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LEFTOVERS

By: Hansern - Munintrollet

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?

Initial Analysis

From the challenge description we know there is a "master class secret" to be found.

The attached terminal.log.txt file contains both commands and the following output from an Ubuntu terminal session:
Terminal Output
During this session the user does some interesting stuff before encrypting a master.txt file using openssl. This is most likely the highly regarded "master class secret" we are on the look out for. Since the log contains both output and command, we actually have the encrypted version of master.txt which is:

U2FsdGVkX1+/39qrCQ9rlxMW2E30ylTUXYS+GTAVDMUK0oXJvkUDBCRbhClK2GKYc50OQZ7zgLPBhkMW8CM5VVnZBrxfyH5CAG8nj5BPDCg=

This seems simple enough; we know how the user encrypted the highly regarded "master class secret", but we don't know why :( and we have the encrypted output. So, let's decrypt this dubious "master class secret".

OpenSSL

Before solving this challenge we need to understand what this argument-ridden openssl command line does.

From the forever helpful man command: The openssl program is a command line program for using the various cryptography functions of OpenSSL's crypto library from the shell.

For the arguments, we get this from the lovely man:

  • enc: Enables encryption, decryption, and encoding
  • -aes-256-cbc: Use AES 256-bit size encryption algorithm
  • -pbkdf2: Something called Password-Based Key Derivation Function 2, tries to make brute-force difficult
  • -iter: Number of iterations
  • -in: Input filename
  • -k: Password used to derive the key from
  • -a: Process the data with Base64 after encryption

Now we have memorized all this useful information, we can finally decrypt this sinister "master class secret".

Decryption

Before we start to decrypt this surprising "master class secret", we need to figure out what the $ITER and $PWD variables are.

Lets start with $PWD:

Normally $PWD is our Printed Working Directory, i.e. which directory we currently reside in. But in this case, it's different. In this terminal session it would (probably) be something like /home/USER108/code/openssl, but the 3rd command executed changes this $PWD environmental variable to $(hostname). Where $(hostname) is the output from the command hostname.

The problem is that we do not have the hostname in any of our commands or output! But in the welcome message displayed by the Ubuntu terminal, it displays the hostname at the very top: Hostname
Specifically: ubuntu-s-1vcpu-512mb-10gb-ams3-01

Now for $ITER:

The command run prior to the encryption command itself sets the $ITER value. It's simply the output of date +%s divided by 1000000. But what tf is this date +%s thingy? Turns out it is the number of seconds since January 1, 1970, at 00:00:00 UTC. This is because Linux > everything. So, $ITER is the number of seconds since the 1st of January 1970 at 00:00:00 UTC divided by 1000000. Easy enough.

But when was this encrypted? Well, our user had to check which week and month it was before encrypting this undoubtedly "master class secret". So we do actually have all the information we need. It was encrypted during week 38 in September 2024, between 16th to 22nd September 2024.

Since we have established that Linux > everything, we will decrypt this powerful "master class secret" using a special crafted bash-script, decrypt_leftovers.sh. This script decrypts the discstuing "master class secret" inside the master.txt file.

This script does the following:

  • Sets $PWD and $DATES to values found by the analysis above. I.e. $PWD contains the hostname, and $DATES contains every day in week 38 of 2024.
PWD="ubuntu-s-1vcpu-512mb-10gb-ams3-01"
DATES=("2024-09-16" "2024-09-17" "2024-09-18" "2024-09-19" "2024-09-20" "2024-09-21" "2024-09-22")

DISCLAIMER: Using every day in week 38 turned out to be unnecessary, we will get to that later...

  • Iterate over each day in week 38 2024.
for DATE in "${DATES[@]}"; do
  [...]
done
  • For each iteration, try to decrypt the un-believable "master class secret" using the current day, and check the exit code for decryption command success.
ITER=$(($(date -d "$DATE" +%s) / 1000000))
echo "Trying with ITER=$ITER for date $DATE"

DECRYPTED_OUTPUT=$(openssl enc -aes-256-cbc -pbkdf2 -iter $ITER -d -in $ENCRYPTED_OUTPUT -k "$PWD" -a 2>&1)

if [[ $? -eq 0 && -n "$DECRYPTED_OUTPUT" ]]; then
  echo "Decryption successful for ITER=$ITER on $DATE"
  echo "Decrypted content:"
  echo "$DECRYPTED_OUTPUT"
exit 0  
fi

FLAG

Here we have the output of the decryption script containing the decrypted juicy "master class secret":
Script Output
We have successfully retrieved the eccentric "master class secret":

EPT{Ach13v3m3nt_Unl0ck3d_293857}

Turtle > Rabbit

While writing this writeup explaining what the openssl command and the bash-script does. I quickly realized that, as mentioned in the disclaimer above, we do not need to iterate over all the days in week 38 of 2024. This is because the calculation of $ITER is a bit extreme. Specifically, the division part.

First day of week 38 in 2024 (16th sept.) is 1 726 080 000 seconds after that random day in 1970. So, $ITER will be 1 726 080 000 / 1 000 000 = 1726. Last day of week 38 in 2024 (22nd sept.) is 1 726 144 000 seconds after that day in 1970. So, here $ITER will become 1 726 144 000 / 1 000 000 = 1726. THE SAME VALUE. EACH DAY (AND SOME MORE) RETURNS THE SAME $ITER VALUE. The division value of 1 000 000 is way too big and does not affect the $ITER value nearly enough.

Channelling the inner turtle during a CTF tournament is something we all, at least I, should do more often. Or maybe not the inner turtle, since first bloods > air, but the inner turbit:
Turbit