# Misc - Not really random

## Description

Do you really think random numbers generated by computers are random?

## Downloads

{% file src="<https://4077916634-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F9J6tnLQGbY0Or10P4xUT%2Fuploads%2FGd7FaNvJ3QUD6qiAxgs6%2Fnotreallyrandom.zip?alt=media&token=37bdc0ca-75b7-4548-aa38-27e0e470e77a>" %}

## Solution

We are given 2 files:&#x20;

1. **rand.py**

```python
import random 
import time 
import hashlib

seed = round(time.time())

random.seed(seed, version=2)

while True:
    rand = random.random()
    has = hashlib.sha256(str(rand).encode()).hexdigest()
    flag = f"CTF{{{has}}}"
    if "7a2" in has:
        with open("./flag", "w") as f:
            f.write(flag)
            break
    else:
        print(f"Bad random value: {rand}")
```

2\. **log.txt**

```
Bad random value: 0.33567959567961436
Bad random value: 0.8913897703358419
...
Bad random value: 0.5905894610211082
Bad random value: 0.09018146469820387
Flag created 🎉
```

The weakness in the randomization is in line 5 of the **rand.py** script. According to documentations,

&#x20;

![](https://4077916634-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F9J6tnLQGbY0Or10P4xUT%2Fuploads%2F7knQubkR8p0VOkB2kEro%2Fimage.png?alt=media\&token=6b940b34-15b3-4644-949d-1063a5d023db)

When the seed is a floating point number, it would be very difficult to guess which seed results in the random value in **log.txt**. However in this case, because the seed is rounded to an integer, brute-forcing becomes viable.

First I converted the random values in **log.txt** into a python list using **Sublime** so that the script will be able to compare and know when the brute-forced seed is correct.

```python
target = [0.33567959567961436, 0.8913897703358419, 0.3032054069265432, 0.6860829464688437, 0.2658087107328536, 0.8903005048882441, 0.914630909612433, 0.9688578899818961, 0.7925090397955323, 0.10136501216336935, 0.568451491382639, 0.16898065821921437, 0.5541712073794856, 0.029926361216790154, 0.18218590474521223, 0.49713845657579536, 0.7631162105077507, 0.7386939443532723, 0.5815609491717452, 0.5905894610211082, 0.09018146469820387]
```

Using `seed = round(time.time())`, I get the seed at the current time so that I know the last number to brute-force and the number was **1646540228** when I wrote the script. The starting seed should be a feasible number and in my case, I used **1646430228**. If the number is too small the brute-force will take forever while if the number is too big, you risk missing the number (now to think about it I should probably have brute-forced it in the reverse direction.)

```python
import random 
import time 
import hashlib
import tqdm

#seed = round(time.time())

for seed in tqdm.tqdm(range(1646430228, 1646540228)):
    random.seed(seed, version=2)

    target = [0.33567959567961436, 0.8913897703358419, 0.3032054069265432, 0.6860829464688437, 0.2658087107328536, 0.8903005048882441, 0.914630909612433, 0.9688578899818961, 0.7925090397955323, 0.10136501216336935, 0.568451491382639, 0.16898065821921437, 0.5541712073794856, 0.029926361216790154, 0.18218590474521223, 0.49713845657579536, 0.7631162105077507, 0.7386939443532723, 0.5815609491717452, 0.5905894610211082, 0.09018146469820387]
    br_array = []

    while True:
        rand = random.random()
        has = hashlib.sha256(str(rand).encode()).hexdigest()
        flag = f"CTF{{{has}}}"
        if "7a2" in has:
            with open("flag.txt", "w") as f:
                f.write(flag)
                break
        else:
            br_array.append(rand)

    if (br_array == target):
        print("Seed found: " + str(seed))
        print(flag)
        exit()
```

Flag: `CTF{a13a806d175841731b24a01e9af240bc81750967542550a4b3bb77a29a9d291b}`