protected BigInteger modInverse(BigInteger x) {
int bits = getFieldSize();
int len = (bits + 31) >> 5;
int[] p = Nat.fromBigInteger(bits, q);
int[] n = Nat.fromBigInteger(bits, x);
int[] z = Nat.create(len);
Mod.invert(p, n, z);
return Nat.toBigInteger(len, z);
}
