SealPIR/src/pir.cpp

#include "pir.hpp"

using namespace std;
using namespace seal;
using namespace seal::util;

std::vector<std::uint64_t> get_dimensions(std::uint64_t num_of_plaintexts, std::uint32_t d) {

    assert(d > 0);
    assert(num_of_plaintexts > 0);

    std::uint64_t root = max(static_cast<uint32_t>(2),static_cast<uint32_t>(floor(pow(num_of_plaintexts, 1.0/d))));

    std::vector<std::uint64_t> dimensions(d, root);

    for(int i = 0; i < d; i++){
        if(accumulate(dimensions.begin(), dimensions.end(), 1, multiplies<uint64_t>()) > num_of_plaintexts){
            break;
        } 
        dimensions[i] += 1;
    }

    std::uint32_t prod = accumulate(dimensions.begin(), dimensions.end(), 1, multiplies<uint64_t>());
    assert(prod >= num_of_plaintexts);
    return dimensions;
}

void gen_encryption_params(std::uint32_t N, std::uint32_t logt,
                           seal::EncryptionParameters &enc_params){
    
    enc_params.set_poly_modulus_degree(N);
    enc_params.set_coeff_modulus(CoeffModulus::BFVDefault(N));
    enc_params.set_plain_modulus(PlainModulus::Batching(N, logt+1)); 
    // the +1 above ensures we get logt bits for each plaintext coefficient. Otherwise
    // the coefficient modulus t will be logt bits, but only floor(t) = logt-1 (whp) 
    // will be usable (since we need to ensure that all data in the coefficient is < t).
}

void verify_encryption_params(const seal::EncryptionParameters &enc_params){
    SEALContext context(enc_params, true);
    if(!context.parameters_set()){
        throw invalid_argument("SEAL parameters not valid.");
    }
    if(!context.using_keyswitching()){
        throw invalid_argument("SEAL parameters do not support key switching.");
    }
    if(!context.first_context_data()->qualifiers().using_batching){
        throw invalid_argument("SEAL parameters do not support batching.");
    }

    BatchEncoder batch_encoder(context);
    size_t slot_count = batch_encoder.slot_count();
    if(slot_count != enc_params.poly_modulus_degree()){
        throw invalid_argument("Slot count not equal to poly modulus degree - this will cause issues downstream.");
    }

    return;
}

void gen_pir_params(uint64_t ele_num, uint64_t ele_size, uint32_t d,
                    const EncryptionParameters &enc_params, PirParams &pir_params,
                    bool enable_symmetric, bool enable_batching){
    std::uint32_t N = enc_params.poly_modulus_degree();
    Modulus t = enc_params.plain_modulus();
    std::uint32_t logt = floor(log2(t.value())); // # of usable bits
    std::uint64_t elements_per_plaintext;
    std::uint64_t num_of_plaintexts;

    if(enable_batching){
        elements_per_plaintext = elements_per_ptxt(logt, N, ele_size);
        num_of_plaintexts = plaintexts_per_db(logt, N, ele_num, ele_size);
    }
    else{
        elements_per_plaintext = 1;
        num_of_plaintexts = ele_num;
    }

    vector<uint64_t> nvec = get_dimensions(num_of_plaintexts, d);

    uint32_t expansion_ratio = 0;
    for (uint32_t i = 0; i < enc_params.coeff_modulus().size(); ++i) {
        double logqi = log2(enc_params.coeff_modulus()[i].value());
        expansion_ratio += ceil(logqi / logt);
    }

    pir_params.enable_symmetric = enable_symmetric;
    pir_params.enable_batching = enable_batching;
    pir_params.ele_num = ele_num;
    pir_params.ele_size = ele_size;
    pir_params.elements_per_plaintext = elements_per_plaintext;
    pir_params.num_of_plaintexts = num_of_plaintexts;
    pir_params.d = d;                 
    pir_params.expansion_ratio = expansion_ratio << 1;           
    pir_params.nvec = nvec;
    pir_params.slot_count = N;
}


void print_pir_params(const PirParams &pir_params){
    std::uint32_t prod = accumulate(pir_params.nvec.begin(), pir_params.nvec.end(), 1, multiplies<uint64_t>());

    cout << "PIR Parameters" << endl;
    cout << "number of elements: " << pir_params.ele_num << endl;
    cout << "element size: " << pir_params.ele_size << endl;
    cout << "elements per BFV plaintext: " << pir_params.elements_per_plaintext << endl;
    cout << "dimensions for d-dimensional hyperrectangle: " << pir_params.d << endl;
    cout << "number of BFV plaintexts (before padding): " << pir_params.num_of_plaintexts << endl;
    cout << "Number of BFV plaintexts after padding (to fill d-dimensional hyperrectangle): " << prod << endl;
    cout << "expansion ratio: " << pir_params.expansion_ratio << endl;
    cout << "Using symmetric encryption: " << pir_params.enable_symmetric << endl;
    cout << "slot count: " << pir_params.slot_count << endl;
    cout << "=============================="<< endl;
}


void print_seal_params(const EncryptionParameters &enc_params){
    std::uint32_t N = enc_params.poly_modulus_degree();
    Modulus t = enc_params.plain_modulus();
    std::uint32_t logt = floor(log2(t.value()));

    cout << "SEAL encryption parameters"  << endl;
    cout << "Degree of polynomial modulus (N): " << N << endl;
    cout << "Size of plaintext modulus (log t):" << logt << endl;
    cout << "There are " << enc_params.coeff_modulus().size() << " coefficient modulus:" << endl;

    for (uint32_t i = 0; i < enc_params.coeff_modulus().size(); ++i) {
        double logqi = log2(enc_params.coeff_modulus()[i].value());
        cout << "Size of coefficient modulus " << i << " (log q_" << i << "): " << logqi << endl;
    }
    cout << "=============================="<< endl;
}


