Monday, December 26, 2016

3DS CTF 2016

3DS CTF was an interesting CTF, which unlike normal CTFs, went on for a whole week. I had plenty of time to enjoy the challenges. Below are write-ups for some of them.

Stego 100: Excaliflag

The file is a png image with nothing hidden in the binary data. Quick analysis with Steganabara shows that there's not much distortion with the RGB values, this means the flag is either hidden in the LSB values, or an advanced method is used. For only 100 points, of course the former is true.

Playing with the blue bits in Steganabara's Bit Mask Filter and you'll get the flag:

Stego 300: 0liver "Imaged"

By looking at the magic bytes in the binary data, it's easy to see that there is a png image appended at the end of the jpg image. Looking at the png image, it's clear that the flag is hidden in the R and G values of the first few lines. Extract the R and G values and you'll get an ELF file that prints out the flag.

Stego 300: We also have memes!

The flag is hidden in the image using an algorithm in which p and offset are unknown. However, they are small enough to be brute-forced. The flag format is 3DS{}, so this is more like a known plain-text attack with the image as the ciphertext.

(to be continued)

Sunday, December 18, 2016

WhiteHat Grandprix 2016

This is probably the worst CTF I've played. Server was always overloading (probably DDoS'ed and/or pwned). Also the anti-bruteforce mechanism made sure that you couldn't login when someone else was brute-forcing your account (It's very possible that the hackers already got access to your profile data - hope you didn't use the CTF password anywhere else). There were other problems, but they don't bother me anymore.

Below are some write-ups if you're interested. I don't quite remember the names and because some teams were able to submit flags after the contest ended they took everything down.

Web 100: Bánh bột lọc

If you look at the source you'll see the backed-up page. From there, you need to find a pair of username/password that meets the condition: $username.'1337' = md5($password). Some coding and the job is done (probably Linux experts can do faster than me).

The ones I used were 234417335475b7eb761e5f8accae1337 - huna12

Web 100: Bánh căn

For some stupid reason the page allows you to execute arbitrary php functions using get query. However, many important functions are blacklisted (from the hint). In the end, this query did the job: ?assert=require('php://filter/convert.base64-encode/resource=index.php')

Crypto 100
This is basically a substitution cipher. You lookup the characters corresponding to your numbers using the table. For some numbers that are multiplication of 2 other numbers, use them as row/column indices.

I solved this one by replacing the high-frequency numbers with characters and used SCBSolvr to do the rest.

Crypto 300

The key is generated from a 8-byte seed so it has a very big weakness: it's repeating after every 72 bytes. Using known plain-text attack you can recover the key and decrypt the text.

Reverse 100: Nem rán

After decompiling you get the python code. It's basically ROR so just ROL and it's done. Because it's rotation, you can quickly define ROL as ROR(bitsize - shifted_bits)

Forensics 100: Bánh giầy

Using Wireshark you can recover the secret file, which is a zip archive. Crack the zip using brute-force attack (password is 4-char long) and you'll get the flag.

Misc 100: Bánh đa kê

The flags are hidden in 32 files among 10000 folders. The server allows you to execute some Linux commands, and of course some important commands are blacklisted. In the end, the command I used was egrep -r '.' . | sort

Tuesday, December 6, 2016


I'm back, somehow.

Been busy, hard disk crashed, bad things happened, etc.

Restarting things from scratch, somehow.

Tuesday, May 13, 2014

ASIS CTF Quals 2014

A great CTF with a lot of interesting steganos has ended. Too bad the event took place on workdays, so our team didn't have much time for it (probably many other teams shared the same problem). We ended up at #10, which wasn't too bad :P Below you can find write-ups for a few challs I solved.

Trivia 50: Image

The file was actually an amazing image of the once popular NES game Battle City, and as a fan of Nintendo I had the emulator ready to play it :P Just complete the first stage and you'll get the flag: 8BIT_RULEZ (although it is a bit different from what is written and caused a lot of confusions for everyone :P)

Web 75: Hidden flag

A web challenge with barely any description, however the title suggests that the flag should be hidden somewhere. It didn't take us much time to notice the HTTP header named x-flag with the value ASIS_b6b?244608c2?c2e869cb56?67b64?b1. Now obviously the task was to find the full hash. My first thought was using a dictionary attack to find a string that generates a hash with the same pattern, but because of work I didn't really have any time to try it :P

