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I took a look at the implementation of Kyber, which can be found in this GitRepo. I made a small discovery in the indcpa.c in the function indcpa_keypair, starting at line 205, which I can't quite interpret. The line is given below.

  // matrix-vector multiplication
  for(i=0;i<KYBER_K;i++) {
    polyvec_basemul_acc_montgomery(&pkpv.vec[i], &a[i], &skpv);
    poly_tomont(&pkpv.vec[i]);
  }

What we do there, we perform a matrix-vector multiplication of the matrix a with the vector skpv, which is done via the first line in the loop. Then, in the next line, pkpv is converted into the Montgomery representation with poly_tomont. Why are we doing the latter?

My thoughts:

  • From the description of polyvec_basemul_acc_montgomery, see line 202 here, we got: "Pointwise multiply elements of a and b, accumulate into r, and multiply by 2^-16". If I remember correctly, the Montgomery factor was $2^{16}$, so if polyvec_basemul_acc_montgomery produces an output that is multiplied by $2^{-16}$, then we have to work with poly_tomont afterwards (because we multiply by $2^{16}$ again when converting to Montgomery representation) to get rid of the $2^{-16}$, right?

Further question:

  • In conext of polyvec_basemul_acc_montgomery, however, it is another question where the multiplication by $2^{-16}$ takes place...
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