tryyourluck (misc)

Here’s the challenge source:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char const *argv[])
{
	// Generate number using random seed
	unsigned char seed[4];
	int fd = open("/dev/urandom", O_RDONLY);
	read(fd, seed, 4);
	close(fd);
	srand(*(unsigned int *)seed);

	float money = 1000.00f;
	float bet = 0;
	int number = 0;
	int guess = 0;

	printf("Try your luck with this simple guessing game!\n");
	printf("You only have 10 chances to bet and guess\n");
	printf("Get the flag if you win one billion\n");
	printf("Good Luck!!\n\n");
	fflush(stdout);

	for (int i = 0; i < 10; ++i)
	{
		printf("Enter bet: ");
		fflush(stdout);
		scanf("%f",&bet);
		// Check if bet is too less or too high
		if (bet > 499999999 ||  bet < 1)
		{
			printf("Hacker Alert!!\n");
			return 0;
		}
		number = rand() % 0x1337;
		printf("Guess number: ");
		fflush(stdout);
		scanf("%i",&guess);
		if(number == guess){
			money += bet;
			printf("Correct guess!\n");
		}else{
			money -= bet;
			printf("Wrong guess.. money left %.2f\n",money);
		}
	}

	// If money more than 1 billion will print the flag
	if (money < 1000000000){
		printf("See you next time..\n");
	}else{
		char flag[40];
		int fd = open("./flag.txt", O_RDONLY);
		read(fd, flag, 40);
		close(fd);
		printf("Congrats!! Flag is %s\n",flag);
	}
	return 0;
}

In this challenge, we got 10 tries to bet and guess.

• If we guess correct, we earn money.
• If wrong, lose money. The bet amount is limited to the range (1, 499999999).
• After 10 tries, if our money is not less than 1000000000, we get flag.

One of the things to consider when dealing with floating point values is the NaN and infinity values. A property of NaN values is that all comparisons involving them will result in false.

See this:

	if (money < 1000000000){
		printf("See you next time..\n");
	}else{
		char flag[40];
		int fd = open("./flag.txt", O_RDONLY);
		read(fd, flag, 40);
		close(fd);
		printf("Congrats!! Flag is %s\n",flag);
	}

If money is NaN 😉

So enter a bet that is NaN, then speed through the rest.

Try your luck with this simple guessing game!
You only have 10 chances to bet and guess
Get the flag if you win one billion
Good Luck!!

Enter bet: nan
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Enter bet: 2
Guess number: 2
Wrong guess.. money left nan
Congrats!! Flag is wgmy{e537f4308d96e369bef7e7f83bf79209}