inline vector PIRServer::expand_query()

in pir_server.cpp [260:348]

• 55 lines of code
• 7 McCabe index (conditional complexity)

inline vector<Ciphertext> PIRServer::expand_query(const Ciphertext &encrypted, uint32_t m,
                                           uint32_t client_id) {

#ifdef DEBUG
    uint64_t plainMod = params_.plain_modulus().value();
    cout << "PIRServer side plain modulus = " << plainMod << endl;
#endif

    GaloisKeys &galkey = galoisKeys_[client_id];

    // Assume that m is a power of 2. If not, round it to the next power of 2.
    uint32_t logm = ceil(log2(m));
    Plaintext two("2");

    vector<int> galois_elts;
    auto n = params_.poly_modulus_degree();
    if (logm > ceil(log2(n))){
        throw logic_error("m > n is not allowed."); 
    }
    for (int i = 0; i < ceil(log2(n)); i++) {
        galois_elts.push_back((n + exponentiate_uint64(2, i)) / exponentiate_uint64(2, i));
    }

    vector<Ciphertext> temp;
    temp.push_back(encrypted);
    Ciphertext tempctxt;
    Ciphertext tempctxt_rotated;
    Ciphertext tempctxt_shifted;
    Ciphertext tempctxt_rotatedshifted;


    for (uint32_t i = 0; i < logm - 1; i++) {
        vector<Ciphertext> newtemp(temp.size() << 1);
        // temp[a] = (j0 = a (mod 2**i) ? ) : Enc(x^{j0 - a}) else Enc(0).  With
        // some scaling....
        int index_raw = (n << 1) - (1 << i);
        int index = (index_raw * galois_elts[i]) % (n << 1);

        for (uint32_t a = 0; a < temp.size(); a++) {

            evaluator_->apply_galois(temp[a], galois_elts[i], galkey, tempctxt_rotated);

            //cout << "rotate " << client.decryptor_->invariant_noise_budget(tempctxt_rotated) << ", "; 

            evaluator_->add(temp[a], tempctxt_rotated, newtemp[a]);
            multiply_power_of_X(temp[a], tempctxt_shifted, index_raw);

            //cout << "mul by x^pow: " << client.decryptor_->invariant_noise_budget(tempctxt_shifted) << ", "; 


            multiply_power_of_X(tempctxt_rotated, tempctxt_rotatedshifted, index);

            // cout << "mul by x^pow: " << client.decryptor_->invariant_noise_budget(tempctxt_rotatedshifted) << ", "; 


            // Enc(2^i x^j) if j = 0 (mod 2**i).
            evaluator_->add(tempctxt_shifted, tempctxt_rotatedshifted, newtemp[a + temp.size()]);
        }
        temp = newtemp;
        /*
        cout << "end: "; 
        for (int h = 0; h < temp.size();h++){
            cout << client.decryptor_->invariant_noise_budget(temp[h]) << ", "; 
        }
        cout << endl; 
        */
    }
    // Last step of the loop
    vector<Ciphertext> newtemp(temp.size() << 1);
    int index_raw = (n << 1) - (1 << (logm - 1));
    int index = (index_raw * galois_elts[logm - 1]) % (n << 1);
    for (uint32_t a = 0; a < temp.size(); a++) {
        if (a >= (m - (1 << (logm - 1)))) {                       // corner case.
            evaluator_->multiply_plain(temp[a], two, newtemp[a]); // plain multiplication by 2.
            // cout << client.decryptor_->invariant_noise_budget(newtemp[a]) << ", "; 
        } else {
            evaluator_->apply_galois(temp[a], galois_elts[logm - 1], galkey, tempctxt_rotated);
            evaluator_->add(temp[a], tempctxt_rotated, newtemp[a]);
            multiply_power_of_X(temp[a], tempctxt_shifted, index_raw);
            multiply_power_of_X(tempctxt_rotated, tempctxt_rotatedshifted, index);
            evaluator_->add(tempctxt_shifted, tempctxt_rotatedshifted, newtemp[a + temp.size()]);
        }
    }

    vector<Ciphertext>::const_iterator first = newtemp.begin();
    vector<Ciphertext>::const_iterator last = newtemp.begin() + m;
    vector<Ciphertext> newVec(first, last);
    return newVec;
}