Hello,

i am writing about an image you created for the article about the Cast-128 crypto algorithm. As i studied the specific algorithm, i realized that there is a slight difference between the image and the Request For Comments 2144 where the algorithm is presented. Copying form the RFC:

      Type 1:  I = ((Kmi + D) <<< Kri)
               f = ((S1[Ia] ^ S2[Ib]) - S3[Ic]) + S4[Id]

      Type 2:  I = ((Kmi ^ D) <<< Kri)
               f = ((S1[Ia] - S2[Ib]) + S3[Ic]) ^ S4[Id]

      Type 3:  I = ((Kmi - D) <<< Kri)
               f = ((S1[Ia] + S2[Ib]) ^ S3[Ic]) - S4[Id]

The RFC also states:

  "CAST-128 uses a pair of subkeys per round:  a 32-bit quantity Km is
  used as a "masking" key and a 5-bit quantity Kr is used as a
  "rotation" key."

So i believe that the two sub-keys (Kmi and Kri) should be switched in place in the image. I believe that now the image depicts the opposite argument.

ICHTHUS

January 2012

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Hey Crypto Matt,

Wanted to say Hi :)