Tags: cbc crypto
Rating:
# ᒣ⍑╎ϟ ╎ϟ リᒷᖋᒣ
## Description
```
We have reason to believe that Mortimer is rebuilding the Armageddon Machine.
We managed to intercept a message sent from his personal computer.
After consulting with the Oracle, we were only able to discover two things:
The tool used to encrypt messages (encrypt.py)
All messages sent by Mortimer begin with Mortimer_McMire:
Can you decrypt the message?
```
## Provided Files
```
- encrypt.py
- message.enc
```
## Writeup
Looking at `encrypt.py` we can see the encryption used for the plaintext.
```py
from Crypto.Cipher import AES
import binascii, os
key = b"3153153153153153"
iv = os.urandom(16)
plaintext = open('message.txt', 'rb').read().strip()
cipher = AES.new(key, AES.MODE_CBC, iv)
encrypted_flag = open('message.enc', 'wb')
encrypted_flag.write(binascii.hexlify(cipher.encrypt(plaintext)))
encrypted_flag.close()
```
`message.enc` contains the output of `encrypt.py`.
```
2a21c725b4c3a33f151be9dc694cb1cfd06ef74a3eccbf28e506bf22e8346998952895b6b35c8faa68fac52ed796694f62840c51884666321004535834dd16b1
```
To get the flag we actually already have everything we need.
- `key` - Key used for encryption and decryption, `b'3153153153153153'`.
- `iv` - Initialization vector used for encryption algorithm (first 16 bytes of ciphertext), `b'2a21c725b4c3a33f'`.
- `ciphertext` - Ciphertext containing the flag `151be9dc694cb1cfd06ef74a3eccbf28e506bf22e8346998952895b6b35c8faa68fac52ed796694f62840c51884666321004535834dd16b1`.
Putting these things together in a solution script:
```py
from Crypto.Cipher import AES
import binascii
key = b"3153153153153153"
with open('message.enc', 'rb') as file:
hex_data = file.read()
cipher_text_bytes = binascii.unhexlify(hex_data)
iv = cipher_text_bytes[:16] # IV is first 16 bytes of ciphertext
cipher_text = cipher_text_bytes[16:] # The rest is the actual ciphertext
cipher = AES.new(key, AES.MODE_CBC, iv)
plain_text = cipher.decrypt(cipher_text)
print(plain_text.decode('utf-8'))
```
Running the decryption script concludes this writeup.
```sh
$ python3 decrypt.py
shctf{th1s_was_ju5t_a_big_d1str4ction}
```
