Crypto on Bitsdeep

FCSC 2024 - Write-Ups for the crypto challenges

Tue, 10 Sep 2024 00:00:00 +0000

In this post, I’ll present my write-ups for all the challenges listed in the crypto category, ordered by difficulty rating.

The challenges are:

AdveRSArial Crypto (Baby)
El Gamal Fait 1/2
El Gamal Fait 2/2
The pake is a lie
AdveRSArial Crypto (Kiddo)
Broadcastopol
Salade de fruits
Share a Saucisse
Winternitz is coming
Appellation d’Origine Protégée
Secret SHenanigans
Tight Schedule

Breaking Python's PRNG with a few values and no bruteforce

Mon, 22 Apr 2024 00:00:00 +0000

In this project, our focus will be on Python’s random implementation. We’ll demonstrate that even with a small number of outputs (as few as 6 for a 32-bit seed), it’s feasible to deduce Python’s original seed.

KalmarCTF 2024 - One key to rule them all

Mon, 18 Mar 2024 23:04:35 +0000

Encryption is easy, but key management is hard - I got the solution! Just use one key everywhere, i.e. nothing to manage (just don’t lose it)

FCSC 2022 - Write-Ups for some of the crypto challenges

Thu, 19 May 2022 19:34:04 +0000

In this post, I’ll present my write-ups for some of the challenges listed in the crypto category, in no particular order.

The challenges are:

Shuffled
My Tailor is Rich
T-Rex
Gaston La Paffe
Surface
Hash-ish
Kahl Hash
Share It
IZNOGOOD
Millenium

I didn’t manage to solve two of them, but I will briefly give my thoughts on them.

FCSC 2021 - Write-Ups for the crypto challenges

Mon, 03 May 2021 16:00:49 +0000

In this post, I’ll present my write-ups for all the challenges listed in the Crypto category, in the order I solved them during the competition.

The challenges are:

Macaque
RSA Destroyer
Lost curve
Hashy Parmentier
Revaulting
SmeaLog
Trappy Skippy

Analysis of the ROCA vulnerability

Mon, 04 May 2020 11:20:00 +0000

In this post we will see how the key generation process is done, why it is bad and how other people have been able to rediscover the attack before it was publicly disclosed. We will also see how the detection methods work and most importantly how to perform the whole attack. Hopefully at the end you will understand all the steps required to factor vulnerable keys and how the necessary optimizations of the attack where found. As a bonus, I’ll provide a fully functional multiprocess attack script in Sage.

Solving RE tasks the crypto way

Mon, 19 Aug 2019 08:45:14 +0000

In this post I want to share with you my way of solving reverse engineering (RE) tasks in CTFs involving simple cryptography without reading a single bit of assembly, purely by cryptanalysis.

Of course this can’t be applied on all RE tasks you’ll encounter that make use of cryptography, but sometimes it might save you a lot of pain trying to solve a difficult RE task. You might also have to do it simply because you don’t have access to the cryptographic code (Black Box), like in one of the parts of the Black Badge challenge for Le Hack 2019, which will serve as an example throughout this post.

Attacking RSA for fun and CTF points - part 4

Thu, 27 Jun 2019 08:19:05 +0000

It’s been a long time since part 3 of this series. So to quietly resume our journey in the beautiful world of mathematics I propose you 4 rather simple topics :

Multi-prime RSA
Brute force attack on small secret CRT-Exponents
Fault attack on signatures
Twin primes

Analysing the worst ransomware - part 2

Wed, 26 Sep 2018 11:10:52 +0000

In the last part we have seen that Tupper encrypts “.txt” files using RSA and “.pdf” files using a xor and custom key schedule. In this part we will write decryption scripts to recover encrypted files because Tupper doesn’t provide any decryption function.

Attacking RSA for fun and CTF points - part 3

Sun, 15 Jul 2018 17:24:33 +0000

Today the focus is on oracles ! You already encountered the decipher oracle in part 1 but now you’ll meet :

The LSB oracle
The padding oracle

Attacking RSA for fun and CTF points - part 2

Fri, 25 May 2018 17:37:59 +0000

Before diving right into more advanced attacks, let’s take a minute to do a quick recap because it’s been a long time since the last part. Once your mind is warmed up you can safely move on.

On the program today you have :

Small public exponent
Hastad broadcast attack
Fermat’s attack
Wiener’s attack

Attacking RSA for fun and CTF points - part 1

Fri, 13 Apr 2018 14:00:31 +0000

RSA is my favorite cryptosystem. It’s simple and powerful.

In this series I will try to go through every attacks (that I’m aware of) against RSA which are useful for solving CTF tasks. I’m not going to give you scripts that will do all the work for you but rather explain how the attacks work. The aim of this series is to understand the attacks you use and which one is most appropriate depending on the task. I will try to be beginner friendly and repeat myself in the beginning but afterwards I will assume that the reader has learnt the concepts.

Our first attacks will be :

Common modulus
Decipher oracle

EasyCTF IV - RSA V

Wed, 21 Feb 2018 14:20:07 +0000

Bob is extremely paranoid, so he decided that just one RSA encryption is not enough. Before sending his message to Alice, he forced her to create 5 public keys so he could encrypt his message 5 times! Show him that he still is not secure…

EasyCTF IV - Not OTP

Wed, 21 Feb 2018 13:23:12 +0000

It seems we’ve intercepted 2 strings that were both encrypted with what looks like OTP! Is it possible to decrypt them?

Hackim18 - Web1

Tue, 13 Feb 2018 22:01:20 +0000

The objective of this challenge is to find a way to get logged in as the user “administrator”.

This is the demo application for our new session management system based on military cryptography (namely CBC-MAC). You can log in as any user (except ‘administrator’) with the password ‘Password1’. We blocked access to ‘administrator’ account for security reasons.

Hackim18-What did he said ?

Tue, 13 Feb 2018 21:10:23 +0000

By his grace, we have been successfully organizing nullcon, year on year and this is the Nullcon9. At this juncture we cannot forget what did he had said, the one panacea given by him…

In 2010 when we were all worried, how can we be successful with this first ever event…