Return a random prime up to a certain length.

    This function uses random.SystemRandom.

def get_prime(bits, k=64):
    """
    Return a random prime up to a certain length.

    This function uses random.SystemRandom.

    """
    if bits % 8 != 0 or bits == 0:
        raise ValueError("bits must be >= 0 and divisible by 8")
    while True:
        n = int.from_bytes(os.urandom(bits // 8), "big")
        if is_prime(n, k):
            return n