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Why are AES keys increments of 64-bits? Rijndael, which subsequently became AES, allowed key increments in 32-bits (128, 160, 192, 224, and 256 bits). However, AES made it 128, 192, or 256.

My guess would be that a 64-bit default increment may be good to increase the lifetime of a particular key size for a particular application. For example, assume that an application is using 128-bits and then the application owners feel they have to increase key size because of threats. An increase to 192-bits (cf. 160-bits) may give this upgrade a longer life before technology advancement catches up and the application owners have to change again. Is this speculation in the right direction?

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    $\begingroup$ In hindsight, the 192 bit key size was probably a mistake. 192 is not a multiple of 128, which makes things like key wrapping a tricky thing to do. Kudo's though for at least restricting the amount of choices, like they did not do for e.g. KDF's, which are a frickin' mess. When it comes to standardization the limiting of choices is almost a goal in itself. Less testing, more compatible implementations... $\endgroup$ – Maarten Bodewes Jan 21 '20 at 17:59
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When NIST made the call for AES, they were trying to address short comings of DES that had a 56-bit key (64-bits with parity). I got the following information from NIST while at the NIST 8114 conferences because I was complaining about having to keep 192-bit key compatibility in my SIMON hardware when no one would likely use it.

The summary:

In 1997, the idea was to double the key length (to 128-bits) and then double the key length again (to 256-bits). The reason that 192 bits was there was because it was thought that an intermediate key length would be good for systems that couldn't handle the complete 256-bit length. You need to keep in mind that at the time of specifying 128/192/256 bits, and algorithm had not yet been picked. On 1997 hardware, 192-bits takes more time than 256-bits, and it was hoped that by having a larger key size the life of AES would be more than 20-years, unlike DES.

As mentioned in the comments, retrospectively, the 192-bit requirement was likely not optimal as pretty much no one uses it. You have to include a 192-bit key option in your hardware cores for complete AES compatibility. I had a bug in my netlist verification for AES-192 for 10 years, and I only found it recently, which really means... no one every uses AES-192.

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    $\begingroup$ As an aside, I have one user of SIMON128/192, and this is only because the fast clock on their RF system is not fast enough to do SIMON128/256. $\endgroup$ – b degnan Jan 23 '20 at 14:08

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