FesseMisk

Employee of the Week

Challenge Overview

This was an easy and fun challenge to get to do some scripting early on in the competition on my part. It is a challenge that is solved when finding the needle in a heystack, which fortunately is easy with computers.

Challenge extract:

"Our 'employee of the week' eloped. He was the only one that knew the master class secret. We managed to extract logs from his last activities. Can you find anything from this?"

This "last activity" log contained some crucial details:

• The encryption command, using AES via OpenSSL to encrypt master.txt which we must assume is the flag with an iteration count (ITER) and a password $PWD.

Log excerpt:

> openssl enc -aes-256-cbc -pbkdf2 -iter $ITER -in master.txt -k $PWD -a
U2FsdGVkX1+/39qrCQ9rlxMW2E30ylTUXYS+GTAVDMUK0oXJvkUDBCRbhClK2GKYc50OQZ7zgLPBhkMW8CM5VVnZBrxfyH5CAG8nj5BPDCg=

I saved this output to a file such that we could script over it later on.

Then I created what I imagined would be the correct command to decrypt this later on.

openssl enc -d -aes-256-cbc -pbkdf2 -iter ($(iter_value)) -in enc.enc -k ($(PWD)) -a

Here we are missing two pieces of information. iter_value and $PWD.

• The ITER was generated from a UNIX timestamp, calculated by dividing by one million.

export ITER=$(($(date +%s) / 1000000))

It would be crazy to search all seconds throughout all time, but thankfully the log shows the admin using the week and cal commands which both show the week number and the year. This is doable to find.

That leaves us with just $PWD, normally this is the current working directory of a shell, but investigating the shell log further showed this.

export PWD=$(hostname)

• The password was derived from the hostname (ubuntu-s-1vcpu-512mb-10gb-ams3-01).

This leaves us with

openssl enc -d -aes-256-cbc -pbkdf2 -iter ($(iter_value)) -in enc.enc -k "ubuntu-s-1vcpu-512mb-10gb-ams3-01" -a

Finding the solution

Now we just needed to find the needle (the correct second) in the heystack (week 38 of 2024).

I was looking forward to coding this, but I also wanted to finish quickly to be able to move on to other challenges, so I used the friendly chat-bot (I'm sorry).

Chatbot output:

1. **Generate timestamps** for each second in week 38.
2. **Calculate `ITER`** by dividing each timestamp by 1,000,000.
3. **Test each possible `ITER`** to decrypt the encoded file, hoping to land on the one used in encryption.

Using Python, I scripted a loop to automatically generate timestamps for the week and attempt decryption for each one. Here’s a closer look at the script:

import os
import subprocess
import datetime

# Define week 38 in September 2024
start_date = datetime.datetime(2024, 9, 15)
week_start = start_date
week_end = week_start + datetime.timedelta(days=7)

# Generate timestamps within that week
timestamps = []
current_date = week_start
while current_date < week_end:
    timestamps.append(int(current_date.timestamp()))
    current_date += datetime.timedelta(seconds=1)

# Iterate over possible ITER values
for timestamp in timestamps:
    iter_value = timestamp // 1000000  # Calculate ITER from timestamp
    env = os.environ.copy()
    env['ITER'] = str(iter_value)

    command = [
        "openssl", "enc", "-d", "-aes-256-cbc", "-pbkdf2",
        "-iter", str(iter_value), "-in", "enc.enc", "-k", "ubuntu-s-1vcpu-512mb-10gb-ams3-01", "-a"
    ]

    try:
        output = subprocess.check_output(command, env=env, stderr=subprocess.DEVNULL)
        print(f"Success with ITER = {iter_value}")
        print("Decrypted content:", output.decode('utf-8'))
        break  # Stop if decryption succeeds
    except subprocess.CalledProcessError:
        continue

When the friendly chat-bot gets enough information it often manages to write functional code. I'm not really sure if this is good code or not, but it worked and I havent looked back at it since. I guess I could have tried to style it nicely for the writeup, but I didnt bother. (im sorry)

Success with ITER = 1726
Decrypted content: Master class secret: EPT{Ach13v3m3nt_Unl0ck3d_293857}

Afterthoughts

After writing my own writeup I wanted to see what other writeups did. My oh my. It's not hard to find the needle in the heystack if the needle is heystack sized and there is no heystack. I never looked too close at the code to realise what dividing the second by 1 million in reality meant. It shows that every second that whole week will all result in 1726. Perfect. Anyhow, I'm sure this code can in some way be used again in CTFs later on, so Its not always bad to do it thoroughly™