void PAlgebraModDerived<type>::decodePlaintext(
vector<RX>& alphas, const RX& ptxt, const MappingData<type>& mappingData) const
{
long nSlots = zMStar.getNSlots();
if (isDryRun()) {
alphas.assign(nSlots, RX::zero());
return;
}
// First decompose p into CRT components
vector<RX> CRTcomps(nSlots); // allocate space for CRT component
CRT_decompose(CRTcomps, ptxt); // CRTcomps[i] = p mod facors[i]
if (mappingData.degG==1) {
alphas = CRTcomps;
return;
}
alphas.resize(nSlots);
REBak bak; bak.save(); mappingData.contextForG.restore();
for (long i=0; i<nSlots; i++) {
REX te;
conv(te, CRTcomps[i]); // lift i'th CRT componnet to mod G(X)
te %= mappingData.rmaps[i]; // reduce CRTcomps[i](Y) mod Qi(Y), over (Z_2[X]/G(X))
// the free term (no Y component) should be our answer (as a poly(X))
alphas[i] = rep(ConstTerm(te));
}
}
void FSPRG_Seek(void *state, uint64_t epoch, const void *msk, const void *seed, size_t seedlen) {
gcry_mpi_t p, q, n, x, xp, xq, kp, kq, xm;
uint16_t secpar;
initialize_libgcrypt();
secpar = read_secpar(msk + 0);
p = mpi_import(msk + 2 + 0 * (secpar / 2) / 8, (secpar / 2) / 8);
q = mpi_import(msk + 2 + 1 * (secpar / 2) / 8, (secpar / 2) / 8);
n = gcry_mpi_new(0);
gcry_mpi_mul(n, p, q);
x = gensquare(n, seed, seedlen, RND_GEN_X, secpar);
CRT_decompose(&xp, &xq, x, p, q); /* split (mod n) into (mod p) and (mod q) using CRT */
kp = twopowmodphi(epoch, p); /* compute 2^epoch (mod phi(p)) */
kq = twopowmodphi(epoch, q); /* compute 2^epoch (mod phi(q)) */
gcry_mpi_powm(xp, xp, kp, p); /* compute x^(2^epoch) (mod p) */
gcry_mpi_powm(xq, xq, kq, q); /* compute x^(2^epoch) (mod q) */
CRT_compose(&xm, xp, xq, p, q); /* combine (mod p) and (mod q) to (mod n) using CRT */
store_secpar(state + 0, secpar);
mpi_export(state + 2 + 0 * secpar / 8, secpar / 8, n);
mpi_export(state + 2 + 1 * secpar / 8, secpar / 8, xm);
uint64_export(state + 2 + 2 * secpar / 8, 8, epoch);
gcry_mpi_release(p);
gcry_mpi_release(q);
gcry_mpi_release(n);
gcry_mpi_release(x);
gcry_mpi_release(xp);
gcry_mpi_release(xq);
gcry_mpi_release(kp);
gcry_mpi_release(kq);
gcry_mpi_release(xm);
}
