Java Puzzle: Square Root

Consider that SecureRandom is really a facade around multiple providers that can plug in varying implementations. :)

Force a race condition on the variable `root'.<p><pre><code> if (n.divide(root).equals(root)) { </code></pre> Try to set it to 2^20001 on the first funcall and 0 on the second.

I'm not sure this is interesting. Unless I'm missing something, the question is "find what way of breaking the compartmentalization of the JVM we haven't forbidden in these English-language rules".

I'm surprised nobody's given the obvious algorithm yet: while(i &#60; BigInt(2).pow(10000)) ++i;<p>Before anyone complains, this algorithm is correct and does not break any of the rules as far as I can tell. :)<p>I think the timing attack is probably what he's really looking for.<p>Edit: as mattvanhorn pointed out, answer() is void, but that's ok... changed to a "constant time" algorithm. :)

Someone should just modify the StackSort algorithm that was posted yesterday (made up by xkcd) to search for Square Root functions instead and run them.<p><a href="http://gkoberger.github.com/stacksort/" rel="nofollow">http://gkoberger.github.com/stacksort/</a>

Not really a Java guy, but could you subclass BigInteger in such a way that when .divide is called and accesses root's representation of the data, that property access reflectively examines the calling expression to find out n's representation of the data, then just square roots that and sets it as its own representation before allowing the expression to evaluate? Or something along those lines, anyway?

Maybe read /proc/self/maps and /proc/self/mem and try to locate the BigInteger value in the JVM heap?

I'd start by trying to create an evil subclass of BigInteger, as it is not a final class.

I hate to break it to you but "n" is not final. Hence, you can reassign any BigInteger to it. Why not just set n = 9 and the root = 3...<p>Immutability is your friend. Use it or lose it.

Can someone explain what this is asking? I don't know java

1: Pause debugging session. 2: Look into SquareRoot.n

Are they asking for a timing attack, then?

I think you can create a new instance of SecureRandom() that provides the same value that we are looking for.

Hint: Many pseudo-random number generators take the square of a seed value to advance to the next sequence.

Presumably something from the java.lang.instrument package will allow you to solve this

Subclass BigInteger, and override equals() on your subclass to "return true;"?

Use reflection to read n. Reflection lets you bypass |private|.

is using the ASM library not a legitimate way or would that violate the rules in some way?

The only possibilities seem to be a timing attack, exploiting a bug or taking advantage of an implementation that puts the dividend value in the exception when dividing by zero.<p>I'm not sure anyone will bother writing the timing attack since he apparently gives out no prize.

Is this just like String that are actually mutable on most JVM implementations out there where the underlying char[] can be accessed using reflection?