Tags: optimization crypto
Rating:
# Homecooked
Author: [roerohan](https://github.com/roerohan)
All you have to do is optimize the code you already got.
# Requirements
- Python
# Source
```
I cannot get this to decrypt!
Download the file below.
[decrypt.py](https://ctf.nahamcon.com/files/6997fd70f4c38f9efcc4984193bcd541/decrypt.py?token=eyJ1c2VyX2lkIjoyMzk2LCJ0ZWFtX2lkIjpudWxsLCJmaWxlX2lkIjoyfQ.XuYscg.X77rndeap6B6BGNQQulUqJbzLEE)
```
```python
# decrypt.py
import base64
num = 0
count = 0
cipher_b64 = b"MTAwLDExMSwxMDAsOTYsMTEyLDIxLDIwOSwxNjYsMjE2LDE0MCwzMzAsMzE4LDMyMSw3MDIyMSw3MDQxNCw3MDU0NCw3MTQxNCw3MTgxMCw3MjIxMSw3MjgyNyw3MzAwMCw3MzMxOSw3MzcyMiw3NDA4OCw3NDY0Myw3NTU0MiwxMDAyOTAzLDEwMDgwOTQsMTAyMjA4OSwxMDI4MTA0LDEwMzUzMzcsMTA0MzQ0OCwxMDU1NTg3LDEwNjI1NDEsMTA2NTcxNSwxMDc0NzQ5LDEwODI4NDQsMTA4NTY5NiwxMDkyOTY2LDEwOTQwMDA="
def a(num):
if (num > 1):
for i in range(2,num):
if (num % i) == 0:
return False
break
return True
else:
return False
def b(num):
my_str = str(num)
rev_str = reversed(my_str)
if list(my_str) == list(rev_str):
return True
else:
return False
cipher = base64.b64decode(cipher_b64).decode().split(",")
while(count < len(cipher)):
if (a(num)):
if (b(num)):
print(chr(int(cipher[count]) ^ num), end='', flush=True)
count += 1
if (count == 13):
num = 50000
if (count == 26):
num = 500000
else:
pass
num+=1
print()
```
- [decrypt.py](./decrypt.py)
# Exploitation
As you see in the source, there are 2 functions, `a()` and `b()`. When you run the code you got from the site, you see the starting of the flag is printed automatically: `flag{pR1m3s_4re_co0ler_Wh3`. Now, it takes a while to print the rest of the flag. Well, a long time XD. So it's best if you optimize the `a()` function a little bit.
```python
import base64
num = 0
count = 0
cipher_b64 = b"MTAwLDExMSwxMDAsOTYsMTEyLDIxLDIwOSwxNjYsMjE2LDE0MCwzMzAsMzE4LDMyMSw3MDIyMSw3MDQxNCw3MDU0NCw3MTQxNCw3MTgxMCw3MjIxMSw3MjgyNyw3MzAwMCw3MzMxOSw3MzcyMiw3NDA4OCw3NDY0Myw3NTU0MiwxMDAyOTAzLDEwMDgwOTQsMTAyMjA4OSwxMDI4MTA0LDEwMzUzMzcsMTA0MzQ0OCwxMDU1NTg3LDEwNjI1NDEsMTA2NTcxNSwxMDc0NzQ5LDEwODI4NDQsMTA4NTY5NiwxMDkyOTY2LDEwOTQwMDA="
pl = {}
p = open('primes1.txt').read().split()
for i in p:
pl[int(i)] = True
def a(num):
if (num > 1):
for i in range(2,num):
if (num % i) == 0:
return False
break
return True
else:
return False
def primes(num):
return num in pl
def b(num):
my_str = str(num)
rev_str = my_str[::-1]
if my_str == rev_str:
return True
else:
return False
cipher = base64.b64decode(cipher_b64).decode().split(",")
while(count < len(cipher)):
if (primes(num)):
if (b(num)):
print(chr(int(cipher[count]) ^ num), end='', flush=True)
count += 1
if (count == 13):
num = 50000
if (count == 26):
num = 500000
else:
pass
num+=1
print()
```
So, we create a new function `primes()`, which can now replace `a()`. All it does is basically, read the first million primes from a file called [`primes1.txt`](./primes1.txt). Now, create a python dictionary of primes and check if the parameter is in that dictionary. This prints out the flag: `flag{pR1m3s_4re_co0ler_Wh3n_pal1nDr0miC}`.
