Wolv CTF 2024
Crypto Yors Truly [50 pts]
I have encrypted some text but it seems I have lost the key! Can you find it?
We’re provided a Python source file:
import base64
plaintext = "A string of text can be encrypted by applying the bitwise XOR operator to every character using a given key"
key = "" # I have lost the key!
def byte_xor(ba1, ba2):
return bytes([_a ^ _b for _a, _b in zip(ba1, ba2)])
ciphertext_b64 = base64.b64encode(byte_xor(key.encode(), plaintext.encode()))
ciphertext_decoded = base64.b64decode("NkMHEgkxXjV/BlN/ElUKMVZQEzFtGzpsVTgGDw==")
print(ciphertext_decoded)
Note that XOR is a reversible operation, since a ^ a = 0 and XOR is associative and commutative.
Therefore, to recover the key, it we can simply XOR the plaintet and the ciphertext. See below:
\(plaintext=pt\)
\(ciphertext=ct\)
\(ct=pt \oplus key\)
\(ct \oplus pt = pt \oplus key \oplus pt = key\)
Therefore, here is the solve script:
import base64
plaintext = "A string of text can be encrypted by applying the bitwise XOR operator to every character using a given key"
key = "" # I have lost the key!
def byte_xor(ba1, ba2):
return bytes([_a ^ _b for _a, _b in zip(ba1, ba2)])
ciphertext_b64 = base64.b64encode(byte_xor(key.encode(), plaintext.encode()))
ciphertext_decoded = base64.b64decode("NkMHEgkxXjV/BlN/ElUKMVZQEzFtGzpsVTgGDw==")
print(byte_xor(plaintext.encode(), ciphertext_decoded))
Run the script to get the flag!
wctf{X0R_i5_f0rEv3r_My_L0Ve}