) so that "multiplied" applies. $\endgroup$
    – fgrieu
    Mar 28, 2022 at 7:29
  • $\begingroup$ @fgrieu I forgot to put "with random independent subkeys", thanks, you answered my question. $\endgroup$ Mar 30, 2022 at 2:38
  • $\begingroup$ I'm afraid there is much more to it than in my comment (like, at some point, the width of the message space becomes an issue), but again I'm not that comfortable with the subject. $\endgroup$
    – fgrieu
    Mar 30, 2022 at 5:39
  • $\begingroup$ @fgrieu I changed the title and the description of my question. $\endgroup$ Mar 31, 2022 at 7:18

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Increased? yes. Not in the formal proof sense but as in it's hard to imagine a Feistal cipher not tailored to have an issue becoming any weaker by adding rounds with independent sub keys.

Doubled indefinitely? no. For instance some attacks only depend on block size. If I have a 64 bit block size we will have an issue even if we have a huge key and many rounds. An attacker may just collect plain text cipher text pairs.

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