seed

seed

prompt

Every seed has it's own way

Retrieve the flag from the script below

Note: flag is a python file, which was imported....it is not a module

read [this](https://csatlas.com/python-import-file-module/) for more info

hints

No hints provided

files

• source.py

solution

The intention of this chall is to introduce random.seed(). Usually when we use random module, the actions are random enough not to be predicted.

But using seed() sets the state of random function which ensures no matter how many times the code is executed, it generates the same result everytime.

One more thing here is

from flag import FLAG

This line doesn't necessarily mean that flag has to be a python module. It can be a .py file

FLAG = "cyberZ{n0th1ng_1n_c0mput1ng_1s_tru3_r4nd0m_4dd1ng_th1s_t0_4v01d_gu3ss1ng}"

Here the code works like this

seed(0x1337) # set the seed
flag_character_list = list(FLAG) # create a list with the flag characters
shuffle(flag_character_list) # shuffle them
shuffled_flag = ''.join(x for x in flag_character_list) # combine the shuffled characters together
print(b64encode(shuffled_flag.encode())) # base64 encode the shuffled flag 

So the solution is simple:

• take a fake flag shuffle it with setting 0x1337 as seed
• retrace how the characters are being moved
• combine in that order

enc = b"X3RnX25fc20xZGJ0dHJaZTF5MG5fdW5udG0wfWcwaDFkM2RwbnUwX3MxbjExXzM0c3JfdF9nZF9ndXJjdjQxMWdfczRuMGNoew=="
enc = b64decode(enc).decode()

This gives base64 decoded output

_tg_n_sm1dbttrZe1y0n_unntm0}g0h1d3dpnu0_s1n11_34sr_t_gd_gurcv411g_s4n0ch{

init = list(x for x in range(length)) 

# list init contains numbers equal to the length of the flag [0, 1, 2, 3, ... 71, 72]

seed(0x1337) # set the same seed

shuffle(init)

# init after shuffle becomes [27, 31, 71, 30, 12, ... 7, 8, 18, 10, 6]

This means the first character in the shuffled flag _ is 27th character in the actual flag.

Second character t is 31st character in the actual flag.

Now that we know which character is at what index we can basically assemble the flag.

for i in range(length):
    index = init.index(i)
    flag += enc[index]

The above snippet is finding where the index is in the shuffled list and add that character to flag.

So here the first number 0 is at 59th index so the 59th character in the shuffled flag c is the zeroth character in the actual flag. flag becomes

c

1 is at 17th index so the 17th character y becomes first character. flag becomes

cy

2 is at 10th index so the 10th character b becomes second character. flag becomes

cyb

Like this you can traceback each character and get the flag.

solve.py and flag.py are added in ./assets for reference.

flag

cyberZ{n0th1ng_1n_c0mput1ng_1s_tru3_r4nd0m_4dd1ng_th1s_t0_4v01d_gu3ss1ng}