ASIS Final - Simple crypto Write up

Đề:
Begining always needs an interesting challenge, we can assure you, this challenge is an interesting one to begin the CTF!

Ta nhận được 1 file flag.enc và 1 file simple.py
Sau 1 hồi quan sát ta thấy rằng mã hóa này có điểm yếu ở dòng 

enc = xor_str(key * (len(flag) // len(key) + 1), flag).encode('hex')

 Ta có thể phục lại được flag bằng cách check length() của file flag.enc là 25280
đặt phần ( len(flag) // len(key) + 1 ) = x
lúc này mã hóa sẽ thành

 enc = x_str(key*x,flag).encode('hex')

 -> ta có thể biết được độ dài của x vì function xor_str có điểm yếu là

 if len(x) > len(y):
        return ''.join([chr(ord(z) ^ ord(p)) for (z, p) in zip(x[:len(y)], y)])
    else:

        return ''.join([chr(ord(z) ^ ord(p)) for (z, p) in zip(x, y[:len(x)])])

 Dễ dàng thấy độ dài của file = độ dài của file mã hóa / len( key.encode('hex') = 25280 / 40 = 632 
ta đã có x
lúc này phục hồi bằng cách 

flag = xor_str( key*x , flag.enc )
ta tìm được 1 file ảnh là flag
đây là Script mình viết để làm bài



Nhận xét

Đăng nhận xét

Bài đăng phổ biến từ blog này

SHACTF 2017 Write up for MISC Junior (200)

Hình ảnh
Crypt1:All about the base   V2VsbCBkb25lLAoKdGhpcyBmaWxlIGlzIGVuY3J5cHRlZCB3aXRoIEJhc2U2NC4gT2Z0ZW4gdXNl ZCBpbiBDVEYncyB0byBkaXNwbGF5IGJpbmFyeSBkYXRhIGluIGEgbW9yZSBmcmllbmRseSB3YXku IAoKVGhlIGZsYWcgZm9yIHRoaXMgY2hhbGxlbmdlIGlzIGZsYWd7YjNlOWMzZWVlNjA5YmFjNDZm YWQ0NDM5Y2YzMjFmZTV9Cg== just base64 decode -> flag Crypt 1: Rotation Seems someone rotated the alphabet, can you get the original message back? Ykksy eua ckxk ghrk zu mkz znk zkdz hgiq. Znk lrgm oy lrgm{30j3g1gg0ijg9l08ijlg52668hi6854g} Okay, must be caesar right? i took the part " lrgm{30j3g1gg0ijg9l08ijlg52668hi6854g}" Because i know for sure lrgm must be flag in some ROT    kt.pe is dead :v so i wrote a script to do the job :v    flag: flag{30d3a1aa0cda9f08cdfa52668bc6854a} Crypt2: Substitute Teacher Hello, my name is Mr. Smith, I will be your  substitute  teacher for today. I only have one assignment for you today, if you solve it you may go home. Now be silen...
Đọc thêm

SECT CTF - ACID BURN ft REPORT Misc 200 & 200 Write up

Hình ảnh
Report - Misc ( 200 + 0 ) Mr. Belford sucks, is he hiding something.... Download:  http://dl.ctf.rocks/Report.tar.gz We are given a PDF file, at first i though maybe they hide some image or hidden text inside the pdf, so i use this site to extract   But wow, the are neither image nor font ,  nothing but text and metadata, cool  :D  So i was googling to see if the anyway to hide data in PDF We know that hidden data can be embeded in  But in SHA2017 CTF, i have solved something encrypted with AES , to recover, we must know how the PDF Structure to get key, so i think maybe the data is embeded in the OBJ section like  and below is the hidden data that we need to find.  So I used binwalk to extract data in pdf file ( fore some reason i cant use any pdf tool in my ubuntu, still working on fixing lol ) , then check the obj section ( in .zlib ) as u can see, we know that the prefix "SECT" in hex are : 53 45 43 5...
Đọc thêm

Write up - SHACTF 2017 ( 2For100 + Cryp100 + Network100 + Crypt200)

Hình ảnh
1. Crypt100 : Stackoverflow I had some issues implementing strong encryption, luckily I was able to find a nice example on  stackoverflow  that showed me how to do it. Challenge:  stackoverflow.tgz File contains: - Script At first, when we check the given site above, we know that this pdf was encrypted with AES-CTR mode, and more interesting, we can see that they reuse the Counter -> that make AES-CTR vulnarable to Known-Plaintext attack crypto = AES.new(os.urandom(32), AES.MODE_CTR, counter=lambda: secret) with secret = os.urandom(16) Okay, let take look at the AES-CTR , let see the reason why they are vulnerable to Known-Plaintext attack: 107.50 KB Normally Counter must be UNIQUE , so that the keystream will be different every step. It is more secured than reused , it will make AES-CTR encryption become:  E(m[i]) = message[i] ^ keystream = cipher[i] and when we know the plaintext message[i] and the cipher[i] , we will get t...
Đọc thêm