Rating:
# Either or Neither nor
## Overview
- 100 Points / 271 Solves
## Background
Made by MetaCTF
Oh no! I was working on this challenge and totally forgot to save a backup of the decryption key! Do you think you could take a look and see if you can recover it for me?
NOTE: The flag format is MetaCTF{}
[https://metaproblems.com/6ebee70f0d78d94a4750f9cb70031965/chal.py](https://metaproblems.com/6ebee70f0d78d94a4750f9cb70031965/chal.py)
## Find the flag
**In this challenge, we can download a file:**
```shell
┌[siunam♥earth]-(~/ctf/RITSEC-CTF-2023/Crypto/Either-or-Neither-nor)-[2023.04.01|21:54:34(HKT)]
└> file chal.py
chal.py: Python script, ASCII text executable
```
**It's a Python script:**
```py
#! /usr/bin/env python
flag = "XXXXXXXXXXXXXXXXXXXXX"
enc_flag = [91,241,101,166,85,192,87,188,110,164,99,152,98,252,34,152,117,164,99,162,107]
key = [0, 0, 0, 0]
KEY_LEN = 4
# Encrypt the flag
for idx, c in enumerate(flag):
enc_flag = ord(c) ^ key[idx % len(key)]
```
The flag is being XOR'ed!
**To reverse that process, we can XOR back the `enc_flag`.**
However, I wasn't able to reverse it... Maybe I need to brute force the key??
Hmm... Maybe my Python skill sucks.
**Let's Google "XOR brute force online":**
[dCode](https://www.dcode.fr/xor-cipher) always work for me.
**But first, we need to convert those `enc_flag` ASCII decimal to hex:**
```py
#!/usr/bin/env python3
def main():
enc_flag = [91,241,101,166,85,192,87,188,110,164,99,152,98,252,34,152,117,164,99,162,107]
hexedEnc_flag = '0x'
for c in enc_flag:
hexedEnc_flag += hex(c)[2:]
print(hexedEnc_flag)
if __name__ == '__main__':
main()
```
```shell
┌[siunam♥earth]-(~/ctf/RITSEC-CTF-2023/Crypto/Either-or-Neither-nor)-[2023.04.02|23:17:34(HKT)]
└> python3 solve.py
0x5bf165a655c057bc6ea4639862fc229875a463a26b
```
**Then, copy and paste that to XOR decoder:**
Next, choose "KNOWING THE KEY SIZE (IN BYTES)", and type `4`. **This is because the `KEY_LEN` is `4`.**
**Finally, choose "RESULT FORMAT" to "ASCII (PRINTABLE) CHARACTERS", and click "ENCRYPT/DECRYPT":**
Hmm... Those looks like the flag??
**Since the flag format is `MetaCTF{}`, we can search for that pattern:**
Boom! We found it!
- **Flag: `MetaCTF{x0r_th3_c0re}`**
## Conclusion
What we've learned:
1. Many Time Pad XOR
