So I saw this example of IDEA ..

If the key is : 5a14 fb3e 021c 79e0 6081 46a0 117b ff03

The encryption subkeys are: https://i.stack.imgur.com/suj48.jpg

The decryption subkeys are: https://i.stack.imgur.com/EmCFU.jpg

And I can't figure out how the decryption keys were computed.. How is additive inverse and multiplicative inverse done to calculate the decryption subkeys? plss helppp!!

  • 1
    $\begingroup$ Is anything unclear in this? Or in what justifies it? $\endgroup$
    – fgrieu
    Mar 21, 2017 at 16:49
  • $\begingroup$ I only get the part that the order of the round keys is inverted and the subkeys for the odd rounds are inversed, but how is multiplicative and additive inverse done to get the decryption subkeys? $\endgroup$ Mar 21, 2017 at 17:35
  • 1
    $\begingroup$ In every case, the inverse to be computed is for the law implemented by the box the key enters in. The additive inverse requires only the most basic understanding of modular addition. The multiplicative inverse is even so slightly more difficult, and is discussed there. $\endgroup$
    – fgrieu
    Mar 21, 2017 at 17:42


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