Posts tagged with nactf2019


This program is quite short, but has got printf and gets in it! This shouldn't be too hard, right?

Connect at nc 31283

We are given the libc used, the binary, and the source code.

#include <stdio.h>

void vuln()
	char buf[64];
	fputs("Type something>", stdout);
	fputs("You typed: ", stdout);

int main()
	/* Disable buffering on stdout */
	setvbuf(stdout, NULL, _IONBF, 0);


	return 0;
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This is it. The last group test of the year. Dr. J patched his prng again so numbers won't repeat, so I guess Leaf won't get to know the group test pairs ahead of time... oh WEYL. Who knew middle square could make such a good prng?

nc 31382

We're also given the source of the running application:

#include <stdio.h>
#include <stdint.h>
#include <sodium.h>
#include <stdbool.h>
#include <string.h>
#define NUM_CORRECT 10

uint64_t x = 0, w = 0, s = 0xb5ad4eceda1ce2a9;

uint32_t nextRand() {
  w += s;
  x = x*x + w;
  x = (x>>32) | (x<<32);
  return x % (1UL<<32);

void init_seed() {
  uint64_t r1 = (uint64_t) randombytes_random();
  uint64_t r2 = (uint64_t) randombytes_random();
  x = (r1 << 32) + r2;
  r1 = (uint64_t) randombytes_random();
  r2 = (uint64_t) randombytes_random();
  w = (r1 << 32) + r2;

void print_flag() {
  FILE *f = fopen("flag.txt", "r");
  char flag[100];
  fgets(flag, sizeof(flag), f);
  printf("%s\n", flag);
  fflush( stdout );

const char *messages[NUM_CORRECT] =
{ "\nHmm... lucky guess...\n",
  "\nWow, that was coincidental!\n",
  "\nWhat? How did you guess that?\n",
  "\nThat's right, but you won't be able to guess right again!\n> ",
  "\nStrangely, that's correct...\n"

int main() {
  setvbuf(stdout, NULL, _IONBF, 0);
  printf("\nWelcome to Dr. J's Random Number Generator v3! We have received reports of "
  "a vulnerability involving repetition of output. This vulnerability has since been patched, "
  "and Dr. J's RNG is now 100%% secure. \n"
  "[r] Print a new random number \n"
  "[g] Guess the next ten random numbers and receive the flag! \n"
  "[q] Quit \n\n");
  char line[100];
  while (true) {
    printf("> ");
    fgets (line, sizeof(line), stdin);
    line[strcspn(line, "\n")] = 0;

    if (!strcmp("r", line)) {
      uint64_t r = nextRand();
      printf("%lu\n", r);
    if (!strcmp("g", line)) {
      printf("\nGuess the next ten random numbers for a flag! "
      "The chance of guessing all ten numbers correctly is 1/(2*10^96). I hope you're lucky! "
      "\nEnter Guess 1:\n> ");

      for (int i = 0; i < NUM_CORRECT; i++) {
        uint64_t guess = 0;
        fgets (line, sizeof(line), stdin);
        sscanf(line, "%lu", &guess);
        if (guess == nextRand()) {
          int m = randombytes_uniform(5);
          printf("%s", messages[m]);
          if (i < NUM_CORRECT-1) {
            printf("Enter Guess %d:\n> ", i+2);
          else {
            printf("What sorcery is this? That's impossible! I guess you deserve this flag:\n");
        else {
          printf("That's incorrect. Get out of here!\n");
    if (!strcmp("q", line)) {
  return 0;
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