Find out what a random number generator was seeded with in C++

Question

I've got an unmanaged c++ console application in which I'm using srand() and rand(). I don't need this to solve a particular problem, but was curious: is the original seed passed to srand() stored somewhere in memory that I can query? Is there any way to figure out what the seed was?

Answer 1

The seed is not required to be stored, only the last random number returned is.

Here's the example from the manpage:

       static unsigned long next = 1;

       /* RAND_MAX assumed to be 32767 */
       int myrand(void) {
           next = next * 1103515245 + 12345;
           return((unsigned)(next/65536) % 32768);
       }

       void mysrand(unsigned seed) {
           next = seed;
       }

Answer 2

If you have a simple linear congruential generator, for which you have several values this yields a system of equations:

 v1 = ( seed * a + b ) % m
 v2 = (   v1 * a + b ) % m;
 v3 = (   v2 * a + b ) % m;
... 

If you know the first value, you can go backwards in the sequence:

seed = (v1 - b)/a (mod m)

You don't know the seed uniquely, you only know it mod m (which is usually fine since (0 < seed < m) anyways) If v1 - b is negative you need to add m's until its positive again.

You might also look at the Chinese Remainder Theorem, though its not an exact match.