At one point, some members in our team noticed that when a wrong solution was submitted, the response was almost instant. This suggested that there should be a javascript check somewhere. From here it didn't take much time for us to find the sha256 hash and recover the full flag: ASIS_b6be244608c27c2e869cb56167b649b1

Stego 100: Spy Paper

The image was quite big and it was quite easy for me to overlook every detail :P Fortunately redoc found anomalies in the blue channels and these dots reminded me of punched tape, which was very close to the final solution. We were able to quickly figure out the parity bits and decrypt the second part, however we could not find anything meaningful from the first part:

After a lot of time spending on it, we came to realize that this could be printer steganography, and the first part could be date and time. With that we were able to fully decrypt the flag: 9/6/19 13:22:44 E4sy_0n3.

Crypto 150: Random Image

This isn't a hard chall, the code seems to randomly create a new image based on the flag but in fact there is quite a big correlation between the "random" result and the original one. Specifically, if the value of the pixel is less than 250, the resulting pixel is the result of some operation on the coordinates xored with a random value which is the same for all pixels. We do not know the random value however we know the result of the operation on the coordinates and by xoring the encrypted image with this value all pixels with values less than 250 should stand out. Here is the final result of the decryption:

Stego 175: White noise

This is an easy prey for my powerful Steganabara, and that was the reason why our team quickly became the first solver. A quick histogram analysis shows that the values in the green and blue channels are evenly distributed, and the reason behind this is that they were made to be used as coordinates to rearrange the pixels.

However, the red channel only has 1 value: 128, so it is pointless if you rearrange the whole image, you'll get just a red square. This got me stumped for a little while, until I realized that the order of arrangement could be important too. With this I only used the first 30 lines of the image for rearrangement, and got the flag:

Wednesday, April 30, 2014

0x3004 CTF

Another great CTF just ended and we were the champion! 0x3004 CTF was a 5-day event to celebrate the Liberation Day 30/04/1975 that ended all the miseries and brought about happiness to everyone. It was a pleasure having solved so many challenges of such great qualities thanks to the administrators.

Below you can find a quick write-up of some challs. Because there are so many of them, I'm only writing out the key points, the rest is up to you :P

Crypto 50: Phú Yên :: No Encryption Here

The cipher text:

The = signature at the end makes it fairly easy to identify it as base64 encoded. The decoded text is M,'@S,#`T>W!L96%S95]S=6)M:71?...

This text is still encrypted and because it consists of only printable characters it can be guessed that encryption method is uuencode. It can be decrypted using this online decryption tool but you have to break it into 2 parts to fully decrypt the flag: 0x3004{please_submit_this_sh!t_and_get_your_rewards}

Crypto 50: Quảng Nam :: Chuyển vận lương thực

The encryption is done in the cookie. Trying with different inputs should bring you to a conclusion that the username and the timestamp are combined, xored with a binary key and then base64 encoded.
The string encrypted in the cookie is something like "username=admin;time=2014-04-27T16:27:24.644158", to solve the challenge you have to append ";admin=true" to it and get the flag: 0x3004{you_control_the_world}

Web 50: Yên Bái :: Injection1

Someone else in our team solved it, basically the session is managed by the serialized data in the cookie and you can exploit the deserialization to do SQL injection and get the flag.

Misc 50: Bạc Liêu :: Hidden1

The flag is hidden somewhere in the source code of the challenge page: 0x3004{_haha_you_found_it_:D}

Misc 50: Đà Nẵng :: Áo Dài

This is a BMP stegano. Analysis with my Steganabara's Bit Mask Filter (or Caesum's Stegsolve) will reveal that there is data hidden in the LSB of the pixels:

In fact the authors herd you like BMP so they put a BMP in your BMP so you can extract while you extract :P xp45g did this with a "murderous unreadable 1-liner", and here's the final result:

Misc 50: Vĩnh Long :: HeartBeat

This is similar to the famous HeartBleed bug in OpenSSL. It is even easier because you can input the length directly into the URL. If you input a bigger length than the actual string the rest of the memory will be printed out. We used a length of -1 to dump out everything and after some tries we were able to get the flag: 0x3004{He4rtBle3d_works_this_way}

Misc 50: Lâm Đồng :: Hidden2

The flag is again hidden in the source code of the home page: 0x3004{hidden2_hidden_everywhere}

