byte* TrapdoorPermutationAbs::hardCoreFunction(TPElement * tpEl) {
if (!isKeySet())
throw IllegalStateException("keys aren't set");
/*
* We use this implementation both in RSA permutation and in Rabin permutation.
* Thus, We implement it in TrapdoorPermutationAbs and let derived classes override it if needed.
*/
// gets the element value as byte array
biginteger elementValue = tpEl->getElement();
byte* bytesValue=NULL;
int bytesSize = allocateAndEncodeBigInteger(elementValue, bytesValue);
// the number of bytes to get the log (N) least significant bits
double logBits = NumberOfBits(modulus) / 2.0; //log N bits
int logBytes = (int)ceil(logBits / 8); //log N bites in bytes
// if the element length is less than log(N), the return byte[] should be all the element bytes
int size = min(logBytes, bytesSize);
byte* leastSignificantBytes = new byte[size];
// copies the bytes to the output array
for (int i = 0; i < size; i++)
leastSignificantBytes[i] = bytesValue[bytesSize - size + i];
return leastSignificantBytes;
}
RSAElement::RSAElement(biginteger modN){
/*
* samples a number between 1 to n-1
*/
mt19937 generator = get_seeded_random();
biginteger randNumber;
int numbit = NumberOfBits(modN);
biginteger expo = mp::pow(biginteger(2), numbit-1);
boost::random::uniform_int_distribution<biginteger> ui(0, expo);
do {
randNumber = ui(generator); // samples a random BigInteger with modN.bitLength()+1 bits
} while (randNumber > (modN - 2)); // drops the element if it's bigger than mod(N)-2
// gets a random biginteger between 1 to modN-1
randNumber += 1;
// sets it to be the element
element = randNumber;
}
RSAElement::RSAElement(biginteger modN){
/*
* samples a number between 1 to n-1
*/
mt19937 generator = get_seeded_random();
biginteger randNumber;
int numbit = NumberOfBits(modN);
biginteger expo = mp::pow(biginteger(2), numbit-1);
do {
// samples a random BigInteger with modN.bitLength()+1 bits
randNumber = getRandomInRange(0, expo, generator);
} while (randNumber > (modN - 2)); // drops the element if it's bigger than mod(N)-2
// gets a random biginteger between 1 to modN-1
randNumber += 1;
// sets it to be the element
element = randNumber;
}
/**
* Checks the validity of the given soundness parameter.<p>
* t must be less than a third of the length of the public key n.
* @return true if the soundness parameter is valid; false, otherwise.
*/
bool checkSoundnessParam(const biginteger & modulus) {
//If soundness parameter is not less than a third of the publicKey n, return false.
int third = NumberOfBits(modulus) / 3;
return (t < third);
}
