Tags: beginner crypto xor
Rating:
I have encrypted some text but it seems I have lost the key! Can you find it?
[yors-truly.py](https://github.com/Nightxade/ctf-writeups/blob/master/assets/CTFs/Wolv-CTF-2024/beginner/yors-truly.py)
---
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src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"
type="text/javascript">
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We're provided a Python source file:
```py
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:
```py
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}
