Signature is a crypto challenge from TSJ CTF 2022, which ended up having two solves. From this challenge, we can see how ECDSA private keys can be recovered by having a (weak) deterministic ephemeral key, $k$.

@blackb6a played TSJ CTF and CODEGATE CTF this weekend. Both of the CTFs had a bunch of epic (and hard-ish) crypto challenges, which made our head scratched for days.

We were two points behind @balsnctf few minutes before the game ends and I found one more flag. It is proud to tell that we won TSJ CTF 🎉!

Anyway, I will compile writeups for (at least) three challenges for TSJ CTF, namely Cipher Switching Service, Signature and Genie. I will go through Cipher Switching Service as the first part of the series.

@blackb6a helped preparing some challenges for MOCSCTF, a 8.5-hour long CTF in Macau. This time I wrote nine challenges and @hoifanrd made one of them (3-AES). This blog post covers the intended solution for all of them.

This is another round of @intigriti’s XSS challenge, and this time it is written by @aszx87410. I spent around four hours solving it.

DiceCTF is an annual CTF competition prepared by @dicegangctf. The challenges are great and we had a lot of fun solving them. I might be compiling writeup for a number of them, which depends if I had time.

I will first cover on a crypto challenge called commitment-issues, which had 16 solves (out of 1127 participating teams). @grhkm2023 and I spent a good 3 to 4 hours working on this challenge.

This is the time that Firebird Internal CTF happens. I made three crypto challenges this year - Lack of Entropy (⭐), Authenticator (⭐⭐) and Collider (⭐⭐). I will discuss the solution for all of them in the blog post.

@intigriti has a XSS challenge every month. The challenge is not hard this time and I am able to solve it in an hour or two. The best thing I learned is to recover source code using the source map file.

@blackb6a played Dragon CTF 2021 last weekend and I spent most of the time solving the CRC duo. They are very fun but unfortunately that we were close enough while unable to get the second flag.

@blackb6a played Balsn CTF 2021 last weekend. There are three crypto challenges, and they are all pretty hard. In this blog post we will cover the dlog challenge, which is a timing attack on the CPython’s pow method. @grhkm2023 and I spent a lot of time working on this challenge. Although we did not solve it, we actually learned a lot from the challenge and lost a pile of hair (maybe it is just me).

We will cover the remaining challenges I wrote in this part: Flag Checker™, The Wilderness and Potion of Ciphermath.

As the third part of the series, three reversing challenges will be included: The Hardest Path, A Junior Mathematician and Let’s Chill.

In this part, three harder crypto challenges will be covered: Tenet: The Plagarism, Sratslla SEA and Sign in Please, Again.

Black Bauhinia coorganized HKCERT CTF 2021 and helped 95% of the technical stuffs, including challenge setting, platform development, infrastructure and etc. I will be writing a series of blog posts talking about the contest, and the first four would be the writeups of the challenges those I wrote.

In the first blog post, we will be going through four easier crypto challenges: A Joke Cipher, Cipher Mode Picker, Key Backup Service 1 and Key Backup Service 2.

No, we did not win. I see that coming and took a screenshot during the game.

BSides Ahmedabad CTF 2021 is organized by @zer0pts. That implies that players could spend a day happily working on the challenges. I enjoyed working on the challenges a lot. I’ll go through the harder crypto challenges and the reversing challenge called Collapsed License in this blog post.

LagLeg is a crypto challenge in ASIS CTF Quals 2021. We are asked to factor a given $n$ with $n = (r^5 + s)(r + s)$ as part of the challenge. Hereby $r$ and $s$ are respectively 256 and 64 bits long. I think my approach used is very unique and creative - and definitely worth mentioning. It does not mean that it is a good approach, however.

I will record the exact steps I solved the challenge. However, I will update the bounds and prove the whole thing so that it looked more rigorous. The full code will be attached at the end of the blog post, too.

