Relations

Two completely unrelated operations on completely unrelated values, right?

Time

30 mins

Behavior

.. nc arcade.fluxfingers.net 1821
------------------------------
Welcome to theory world
------------------------------

------------------------------
Possible Oracles
(XOR) Choose XOR Oracle
(ADD) Choose ADD Oracle
(DEC) For trying to decrypt
-----------------------------*
XOR

Please choose the operand in hex >>> 0
Ciphertext is  LaIED+tNb4H2B3YQi97GNNqndX8BuFh8Al1/z/AIXtiYas5LCK2SvYsO4vJAad2g+2TnJKcrwmSw
YOZo8enlVw==

------------------------------
Possible Oracles
(XOR) Choose XOR Oracle
(ADD) Choose ADD Oracle
(DEC) For trying to decrypt
-----------------------------*
ADD

Please choose the operand in hex >>> 0
Ciphertext is  LaIED+tNb4H2B3YQi97GNNqndX8BuFh8Al1/z/AIXtiYas5LCK2SvYsO4vJAad2g+2TnJKcrwmSw
YOZo8enlVw==

------------------------------
Possible Oracles
(XOR) Choose XOR Oracle
(ADD) Choose ADD Oracle
(DEC) For trying to decrypt
-----------------------------*
DEC

Enter the key base64 encoded >>> 0000
Traceback (most recent call last):
  File "/home/chall/rka.py", line 113, in <module>
    main()
  File "/home/chall/rka.py", line 107, in main
    aes = pyaes.AESModeOfOperationECB(key)
  File "/home/chall/pyaes/aes.py", line 304, in __init__
    self._aes = AES(key)
  File "/home/chall/pyaes/aes.py", line 134, in __init__
    raise ValueError('Invalid key size')
ValueError: Invalid key size

Solution

The server provide three operations:

• Encryption(aeskey ^ operand)
• Encryption(aeskey + operand)
• AESDec(inputkey, AESEnc(aeskey, flag))

When bit $i$ in aeskey is 0, $\begin{aligned} & \text{aeskey} \oplus 2^i = \text{aeskey} + 2^i \\ \implies & \text{Encryption}(\text{aeskey} \oplus 2^i) = \text{Encryption}(\text{aeskey} + 2^i) \\ \end{aligned}$ Otherwise those are not equal. We can recover the key bit-by-bit to get key for decrypting the flag.