Method MontgomeryMult

secp256k1/IntMod.cpp:777–813

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// ------------------------------------------------
void Int::MontgomeryMult(Int *a) {

  // Compute a*b*R^-1 (mod n),  R=2^k (mod n), k = Msize*64
  // a and b must be lower than n
  // See SetupField()

  Int t;
  Int pr;
  Int p;
  uint64_t ML;
  uint64_t c;

  // i = 0
  imm_umul(a->bits64, bits64[0], pr.bits64);
  ML = pr.bits64[0] * MM64;
  imm_umul(_P.bits64, ML, p.bits64);
  c = pr.AddC(&p);
  memcpy(t.bits64, pr.bits64 + 1, 8 * (NB64BLOCK - 1));
  t.bits64[NB64BLOCK - 1] = c;

  for (int i = 1; i < Msize; i++) {

    imm_umul(a->bits64, bits64[i], pr.bits64);
    ML = (pr.bits64[0] + t.bits64[0]) * MM64;
    imm_umul(_P.bits64, ML, p.bits64);
	  c = pr.AddC(&p);
    t.AddAndShift(&t, &pr, c);

  }

  p.Sub(&t,&_P);
  if (p.IsPositive())
    Set(&p);
  else
    Set(&t);

}

void Int::MontgomeryMult(Int *a, Int *b) {