I am playing as a part of @blackb6a this time for perfect blue’s annual pbctf. This time there are six crypto challenges and I first blooded 🩸 half of them. I solved five of them, and collaborated with TWY (who made 99% of the process) for Seed Me. In this blog post, I will cover Seed Me and Yet Another RSA. I tried to make the whole post beginner friendly, hence included more details than necessary.

I was originally going to discuss all of the crypto challenges, but I found it is too demanding and tiring.

This is another time @blackb6a unites and plays UIUCTF together. It was my fourth time playing UIUCTF, and I still found the challenges fun. Although the crypto challenges are relatively easy, I had a lot of fun solving phpfuck with @02E774.

This is me playing another Google CTF with @blackb6a, ending up on the 25th place. I aced the crypto challenges and solved some challenges with my teammates. I have a strong feeling that Black Bauhinia grown a lot between the two Google CTFs and I am often backed by my teammates this time. Anyway, I will be covering all of the crypto challenges in this blog post.

This is the summary for me on the second day I played DEF CON CTF. The commentary for day 1 is available here. In this blog post, only the solutions for qoo-or-ooo, back-to-qoo and pooow-buddy are written. Since there are a lot of new stuff, I may not be able to explain them well. Please bear with me…

I played DEF CON CTF Quals once again with @Shellphish and we ended up at the 10th place. This blog post describes what actually was going on from my side. If you are looking for an informative blog post, this is not a good-read for you. There would be a little useful information, since I am just hanging around most of the time. I will even shamelessly copy some memes online to make the blog post looked rich in content.

After all, I am only able to solve nooombers (on day 1), qoo-or-ooo and back-to-qoo (on day 2). What happened on day 2 from my point of view is considerably more interesting.

PlaidCTF this year had very insane challenges. Although I have spent a lot of time working on those crypto challenges, I was only able to solve leaky block cipher.

This completely legitimate™ block cipher looks a bit like GCM, but my computer plumber keeps complaining about water residue. Can you help me spot the leak? The challenge has a hand-crafted AEAD scheme. We are required to go through 20 rounds of challenges, and this is how each round proceeds…

I played ångstromCTF 2021 for @blackb6a to spend my Easter holiday. I solved most of the reverse and cryptography challenges alone. In particular, Cache Money is one of the harder crypto challenges that I spent more than one day dealing with. It is very rewarding, and eventually four teams ended up solving it.

This challenge reimplements the Advanced Encryption Standard (AES) on 128, 192 and 256-bit keys. The encryptor is equipped with caches and we are given a service to encrypt (or decrypt) our messages. In short, there are four oracles provided by the service ($k_0$ is the fixed secret key and $b \in \{128, 192, 256\}$).

On August 2020, @blackb6a was invited to co-organize HKCERT CTF 2020 (which is held on November 2020) as one of the challenge authors. This is a CTF for secondary and tertiary students in Hong Kong. Although I had experience preparing CTFs earlier, this is actually the first CTF officially prepared by Black Bauhinia. I have written four challenges for this CTF - Sanity Check II (Web), LF2 (Reverse), Sign In Please (Crypto) and Calm Down (Crypto). There will also be some stories behind the scenes.

This time I am playing alone for @blackb6a and had all the crypto challenges solved (and nothing else). I found the crypto challenges in many of the CTFs this year are worth-trying, and these are no exceptions. I ended up at the 9th place. By the way, @SuperGuesser is the first to solve for all of the crypto challenges. Can we nerf @RBTree_ and @rkm0959?

We are given log.txt that contains the full commit history of the flag-containing repository. We are also given the content of the only file, namely flag.txt, for the first three commits. Also, three characters are filled in each subsequent commit. The objective is to recover the flag.

I was playing Union CTF with @blackb6a last week. Since this CTF is prepared by cr0wn where Jack and hyperreality were in (they made CryptoHack), I am convinced that the CTF will be fun. Turns out I wasn’t disappointed at all. I will be writing three or four posts on the challenges, and the first of the series covers on exah, the reversing challenge I spent most of my time (yet not solving it).

