Tutor CTF: secedu

Contest Source: COMP6[84]41 CTF

Note that all flags have been replaced with “COMP6841{REDACTED}”. This is to discourage you from just blindly submitting the final answer, and to encourage you to follow along and learn something along the way.

We are given a file called secedu.txt. Opening it, we see it contains a single line containing the following:

secedusecedusecedueduedu secedueduseceduedueduedu secedueduedusecedueduedu secseceduseceduedusecsec
secsecedusecsecsecsecsec seceduedueduedusecsecedu secedueduseceduedueduedu secedueduedusecedusecedu
secsecedusecsecsecsecsec seceduedusecseceduedusec seceduedusecedusecsecedu seceduedusecsecedueduedu
...

Looking more closely we can see that it’s made up of blocks of text, each of which contains either “sec” or “edu” repeated 8 times. This looks suspiciously like binary, so I wrote a script to extract out the binary and convert it into ASCII:

import binascii

with open("secedu.txt", "r") as f:
    lines = f.readlines()

line = lines[0]
line = line.replace("sec", "0")
line = line.replace("edu", "1")

# print(line)
data = line.split()
data = map(lambda x : int(x, 2), data)
data = map(chr, data)
print("".join(data))

Running this gets:

Wow, you figured it out! Great work! How long it it take for you to figure it out?! I would share with you some extra secrets... but... (sorry)
QWggd2VsbCB0aGF0J3Mgbm8gZmFpciEgSGVyZSwganVzdCB0YWtlIHRoZSBzZWNyZXQuIFJlbWVtYmVyIGl0IGlzIGEgc2VjcmV0LCBzbyBrZWVwIGl0IGh1c2ggaHVzaCEgQ09NUDY4NDF7UzNjX0VkdV8xc19jMDBsfQ==

We know from before (see the Warmup Challenge) that this is base64, so running that through an online converter gives us:

Ah well that's no fair! Here, just take the secret. Remember it is a secret, so keep it hush hush! COMP6841{REDACTED}