get random BigInteger in range (x, y) [duplicate]

Question

I need to get a random BigInteger that is bigger than 2^511 and lower than 2^512.

Answer 1

byte[] bytes = new byte[64];    // 512 bits
new Random().nextBytes(bytes);
bytes[0] |= 0x80;               // set the most significant bit
return new BigInteger(1, bytes);

Answer 2

From the doc :

BigInteger(int numBits, Random rnd)

Constructs a randomly generated BigInteger, uniformly distributed over the range 0 to (2numBits - 1), inclusive.

So something like that should work :

    BigInteger number = new BigInteger(512, new Random()); //Give you a number between 0 and 2^512 - 1
    number = number.setBit(0); //Set the first bit so number is between 2^511 and 2^512 - 1