# CTF: Solving nullcon crypto question 2

##### name:

Crypto Question 2

##### category:

crypto

##### points:

350

#### Writeup

The nullcon ctf competition ran this weekend, and the organizers managed to completely give away one of the crypto challenges by giving out a hint that made it trivial.

The problem was given as this image: problem

And the hint that gave the problem away was: Hint 2 : ‘a’ and ‘b’ both are less than 1000

By doing a simple iteration from 0 to 1000 we got two possible answers for both a and b.

```
BigInteger q = new BigInteger("541");
BigInteger g = new BigInteger("10");
for(int i = 0; i < 1000; i++) {
BigInteger a = g.modPow(BigInteger.valueOf(i), q);
if(a.equals(BigInteger.valueOf(298)))
System.out.println("a = " + i);
if(a.equals(BigInteger.valueOf(330)))
System.out.println("b = " + i);
}
```

gave us the output:

```
a = 170
b = 268
a = 710
b = 808
```

And that gave us four possible values for the flag, which was: flag{170,808}