aes

cryptography/level-2/DESCRIPTION.md

@@ -1,3 +1,11 @@
-In this challenge you will decrypt a secret encrypted with a one-time pad.  
+In this challenge you will decrypt a secret encrypted with a [one-time pad](https://en.wikipedia.org/wiki/One-time_pad).
 Although simple, this is the most secure encryption mechanism, if a) you can securely transfer the key and b) you only ever use the pad _once_.
 It's also the most simple encryption mechanism: you simply _XOR_ the bits of the plaintext with the bits of the key one by one!
+
+This challenge encrypts the flag with a one-time pad and then gives you the key.
+Luckily, a one-time pad is a _symmetric_ cryptosystem: that is, you use the same key to encrypt and to decrypt, so you have everything you need to decrypt the flag!
+
+----
+**Fun fact:** the One-time Pad is the _only_ cryptosystem that humanity has been able to _prove_ is perfectly secure.
+If you securely transfer the key, and you only use it for one message, it cannot be cracked even by attackers with infinite computational power!
+We have not been able to make this proof for any other cryptosystem.

cryptography/level-4/DESCRIPTION.md

@@ -0,0 +1,22 @@
+So, One Time Pads fail when you reuse them.
+This is suboptimal: given how careful one has to be when transferring keys, it would be better if the key could be used for more than just a single message!
+
+Enter: the [Advanced Encryption Standard](https://en.wikipedia.org/wiki/Advanced_Encryption_Standard), AES.
+AES is relatively new: coming on the scene in 2001.
+Like a One-time Pad, AES is _also_ symmetric: the same key is used to encrypt and decrypt.
+Unlike a One-time Pad, AES maintains security for multiple messages encrypted with the same key.
+
+In this challenge you will decrypt a secret encrypted with Advanced Encryption Standard (AES).  
+AES is what is called a "block cipher", encrypting one plaintext "block" of 16 bytes (128 bits) at a time.
+So `AAAABBBBCCCCDDDD` would be a single block of plaintext that would be encrypted into a single block of ciphertext.
+
+AES _must_ operate on complete blocks.
+If the plaintext is _shorter_ than a block (e.g., `AAAABBBB`), it will be _padded_ to the block size, and the padded plaintext will be encrypted.
+
+Different AES "modes" define what to do when the plaintext is longer than one block.
+In this challenge, we are using the simplest mode: "[Electronic CodeBook (ECB)](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_codebook_(ECB))".
+In ECB, each block is encrypted separately with the same key and simply concatenated together.
+So if you are encrypting something like `AAAABBBBCCCCDDDDEEEEFFFFGGGGHHHH`, it will be split into two plaintext blocks (`AAAABBBBCCCCDDDD` and `EEEEFFFFGGGGHHHH`), encrypted separately (resulting, let's imagine, in `UVSDFGIWEHFBFFCA` and `LKXBFVYASLJDEWEU`), then concatenated (resulting the ciphertext `UVSDFGIWEHFBFFCALKXBFVYASLJDEWEU`).
+
+This challenge will give you the AES-encrypted flag and the key used to encrypt it.
+Decrypt the flag and score!

cryptography/level-4/run

@@ -0,0 +1,15 @@
+#!/opt/pwn.college/python
+
+import base64
+from Crypto.Cipher import AES
+from Crypto.Util.Padding import pad
+from Crypto.Random import get_random_bytes
+
+flag = open("/flag", "rb").read()
+
+key = get_random_bytes(16)
+cipher = AES.new(key=key, mode=AES.MODE_ECB)
+ciphertext = cipher.encrypt(pad(flag, cipher.block_size))
+
+print(f"aes key: {base64.b64encode(key).decode()}")
+print(f"secret ciphertext: {base64.b64encode(ciphertext).decode()}")

cryptography/module.yml

@@ -4,12 +4,11 @@ challenges:
 - id: level-1
   name: base64
 - id: level-2
-  name: One Time Pad
+  name: One-time Pad
 - id: level-3
-  name: Many Time Pad
+  name: Many-time Pad
 - id: level-4
-  name: level4
-  description: Decrypt a secret encrypted with AES using the ECB mode of operation
+  name: AES
 - id: level-5
   name: level5
   description: Decrypt a secret encrypted with AES-ECB, where arbitrary data is appended to the secret and the key is reused. This level is quite a step up in difficulty (and future levels currently do not build on this level), so if you are completely stuck feel free to move ahead. Check out [this lecture video](https://youtu.be/YO5bgKjqW00?t=1901) on how to approach level 5.