```
#!/usr/bin/env python3
from pwn import *

#establishing remote connection and makind the connection interactive
connection = remote('crypto.chal.csaw.io', 5001)
answerList=[]#contains answer list

#used to loop through the answer list
def answerFiller():
for i in answerList:
connection.sendlineafter(":","a", timeout=1)
connection.sendlineafter("?",i)

#used to convert "ECB"=>0 and "CBC"=>1 and printing flag
def converter():
binFlag=""
flag=""
#convert "ECB"=>0 and "CBC"=>1
for y in answerList:
if y == "ECB":
binFlag+="0"
else:
binFlag+="1"
#take 8 bits change to character and append to flag
for x in range(0, len(answerList), 8):
flag+=chr(int(binFlag[x:x+8], 2))
print(flag)

i=0
#looping to find the answerList
while True:
try:
connection.sendlineafter(":","a", timeout=1)
answerList.append("ECB")#appending ECB at the end and checking whether it's correct if it is correct proceed and save to answerList
connection.sendlineafter("?",answerList[i])
i+=1
#connection breaks or wrong answer
except EOFError:
answerList.pop()#delete the last answer("ECB") in answerList
answerList.append("CBC")#append "CBC"
print("*"*50)
converter()
print("*"*50)
connection = remote('crypto.chal.csaw.io', 5001)#establish connection again
answerFiller()#loop through the answer and continue ones the answerList is finished
continue

connection.interactive()
```
[original writeup](https://github.com/rithik2002/CSAW_CTF_2020/blob/master/modulusOPerandi/getFlag.py)