Skip to content CTF writeup: level 6 – random

This one turned out to be surprisingly easy. What’s required is an understanding of random number generators, and how they’re not really random. Implementations of random number generators are known as PRNGs, acknowledging that they are pseudo random number generators. They use various tricks in order to produce streams of not obviously connected numbers, but without an understanding of how they’re meant to work it can be easy to produce a problem similar to the one described in this CTF.



int main(){
unsigned int random;
random = rand(); // random value!

unsigned int key=0;
scanf("%d", &key);

if( (key ^ random) == 0xdeadbeef ){
system("/bin/cat flag");
return 0;

printf("Wrong, maybe you should try 2^32 cases.\n");
return 0;

Pretty simple to understand, we have two variables, random and key. A decimal value is read into key through scanf, and rand() assigns a “random” number to the variable ‘random’. We then XOR the values and if the result is 0xdeadbeef we give up the flag. In theory, this should be hard to beat. Wouldn’t we have to keep trying values until we happen to get the right one? And with such a large problem space, that could take a very long time.

Key Insight

The most important thing to recognise is that this is incorrect use of the rand() function. Using PRNG correctly involves “seeding” the generator with a value that changes frequently, a common tactic is to use the current timestamp supplied to srand (there are some issues with this too, see if you can figure them out). This leads to predictable behaviour, which becomes a problem when knowing the random value allows us to exploit the behaviour. A list of historical examples of where this has been used in real-world attacks can be found on wikipedia’s page on random number generator attacks

Since the rand() call isn’t seeded, it’s fairly trivial to check the value of a few runs of the program in GDB. Setting a breakpoint for the next line after the call to rand() makes it easy to observe what’s returned from the function into the RAX register (the one used to store return values traditionally). A few runs through and it’s easy to see that this value doesn’t change. Due to failure to seed the PRNG it’s returning the exact same value each time, which happens to be 0x6b8b4567. Now we have the following simple equation to solve: something XOR 0x6b8b4567 = 0xdeadbeef. The value we’re looking for will be 0xdeadbeef XOR 0x6b8b4567, which ends up being = 0xb526fb88 which is 3039230856. Let’s try it:

random@ubuntu:~$ ./random 
Mommy, I thought libc random is unpredictable...

Published inReverse EngineeringUncategorizedWalkthroughs

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