### Intuition
We can use a meet-in-the-middle technique to bruteforce the missing bytes.
### Solution
Simple bruteforce script:
```py#!/usr/bin/env python3from Crypto.Cipher import AESimport hashlibfrom threading import Thread
ct = b'7\xcf7\xce\xa6 \xbe\t\xba\x03\xe4\xac\x9e\x86\x85\xf5YZYa_7\xae\xa1\xe6\xc1\xd1\xad\xfb\x9c\x99s'plain = b'This is a non-secret message....'
everything_dec = {}everything_enc = {}
def find_dec():    for i in range(256):        for j in range(256):            for k in range(256):                r = bytes([i, j, k])                k2 = r + b"A"*29                d = AES.new(k2, AES.MODE_ECB)                dec = d.decrypt(ct[:16])                everything_dec[dec] = k2
def find_enc():    for i in range(256):        for j in range(256):            for k in range(256):                r = bytes([i, j, k])                k1 = r + b"A"*29                e = AES.new(k1, AES.MODE_ECB)                enc = e.encrypt(plain[:16])                everything_enc[enc] = k1
# Multi-thread them just for funt1 = Thread(target=find_dec)t2 = Thread(target=find_enc)
print("Starting threads...")
t1.start()t2.start()
t1.join()t2.join()
print("Searching results...")
for d in everything_dec.keys():    if d in everything_enc:        print("Found!")        print(everything_enc[d], everything_dec[d])```
Then apply the transformations explained in the description.
#### Flag
```CTF{91e6611654e4fe66d6876f728b8dfd54999ed752f89239ab82ecd9e520c1e003}```