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How do we expand the abbreviation “Rcon” used in the Rijndael key schedule?

I know what it is, I know how it works; I just do not know for what it is a shortcut for. I mean: if "Sbox" is "Substitution box", then "Rcon" stands for… ?

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    $\begingroup$ All my life, I have been sure that Rcon means round constant. (But I haven't found a credible reference to back that claim so far.) $\endgroup$ – yyyyyyy Apr 2 '16 at 19:07
  • $\begingroup$ @yyyyyyy For a credible reference, check page 11 of csrc.nist.gov/publications/fips/fips197/fips-197.pdf $\endgroup$ – e-sushi Apr 3 '16 at 8:14
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Quoting from FIPS 197 (PDF), on page 11:

Rcon[] The round constant word array.

So, the answer is obviously "round constant".

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RCon == Round Constant

It's a value that slightly modifies how each round operates. Without it, AES might be vulnerable to a (related key) slide attack. Such an attack would probably not be of practical interest, however since it's so easy to avoid it (by including the RCon), it makes sense to.

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