Function mul

FHE/Ring_Element.cpp:161–240

void mul(Ring_Element& ans,const Ring_Element& a,const Ring_Element& b)
{
  CODE_LOCATION_NO_SCOPE

  assert(a.FFTD);
  if (a.rep!=b.rep)   { throw rep_mismatch(); }
  if (a.FFTD!=b.FFTD) { throw pr_mismatch();  }
  if (a.element.empty() or b.element.empty())
    {
      ans = Ring_Element(*a.FFTD, a.rep);
      return;
    }

  if (a.rep==evaluation)
    { // In evaluation representation, so we can just multiply componentwise
      if (&ans == &a)
        {
          ans *= b;
          return;
        }
      else if (&ans == &b)
        {
          ans *= a;
          return;
        }
      ans.prepare(a);
      for (int i=0; i<(*ans.FFTD).phi_m(); i++)
        ans.element.push_back(a.element[i].mul(b.element[i], a.FFTD->get_prD()));
    }
  else if ((*a.FFTD).get_twop()!=0)
    { // This is the case where m is not a power of two

      // Here we have to do a poly mult followed by a reduction
      // We could be clever (e.g. use Karatsuba etc), but instead
      // we do the school book method followed by term re-writing

      // School book mult
      vector<modp> aa(2*(*a.FFTD).phi_m());
      for (int i=0; i<2*(*a.FFTD).phi_m(); i++)
        { assignZero(aa[i],(*a.FFTD).get_prD()); }
      modp temp;
      for (int i=0; i<(*a.FFTD).phi_m(); i++)
        { for (int j=0; j<(*a.FFTD).phi_m(); j++)
	    { Mul(temp,a.element[i],b.element[j],(*a.FFTD).get_prD()); 
              int k=i+j;
              Add(aa[k],aa[k],temp,(*a.FFTD).get_prD());
            }
        }
      // Now apply reduction, assumes Ring.poly is monic
      reduce(aa, 2*(*a.FFTD).phi_m(), (*a.FFTD).phi_m(), *a.FFTD);
     // Now stick into answer
     ans.partial_assign(a);
     for (int i=0; i<(*ans.FFTD).phi_m(); i++)
       { ans.element[i]=aa[i]; }
    }
  else if ((*a.FFTD).get_twop()==0)
    { // m a power of two case
      Ring_Element aa(*ans.FFTD,ans.rep);