“Welcome to the enchanting world of witches and wizards. In this mystical laboratory, you find yourself as Witch Alice, a sorceress seeking an elusive secret potion recipe. Your trusted friend, Witch Bob, has discovered the hidden elixir of wonders and is eager to share it with you. However, an unforeseen obstacle stands in your way – Goblin Eve, determined to steal it for herself.”
At first glance, I can immediately tell that this is a problem for the Diffie-Helman Exchange, I learned this in class, and you can read it at Session 2.
For example:
Person A chooses a secret power a and sends g^a mod p to person B.
Person B chooses a secret power b and sends g^b mod p to person A.
Person A computes s = (g^b mod)^a mod p
Person B computes s = (g^a mod)^b mod p
Person A and B both share a secret encryption key s = g^ab mod p
In this example, Alice is Person A and Bob is Person B. Eve is an outsider — the eavesdropper, so to speak. However, without Alice or Bob’s private key, it is impossible for her to recreate the shared secret key.
Gameplay Portion
I forgot to take photos throughout the game as I was too involved! I found the secret though :)
Key 1
Technical Portion
Once we install openssl, we can start with the challenge. We need to find the flag that is returned after decrypting encrypted_spell.enc. I installed openssl via scoop, with the command scoop install openssl.
Now, we can start~!
The following command generates Diffie Helman parameters, in a dhparams.pen file. It creates a 2048-bit long safe prime.
Now, we need to generate Alice and Bob’s respective private keys, using what we are given.
These commands generate the respective private keys with the parameter files:
Now, we need to generate Alice and Bob’s respective public keys.
These commands generate the respective public keys with the parameter files:
Next, we can use Alice’s private key and Bob’s public key (or vice versa) to find out the shared secret key:
After executing that, we should be left with some more files, including the shared secret key:
Now, we just need to replicate the decryption done by Bob (which we have a hint for, at the bottom of the page, near the submit box. “I encrypted the spell using AES-256-CBC.” -Bob). Easy enough!
To decrypt in openssl, with a specific encryption type, use the following command:
After execution, though, it says that I am using a deprecated key derivation, and to swap, because it couldn’t decrypt it. Instead of using -d, I went with -pbkdf2, which I know well, and is the most recognizable one for me. It is called Password Based Key Derivation Function (2). Now the command looks like this:
And we have our recipe.txt!
If we see what is inside, it should be our flag!
Key 2
THM{525403e42fba51dfd0572025d78062f}