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tisc

TISC is a challenge for cybersecurity enthusiasts. Starting on 8 Aug 20, challengers will have 48 hours to progress through 6 cybersecurity and programming questions. The questions will be of increasing difficulty and will be released only when the previous question is successfully solved. Points will be given based on: (1) the number of questions completed; (2) the time taken to complete them.

I only solved Stage 1

Stage 1 Write-Up

Immediately we go ahead and try to unzip the given file. It's password protected!

pwneeded

To solve this, it's just looking at the information given:

  • Simple password (6 characters, hex)
  • Several (alot) of layers of encoding / compression

Given the simple password, I used a brute force attack on 6 character hex values, with only 16^6 possible combinations. At first, I tried to count up from 0, but it seemed to take forever, so I tried counting down from ffffff.

Evetually there was an output:

$ ./bruteforce.py
Password found:b1aa40

Trying to use cat on the file leads to a jumbled mess.

$ cat temp.mess
BZh91AY&SY�un ܹVyo[[ޯc{뽞_}ｽuگ�y}V.{]y;M{o{ow}T>ݝ};nݎ�7Ynu8O}}wﯾ[{]޽'wۋCwn=Xz/Iݾt
cat: write error: Input/output error

So we can use file on it first to find its type, bzip2 compressed data.

$ file temp.mess
temp.mess: bzip2 compressed data, block size = 900k

Python's own bz2 library is easily used here. Continuing the file technique, we can find zlib, then XZ, then ASCII (which looked like base64 encoding), then ASCII (with only hex values), as well as gzip. Easily found on Google, these can be decompressed/decoded with the Python libraries. However, it seemed to go on forever, and I tried multiple times to do it manually (test.py) but realised this had to have a smarter solution - apparently multiple layers meant a lot of layers. To automate the process, there was more Googling to get shell commands done from within the Python script, as well as learning about how to use awk. The script was eventually done, there was a bit of minor troubleshooting, and finally, an error came up!

$ ./decode.py

error

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/encodings/hex_codec.py", line 19, in hex_decode
    return (binascii.a2b_hex(input), len(input))
binascii.Error: Non-hexadecimal digit found

It was finished, and there it was when using cat!

$ cat temp.mess
{"anoroc": "v1.320", "secret": "TISC20{q1_85d3ba2b674fc9e22023130e245adafb}", "desc": "Submit this.secret to the TISC grader to complete challenge", "constants": [1116352408, 1899447441, 3049323471, 3921009573, 961987163, 1508970993, 2453635748, 2870763221], "sign": "

flag

Flag is easily seen:

TISC20{q1_85d3ba2b674fc9e22023130e245adafb}