// Number of coefficients needed to represent a database element
uint64_t coefficients_per_element(uint32_t logt, uint64_t ele_size) {
    return ceil(8 * ele_size / (double)logt);
}

// Number of database elements that can fit in a single FV plaintext
uint64_t elements_per_ptxt(uint32_t logt, uint64_t N, uint64_t ele_size) {
    uint64_t coeff_per_ele = coefficients_per_element(logt, ele_size);
    uint64_t ele_per_ptxt = N / coeff_per_ele;
    assert(ele_per_ptxt > 0);
    return ele_per_ptxt;
}

// Number of FV plaintexts needed to represent the database
uint64_t plaintexts_per_db(uint32_t logt, uint64_t N, uint64_t ele_num, uint64_t ele_size) {
    uint64_t ele_per_ptxt = elements_per_ptxt(logt, N, ele_size);
    return ceil((double)ele_num / ele_per_ptxt);
}

vector<uint64_t> bytes_to_coeffs(uint32_t limit, const uint8_t *bytes, uint64_t size) {

    uint64_t size_out = coefficients_per_element(limit, size);
    vector<uint64_t> output(size_out);

    uint32_t room = limit;
    uint64_t *target = &output[0];

    for (uint32_t i = 0; i < size; i++) {
        uint8_t src = bytes[i];
        uint32_t rest = 8;
        while (rest) {
            if (room == 0) {
                target++;
                room = limit;
            }
            uint32_t shift = rest;
            if (room < rest) {
                shift = room;
            }
            *target = *target << shift;
            *target = *target | (src >> (8 - shift));
            src = src << shift;
            room -= shift;
            rest -= shift;
        }
    }

    *target = *target << room;
    return output;
}

void coeffs_to_bytes(uint32_t limit, const vector<uint64_t> &coeffs, uint8_t *output, uint32_t size_out, uint32_t ele_size){
    uint32_t room = 8;
    uint32_t j = 0;
    uint8_t *target = output;
    uint32_t bits_left = ele_size * 8;
    for (uint32_t i = 0; i < coeffs.size(); i++) {
        if(bits_left == 0){
            bits_left = ele_size * 8;
        }
        uint64_t src = coeffs[i];
        uint32_t rest = min(limit, bits_left);
        while (rest && j < size_out) {
            uint32_t shift = rest;
            if (room < rest) {
                shift = room;
            }
            
            target[j] = target[j] << shift;
            target[j] = target[j] | (src >> (limit - shift));
            src = src << shift;
            room -= shift;
            rest -= shift;
            bits_left -= shift;
            if (room == 0) {
                j++;
                room = 8;
            }
        }
    }
}

void vector_to_plaintext(const vector<uint64_t> &coeffs, Plaintext &plain) {
    uint32_t coeff_count = coeffs.size();
    plain.resize(coeff_count);
    util::set_uint(coeffs.data(), coeff_count, plain.data());
}

vector<uint64_t> compute_indices(uint64_t desiredIndex, vector<uint64_t> Nvec) {
    uint32_t num = Nvec.size();
    uint64_t product = 1;

    for (uint32_t i = 0; i < num; i++) {
        product *= Nvec[i];
    }

    uint64_t j = desiredIndex;
    vector<uint64_t> result;

    for (uint32_t i = 0; i < num; i++) {

        product /= Nvec[i];
        uint64_t ji = j / product;

        result.push_back(ji);
        j -= ji * product;
    }

    return result;
}

uint64_t invert_mod(uint64_t m, const seal::Modulus& mod) {
  if (mod.uint64_count() > 1) {
    cout << "Mod too big to invert";
  }
  uint64_t inverse = 0;
  if (!seal::util::try_invert_uint_mod(m, mod.value(), inverse)) {
    cout << "Could not invert value";
  }
  return inverse;
}


PirQuery deserialize_query(uint32_t d, uint32_t count, string s, uint32_t len_ciphertext,
shared_ptr<SEALContext> context) {
    vector<vector<Ciphertext>> q;
    std::istringstream input(s);

    for (uint32_t i = 0; i < d; i++) {
        vector<Ciphertext> cs;
        for (uint32_t i = 0; i < count; i++) {
          Ciphertext c;
          c.load(*context, input);
          cs.push_back(c);
        }
        q.push_back(cs);
    }
    return q;
}

string serialize_galoiskeys(Serializable<GaloisKeys> g) {
    std::ostringstream output;
    g.save(output);
    return output.str();
}

GaloisKeys *deserialize_galoiskeys(string s, shared_ptr<SEALContext> context) {
    GaloisKeys *g = new GaloisKeys();
    std::istringstream input(s);
    g->load(*context, input);
    return g;
}