### Analysis
Reading the source code with `cat re.c` presents the following:
```c#include <stdlib.h>
int win(){    int fd = open("./flag.txt",0);    sendfile(1,fd,0,100);    exit(0);}
int main() {    srand(time(0));    int rand_num;    int user_num;     printf("Give me a number: ");    scanf("%d", &user_num);     rand_num = rand();     if (rand_num == user_num) {        printf("Correct \n ");        win();    }     printf("Wrong!\n");    printf("I randomly generated:\n%d", rand_num);}```
From line 22 (`if (rand_num == user_num)`), we can see that in order to fulfill the win condition, we have to input a string to the program which matches the randomly generated number. The `srand()` function used on line 13 seeds the pseudorandom number generator with the current time, meaning that the "random" number will be the same whenever executed in the same millisecond.
### Testing
This can be tested easily in Bash:```bashfor _ in {1..9} do  # Repeat 9 times    echo "test" | ./re  # Run the program with a dummy input string    echo -e "\n"  # Print a blank line to delimit each loop, for readabilitydone```
This provides an output such as the following example:
```bashGive me a number: Wrong!I randomly generated:1606725990
Give me a number: Wrong!I randomly generated:1606725990
Give me a number: Wrong!I randomly generated:1606725990
# <Repeats 6 more times>```
### Exploitation
We can see that the randomly generated number is displayed on the third line each time, so we can extract just the line containing the number using `sed -n 3p`. Then, we can pipe that number back into the program, since the random number will be unchanged, assuming execution happens within the same millisecond.
Our final code is this:
```bashecho "test" | ./re | sed -n 3p | ./re```
Executing it will provide the following output:```bashGive me a number: CorrectSAH{Can_I_Have_Your_Number?}```
### Flag
#### ? `SAH{Can_I_Have_Your_Number?}`