Exemple #1
0
// incrementalZeroTest sets each res[i], for i=0..n-1, to
// a ciphertext in which each slot is 0 or 1 according
// to whether or not bits 0..i of corresponding slot in ctxt
// is zero (1 if not zero, 0 if zero).
// It is assumed that res and each res[i] is already initialized
// by the caller.
// Complexity: O(d + n log d) smart automorphisms
//             O(n d) 
void incrementalZeroTest(Ctxt* res[], const EncryptedArray& ea,
			 const Ctxt& ctxt, long n)
{
  FHE_TIMER_START;
  long nslots = ea.size();
  long d = ea.getDegree();

  // compute linearized polynomial coefficients

  vector< vector<ZZX> > Coeff;
  Coeff.resize(n);

  for (long i = 0; i < n; i++) {
    // coeffients for mask on bits 0..i
    // L[j] = X^j for j = 0..i, L[j] = 0 for j = i+1..d-1

    vector<ZZX> L;
    L.resize(d);

    for (long j = 0; j <= i; j++) 
      SetCoeff(L[j], j);

    vector<ZZX> C;

    ea.buildLinPolyCoeffs(C, L);

    Coeff[i].resize(d);
    for (long j = 0; j < d; j++) {
      // Coeff[i][j] = to the encoding that has C[j] in all slots
      // FIXME: maybe encrtpted array should have this functionality
      //        built in
      vector<ZZX> T;
      T.resize(nslots);
      for (long s = 0; s < nslots; s++) T[s] = C[j];
      ea.encode(Coeff[i][j], T);
    }
  }

  vector<Ctxt> Conj(d, ctxt);
  // initialize Cong[j] to ctxt^{2^j}
  for (long j = 0; j < d; j++) {
    Conj[j].smartAutomorph(1L << j);
  }

  for (long i = 0; i < n; i++) {
    res[i]->clear();
    for (long j = 0; j < d; j++) {
      Ctxt tmp = Conj[j];
      tmp.multByConstant(Coeff[i][j]);
      *res[i] += tmp;
    }

    // *res[i] now has 0..i in each slot
    // next, we raise to the power 2^d-1

    fastPower(*res[i], d);
  }
  FHE_TIMER_STOP;
}
Exemple #2
0
/**
 * @brief Extract coefficients from ciphertext polynomial

 * @param coeffs extracted coefficients
 * @param ctxt ciphertext
 * @param n extract "n" lowest degree coefficients
 */
void extractCoeffs(EncryptedArray& ea, vector<Ctxt>& coeffs, Ctxt& ctxt, long n) {
  long d = ea.getDegree();
  if (d < n) n = d;

  coeffs.clear();

  vector<Ctxt> conj;  
  for (int coeff = 0; coeff < n; ++coeff) {
    vector<ZZX> LM(d);
    LM[coeff] = ZZX(0, 1);

    // "building" the linearized-polynomial coefficients
    vector<ZZX> C(d);
    ea.buildLinPolyCoeffs(C, LM);

    coeffs.push_back(ctxt);
    applyLinPoly1(ea, coeffs[coeff], C, conj);
  }
}