rerere

reversing - Points: 240

re rere rerere. Find the magic key, and encapsulate in SECT{…} to get the flag

chal

We are given a binary and need to find the correct key, I decompiled it in Ghidra and slightly modified and reduced the interesting part to the following code:

void main(void) {
  double data[13] = {-1.61774E-5, -2.438196E-4, 0.001658882, 0.02094839, -0.06191985, -0.62766, 1.013856, 7.805292, -6.77945, -36.45974, 11.32587, 38.7614, 99.0};

  int result;
  double distance;
  char input [88];
  
  gets(input);
  result = 0;
  for (int i = 0; i < 13; i++) {
    distance = 0.00000000;
    for (int j = 0; j < 13; j++) {
      distance = (double)(i - 6) * distance + data[j];
    }
    if (distance <= 0.00000000) {
      distance = distance - 0.50000000;
    }
    else {
      distance = distance + 0.50000000;
    }
    result = result + ((int)input[(long)(int)i] - (int)distance);
    printf("%d, %d, %d\n", (int)distance, (int)input[(long)(int)i], result);
  }
  printf("%d\n", result);
  if (result == 0) {
    puts("Y");
  }
  else {
    puts("N");
  }
  return;
}

The binary contains a data array with 13 values and the for loops iterate over it and calculate distances with our input, sums them up and checks at the end if there were any differences. With the added printf at the end of each iteration it now shows the distance of the entered character and the expected character, so we can now easily infer the correct characters.

flag: SECT{p0l1y_cr4ck3r}