Web 100: Hải Dương :: CRYPOT

The encryption can be broken using differential attack. In each loop the ciphertext is xored with a hash created from a character in the input and a character in the flag so you can use 2 inputs with the same length as the flag (31) but with different characters at the end, xor the results (which is also the xored result of 2 hashes in the last loop) and from there work out the last character in the flag. Continuing backwards and you can recover the flag: 0x3004{p_to_the_h_to_the_p_yo!}

Web 100: Tuyên Quang :: Injection2

You can still exploit the deserialization to inject SQL code. Below was how xp45g solved it :P

10:54:36: $ phpsessid="$(tr -cd a-zA-Z0-9 < /dev/urandom | head -c 32)" ; curl -s -H "Cookie: PHPSESSID=$phpsessid;login=$(./ser.php "wtf',''),(null,(select flag from web100_flag), '$phpsessid') #")" | html2text
10:54:38: Welcome you back. This time, we made it more secure!
10:54:40: Your action has been logged to our DB.
10:54:42: You are logged in as guest.
10:54:44: Your action has been logged to our DB.
10:54:46: Your last logged in time: 0x3004{php_0bj3ct_m4k35_1t_3a5y}.
10:54:48: win \o/

Web 100: Quảng Ninh :: PATH TO PRO

This was quite an annoying chall. At first we found the blind SQL injection with double quotes and substring function and used it to get the admin password YOUWONTBEABLETOGUESSTHISPASS__@#!@(#*!@(#*!@(#*)(!@*# but that wasn't enough to get the flag.

We were having troubles identifying the DBMS because of so many abnormalities and in the end it turned out the challenge was about XPATH injection, something similar to SQL injection in theory but not as popular in practice. The flag was 0x3004{XXXpath}

RE 100: Nghệ An :: PHPVLD

Our team had some troubles with this chall because of misinterpreting the code but it was basically just hash collision. Just generate 2 strings with the same MD5 hashes and the chall is solved.

Crypto 100: Bình Định :: SERICRYPT

The method to solve this chall is similar to the one described in the RSA page: you factorize n, calculate phi(n), then calculate modulus multiplication inverse and decrypt the message. The result is: 6396138900968155672706619512005662088160241943837385041483898733707420105484519573719621312884.

Misc 100: Gia Lai :: Wireshark

This was an easy chall. At first glance we thought the traffic was encrypted using SSL but in the end the flag was transferred in just plain text and you can see it by searching for 0x3004 in a text editor: 0x3004{I_l0v3_wir35h4rk_S0_MUCH!}

Misc 100: Trà Vinh :: f_x

Our analysis was like this: f(12)/f(11) ~= 2.8, f(11)/f(10) ~= 3.1, (12/11)^12 = 2.84094437661548, (11/10)^12 = 3.138428376721 so the function should be a polynomial with the degree of 12. With this the problem becomes solving a system of 13 equations and even though I only had to modify my old program a little I was still a lot slower than xp45g's z3. The flag was 0x3004{M4thz1g}

Crypto 150: Thừa Thiên Huế :: Tàng hình

This is basically a stegano, the flag was hidden insite the KingthingsTrypewriter2 font file. Opening it using a font editor like ttfedit and you'll get the flag: 0x3004{H4Ppy_huNt1Ng} - it was changed later but someone else in our team solved it and I'm too lazy to solve it again :P

Web 150: Lạng Sơn :: XYZ Bank

If you use the default login suggested by the page you'll get a cookie that decrypts to something like ["guest","guest",1234]. Using a python mysql bug you can use ["guest",0,1234] and the session is still valid. Changing the username to admin and brute-forcing the pin you can get the flag: 0x3004{goooo_home_homie}

Crypto 200: An Giang :: Super RSA

Your job is to break an RSA encryption knowing just the public key. However with the source code available you can see that there is a weakness in the encryption: the private key is small (only 1024 bits) while the public key is big (6144 bits). Now the encryption can be broken using Wiener's attack.

Crypto 250: Lai Châu :: CRYPTOWWW

In this chall you need to bypass the hash check to do SQL injection. This can be done using hash length extension attack. At first we were getting no result because of assuming the secret length of 6 but in the end we wrote a program to brute-force the length (which turned out to be more than 20) and got the flag: 0x3004{www_mix_crypto_ftw}

Bonus: Pwn 300: Hà Nội :: Vượt Ngục 2

The nerd term for this type of challenge is "golfing". Below is the result of our python experts' teamwork. Don't ask me for the code explaination :P In fact after the CTF ended they managed to golf it down even more :P

For more information and solutions for other challs, you can visit stypr's write-up page :P