The crypto challenges in DownUnderCTF are very intriguing. Unfortunately I have very limited time during the contest and could only solve apbq rsa ii (26 solves), hhhhh (14 solves) and handshake (4 solves). I will be covering the latter two challenges in this writeup.

After four years of team formation, we organized our first edition of Bauhinia CTF last weekend. There are over 650 teams registered and over 200 teams scored. This year, I made one crypto challenge, How to Stop Time. I also coauthored a web/crypto challenge, Amnesia, with @chthollic_.

This post is served as a short writeup of How to Stop Time, a crypto challenge with six solves during the contest period.

MHK2 is one of the challenges I coauthored for Google CTF 2023 and there were 18 teams solving this challenge during the contest time.

idekCTF 2022* definitely has a fun and inspiring set of cryptography challenges. I played with @blackb6a and we solved 8 of the 10 crypto challenges.

In this blog post, I’ll include my solution on three of them: Primonumerophobia (10 solves), Chronophobia (5 solves) and Decidophobia (3 solves).

Like in 2021 and 2022, I contributed some challenges for Firebird’s internal CTF, which are from the Hong Kong University of Science and Technology. This time I wrote three crypto challenges: Randomsum, Shelter and Threerider.

There were 24 teams participating. There were three solves for Randomsum, while Shelter and Threerider were unsolved during the CTF.

We will continue walking through the remaining crypto challenges I wrote for HKCERT CTF 2022: Mystiz can’t code, Slow keystream and King of Rock, Paper, Scissors.

This is the third year Black Bauhinia co-organized HKCERT CTF. This time I wrote nine challenges: Seven crypto, one reverse and one misc.

Similar to the last year, I have a series of three blog posts walking through the challenges that I wrote. We will discuss the four easier crypto challenges: Flawed ElGamal, Catch-22, Rogue Secret Assistant and Base64 encryption.

HACKING GOOGLE is a documentary of Google’s cybersecurity teams and H4CK1NG G00GL3 is it’s CTF counterpart. Project Zero Adventure is a cryptography challenge I wrote.

In the game, the players control the Security Princess to dodge the obstacles and catch the bugs (a variant of Google Chrome’s dinosaur game). After that, the server will sign messages consisting of the players' name and the score via the /sign API. The players will then submit it to the /highscore API. If the score submitted to the highscore API is negative, they will be given the flag.

However, there is one catch: The server is only willing to sign the results with non-negative scores.

vss is an interesting crypto challenge in BalsnCTF, which ended up having 9 solves. I took around 2.5 hours to solve the challenge. This challenge reminds me the yet another PRNG challenge from pbctf 2021, but with a setting which looked harder. I was pretty surprised that LLL worked, too.

lfsr is another crypto challenge in BalsnCTF with 6 solves. In the challenge, the output bits are computed nonlinearly from the LFSR states. Given that I knew almost nothing about LFSR, I just came up with the attack by myself… Well, I am not quite a paper guy and I couldn’t read.

This is the third year I had a writeup on Google CTF (see my writeup in 2020 and 2021). Yet this time it is the official writeup for a challenge - as the challenge author! There are eventually 35 solvers (out of 382 teams) for the challenge.

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.

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.

@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).

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.

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 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.

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 $b \in \{128, 192, 256\}$).

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?

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.

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…

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.

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.