cryptopals

commit d51c2413fea0bc38bcf09fef04101aa58a2b6d7a
parent 245c17d8ba7951a8feab3a43604a39169d2e1a69
Author: Jack Sangdahl <jack@pngu.org>
Date:   Thu, 23 Oct 2025 22:39:05 -0600

challenge 22: crack an mt19937 seed

Diffstat:
A
src/22_crack_mt19937_seed.c
 | 
47
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1 file changed, 47 insertions(+), 0 deletions(-)
diff --git a/src/22_crack_mt19937_seed.c b/src/22_crack_mt19937_seed.c
@@ -0,0 +1,47 @@
+// Crack an MT19937 seed
+#include "mt19937.h"
+#include "random.h"
+#include <stdio.h>
+#include <time.h>
+#include <unistd.h>
+
+/*
+ * make sure your mt19937 accepts an integer seed value. test it (verify that
+ * you're getting the same sequence of outputs given a seed). write a routine
+ * that performs the following operation: wait a random number of seconds
+ * between, 40 and 1000, seed the rng with the current unix timestamp, waits a
+ * random number of seconds again, and return the first 32 bit output of the
+ * rng. you get the idea. simulate the passage of time, although you're missing
+ * some of the fun of the exercise if you do that. from the 32 bit rng output,
+ * discover the seed
+ */
+
+#define MIN_DELAY 40
+#define MAX_DELAY 1000
+
+void delay(int min, int max) {
+  int wait = random_int_inclusive(min, max);
+  printf("Waiting %d seconds... (%d minutes)\n", wait, wait / 60);
+  sleep(wait);
+}
+
+uint32_t output_seed(uint32_t seed) {
+  struct mt19937_state mt = mt19937_init(seed);
+  return mt19937_extract(&mt);
+}
+
+int main(void) {
+  srand(time(NULL));
+  delay(MIN_DELAY, MAX_DELAY);
+  int seed = (int)time(NULL);
+  uint32_t first = output_seed(seed);
+  printf("Seeded PRNG...\n");
+  delay(MIN_DELAY, MAX_DELAY);
+  int end = (int)time(NULL);
+  uint32_t curr;
+  for (curr = end; output_seed(curr) != first; curr--)
+    if (!(curr % 10))
+      printf(".");
+  printf("\nFound seed: %d\n", curr);
+  return 0;
+}