Miller Rabin primality test

MillerRabin

private boolean passesMillerRabin(int iterations, Random rnd) {
        // Find a and m such that m is odd and this == 1 + 2**a * m
        BigInteger thisMinusOne = this.subtract(ONE);
        BigInteger m = thisMinusOne;
        int a = m.getLowestSetBit();
        m = m.shiftRight(a);

        // Do the tests
        if (rnd == null) {
            rnd = getSecureRandom();
        }
        for (int i=0; i<iterations; i++) {
            // Generate a uniform random on (1, this)
            BigInteger b;
            do {
                b = new BigInteger(this.bitLength(), rnd);
            } while (b.compareTo(ONE) <= 0 || b.compareTo(this) >= 0);

            int j = 0;
            BigInteger z = b.modPow(m, this);
            while(!((j==0 && z.equals(ONE)) || z.equals(thisMinusOne))) {
                if (j>0 && z.equals(ONE) || ++j==a)
                    return false;
                z = z.modPow(TWO, this);
            }
        }
        return true;
    }