BigUnsigned CryptoScheme::genCoprime(const BigUnsigned& n) {
BigUnsigned gen = random(n.bitLength());
while(gcd(gen, n) != 1) {
gen = random(n.bitLength());
}
return gen;
}
bool CryptoScheme::isProbablyPrime(const BigUnsigned& n) {
if (n%2 == 0 || n==1) return n==2;
for (int i = 0; i < 30; i++) {
BigUnsigned a = random(n.bitLength()-1);
if (modexp(a, n-1, n) != 1) return false;
}
return true;
}
BigUnsigned modexp(const BigInteger &base, const BigUnsigned &exponent,
const BigUnsigned &modulus) {
BigUnsigned ans = 1, base2 = (base % modulus).getMagnitude();
BigUnsigned::Index i = exponent.bitLength();
// For each bit of the exponent, most to least significant...
while (i > 0) {
i--;
// Square.
ans *= ans;
ans %= modulus;
// And multiply if the bit is a 1.
if (exponent.getBit(i)) {
ans *= base2;
ans %= modulus;
}
}
return ans;
}
