void Ctxt::multiplyBy2(const Ctxt& other1, const Ctxt& other2)
{
// Special case: if *this is empty then do nothing
if (this->isEmpty()) return;
long lvl = findBaseLevel();
long lvl1 = other1.findBaseLevel();
long lvl2 = other2.findBaseLevel();
if (lvl<lvl1 && lvl<lvl2){ // if both others at higher levels than this,
Ctxt tmp = other1; // multiply others by each other, then by this
if (&other1 == &other2) tmp *= tmp; // squaring rather than multiplication
else tmp *= other2;
*this *= tmp;
}
else if (lvl<lvl2) { // lvl1<=lvl<lvl2, multiply by other2, then by other1
*this *= other2;
*this *= other1;
}
else { // multiply first by other1, then by other2
*this *= other1;
*this *= other2;
}
reLinearize(); // re-linearize after all the multiplications
}
Ctxt& Ctxt::operator*=(const Ctxt& other)
{
FHE_TIMER_START;
// Special case: if *this is empty then do nothing
if (this->isEmpty()) return *this;
// Sanity check: plaintext spaces are compatible
long g = GCD(ptxtSpace, other.ptxtSpace);
assert (g>1);
this->ptxtSpace = g;
Ctxt tmpCtxt(this->pubKey, this->ptxtSpace); // a scratch ciphertext
if (this == &other) { // a squaring operation
modDownToLevel(findBaseLevel()); // mod-down if needed
tmpCtxt.tensorProduct(*this, other); // compute the actual product
tmpCtxt.noiseVar *= 2; // a correction factor due to dependency
}
else { // standard multiplication between two ciphertexts
// Sanity check: same context and public key
assert (&context==&other.context && &pubKey==&other.pubKey);
// Match the levels, mod-DOWN the arguments if needed
long lvl = findBaseLevel();
long otherLvl = other.findBaseLevel();
if (lvl > otherLvl) lvl = otherLvl; // the smallest of the two
// mod-DOWN *this, if needed (also removes special primes, if any)
modDownToLevel(lvl);
// mod-DOWN other, if needed
if (primeSet!=other.primeSet){ // use temporary copy to mod-DOWN other
Ctxt tmpCtxt1 = other;
tmpCtxt1.modDownToLevel(lvl);
tmpCtxt.tensorProduct(*this,tmpCtxt1); // compute the actual product
}
else
tmpCtxt.tensorProduct(*this, other); // compute the actual product
}
*this = tmpCtxt; // copy the result into *this
FHE_TIMER_STOP;
return *this;
}
void Ctxt::multiplyBy2(const Ctxt& other1, const Ctxt& other2)
{
FHE_TIMER_START;
// Special case: if *this is empty then do nothing
if (this->isEmpty()) return;
long lvl = findBaseLevel();
long lvl1 = other1.findBaseLevel();
long lvl2 = other2.findBaseLevel();
if (lvl<lvl1 && lvl<lvl2){ // if both others at higher levels than this,
Ctxt tmp = other1; // multiply others by each other, then by this
if (&other1 == &other2) tmp *= tmp; // squaring rather than multiplication
else tmp *= other2;
*this *= tmp;
reLinearize(); // re-linearize after all the multiplications
return;
}
const Ctxt *first, *second;
if (lvl<lvl2) { // lvl1<=lvl<lvl2, multiply by other2, then by other1
first = &other2;
second = &other1;
}
else { // multiply first by other1, then by other2
first = &other1;
second = &other2;
}
if (this == second) { // handle pointer collision
Ctxt tmp = *second;
*this *= *first;
*this *= tmp;
if (this == first) // cubing operation
noiseVar *= 3; // a correction factor due to dependency
else
noiseVar *= 2; // a correction factor due to dependency
} else {
*this *= *first;
*this *= *second;
}
reLinearize(); // re-linearize after all the multiplications
}
