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# PW Crack 3
```
enc = open("level3.flag.txt.enc", "rb").read()
```
You open the encrypted file in **binary mode (`rb`)** and read all its contents into the variable `enc`. This gives you raw bytes instead of text, which is important because encryption works at the byte level.
```
known = b"picoCTF{"
```
Here you define a **known plaintext prefix**. PicoCTF flags always start with `picoCTF{`, so you’re using that as a reference to recover the key.
```
key = bytes([a ^ b for a, b in zip(enc, known)])
```
This is the key step. You:
* Pair each byte of the ciphertext (`enc`) with each byte of the known plaintext (`known`) using `zip`
* XOR (`^`) them together
Since encryption was likely done as:
```
cipher = plaintext ^ key
```
You reverse it as:
```
key = cipher ^ plaintext
```
This gives you the **first part of the repeating XOR key**, which turns out to be:
```
b'22952295'
```
```
print(key)
```
Just prints the recovered key so you can see it.
```
decrypted = bytes([c ^ key[i % len(key)] for i, c in enumerate(enc)])
```
Now you decrypt the entire ciphertext:
* `enumerate(enc)` gives you both the index `i` and byte `c`
* `key[i % len(key)]` repeats the key cyclically
* Each ciphertext byte is XORed with the corresponding key byte
This reconstructs the original plaintext.
```
print(decrypted.decode())
```
Finally, you convert the decrypted bytes into a readable string using `.decode()` and print it.
```
picoCTF{m45h_fl1ng1ng_6f98a49f}
```
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