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    <p>The standard Oracle JDK 7 implementation uses what's called a Linear Congruential Generator to produce random values in <code>java.util.Random</code>. </p> <p>Taken from <code>java.util.Random</code> source code (JDK 7u2), from a comment on the method <code>protected int next(int bits)</code>, which is the one that generates the random values:</p> <blockquote> <p>This is a linear congruential pseudorandom number generator, as defined by D. H. Lehmer and described by Donald E. Knuth in <i>The Art of Computer Programming,</i> Volume 3: <i>Seminumerical Algorithms</i>, section 3.2.1.</p> </blockquote> <h3>Predictability of Linear Congruential Generators</h3> <p>Hugo Krawczyk wrote a pretty good paper about how these LCGs can be predicted ("How to predict congruential generators"). If you're lucky and interested, you may still find a free, downloadable version of it on the web. And there's plenty more research that clearly shows that you should <strong>never</strong> use an LCG for security-critical purposes. This also means that your random numbers <em>are</em> predictable right now, something you don't want for session IDs and the like.</p> <h3>How to break a Linear Congruential Generator</h3> <p>The assumption that an attacker would have to wait for the LCG to repeat after a full cycle is wrong. Even with an optimal cycle (the modulus m in its recurrence relation) it is very easy to predict future values in much less time than a full cycle. After all, it's just a bunch of modular equations that need to be solved, which becomes easy as soon as you have observed enough output values of the LCG. </p> <p>The security doesn't improve with a "better" seed. It simply doesn't matter if you seed with a random value generated by <code>SecureRandom</code> or even produce the value by rolling a die several times. </p> <p>An attacker will simply compute the seed from the output values observed. This takes <em>significantly less</em> time than 2^48 in the case of <code>java.util.Random</code>. Disbelievers may try out this <a href="http://jazzy.id.au/default/2010/09/20/cracking_random_number_generators_part_1.html" rel="noreferrer">experiment</a>, where it is shown that you can predict future <code>Random</code> outputs observing only two(!) output values in time roughly 2^16. It takes not even a second on a modern computer to predict the output of your random numbers right now.</p> <h3>Conclusion</h3> <p>Replace your current code. Use <code>SecureRandom</code> exclusively. Then at least you will have a little guarantee that the result will be hard to predict. If you want the properties of a cryptographically secure PRNG (in your case, that's what you want), then you have to go with <code>SecureRandom</code> only. Being clever about changing the way it was supposed to be used will almost always result in something less secure...</p>
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    1. PODifference between java.util.Random and java.security.SecureRandom
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    1. COVery helpful, may be you could also explain how SecureRandom works (just like you explain how Random works)..
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    2. COThat defeats the purpose of secureRandom
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    3. CO@azulflame security through obscurity rarely works
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