Is there a way to generate a seed out of a sequence of numbers?

Question

For example if java produces the pseudorandom sequence: 9 3 2 5 6 by using 23 as a seed, how can I do the inverse? i.e. getting 23<

Answer 1

It is certainly possible to recover the seed used by java.util.Random. This post describes the math behind Random's linear congruential formula, and here is a function to discover the current seed from the last two integers returned from nextInt().

public static long getCurrentSeed(int i1, int i2) {
        final long multiplier = 0x5DEECE66DL;
        final long inv_mult = 0xDFE05BCB1365L;
        final long increment = 0xBL;
        final long mask = ((1L << 48) - 1);

        long suffix = 0L;
        long lastSeed;
        long currSeed;
        int lastInt;

        for (long i=0; i < (1<<16); i++) {
                suffix = i;
                currSeed = ((long)i2 << 16) | suffix;
                lastSeed = ((currSeed - increment) * inv_mult) & mask;
                lastInt = (int)(lastSeed >>> 16);

                if (lastInt == i1) {
                        /* We've found the current seed, need to roll back 2 seeds */
                        currSeed = lastSeed;
                        lastSeed = ((currSeed - increment) * inv_mult) & mask;
                        return  lastSeed ^ multiplier;
                }
        }

        /* Error, current seed not found */
        System.err.println("current seed not found");
        return 0;
}

This function returns a value that can be used with rand.setSeed() to generate a pseudorandom sequence of numbers starting with i1 and i2.