On 13 February, 2021, kurenaif has released five challenges celebrating the 1000-subscription in the YouTube channel. Being locked at home in Lunar New Year, I have nothing else to do. Therefore I decided to attempt those challenges, which is available on GitHub.

DiceCTF 2021 had an assortment of challenges and most of them are tough. Although the five crypto challenges which looked unoriginal, I was only able to solve garbled during the contest and I liked the challenges very much. After the CTF, I have found writeups from various sources and learnt a lot. In this writeup, only plagiarism and benaloh will be covered. However, I haven’t look at the remaining questions yet, and they are definitely on my to-do list.

I was playing justCTF 2020 with the new CTFers in Yakitori (Firebird). Oracles is a fun cryptography challenge that I solved during the game, and I thought this is worth compiling the write-up.

This is a beginning of a new series of blog posts that I review CTF challenges those are not publicly accessible. Generally I will spend some time to solve the challenge while discussing with the author, and give some feedback of the challenge. To kick-start, let’s look at hoifanrd from Firebird CTF team has shared a challenge he created some days ago.

I have written four question for Firebird Internal CTF - Oofbleck (Crypto), Obvious Transfer (Crypto), RC4 (Misc) and Prooof-ooof-Wooork (Reverse, Misc). I will be including all of them in this blog post.

TetCTF is the first CTF I have played in 2021. I recalled from last year that they have cool challenges. This year, there are three crypto challenges. In particular, unevaluated is the hardest among them. Although I did not solve them, I dug into rabbit holes and had a lot of struggle, uh, fun.

I was teamed up to play hxp CTF as @blackb6a last week. The hxp team had come up with a collection of hard challenges. In particular, there are two series of crypto challenges with a total of five parts. I will be writing on the hyper challenge and some follow-up and unanswered questions regarding to hyperelliptic curves.

Dragon CTF 2020 is definitely had my best CTF moments. There are big brain moments and I have been mind-blown for multiple times during the game. This time we have teamed up with @blackb6a. I have solved all the crypto challenges and two challenges with my teammates.

Rotten Secured Algorithm is the only challenge that I have written for Firebird Intro CTF. Two people (out of 166) ended up solving the challenge. There is a Python script attached in the challenge, as well as the output file. Let’s see what’s going on in the Python…

This week, we have teamed up as @blackb6a to play CONFidence 2020 CTF. We end up ranked 15, but we are more proud of ourselves able to solve a reversing challenge called Team Trees (395 points, 5 solves).

In particular, we are the first-to-solve to the challenge. It took us around two hours to win the flag. This writeup is written by @harrier_lcc and @mystiz613.

I was teamed-up with @blackb6a on Google CTF this time. I have solved 7 challenges alone and 3 challenges with my teammates.

In particular, Oracle is a crypto challenge with 13 solves. It has got me spending 12 hours. All in all, it was a great experience in terms of learning, but my liver hurts. This piece of writeup may be very computation intensive, just because I would like to make everything clear.

nookcrypt is a netcat service that have no source code released. There are two functions exposed:

1. Gets an encrypted copy of the flag (and the message hello world).
2. Encrypts an arbitrary message.

I was teamed up with @shellphish this time for De1CTF. During the game, I have solved 5 (out of 7) cryptography challenges individually, and a reverse challenge and a misc challenge in cooperation with DuSu and fs0c.

I said that I’ll commit the solution scripts to my Github after I have managed to prettify them. It never happened. I will not make the same promise again.

A Plaid Puzzle is a RPG game written in PuzzleScript. In the game, you are able to control as the player and you could move around and mess with a string. The game is available online on PuzzleScript.net.

It has been a very long time that I’ve compiled a writeup. This time I have played on my own as @blackb6a. Let me write on some particular interesting ideas that I have learnt in the challenges. Bear with me if you find this writeup too math-intensive.

The solution scripts will be committed to my Github repository sooner or later, after I’ve managed to prettify them.