Posts

# Attacking RSA for fun and CTF points – part 3

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

1. The LSB oracle
Posts

# Attacking RSA for fun and CTF points – part 2

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 :

1. Small public exponent
3. Fermat’s attack
4. Wiener’s attack

Spoiler: There will be Maths 😉

Posts

# Attacking RSA for fun and CTF points – part 1

## Introduction

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. Continue reading

Posts

# EasyCTF IV – Recap

## Description

EasyCTF is one of the largest student-run high school cybersecurity events.
It took place on Fri. 10/02/2018 until Sun. 20/02/2018. 1790 teams competed in the event. Continue reading

Write-Ups

# EasyCTF IV – Not OTP

## Description

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

```c1 = 38445d4e5311544249005351535f005d5d0c575b5e4f481155504e495740145f4c505c5c0e196044454817564d4e12515a5f4f12465c4a45431245430050154b4d4d415c560c4f54144440415f595845494c125953575513454e11525e484550424941595b5a4b
c2 = 3343464b415550424b415551454b00405b4553135e5f00455f540c535750464954154a5852505a4b00455f5458004b5f430c575b58550c4e5444545e0056405d5f53101055404155145d5f0053565f59524c54574f46416c5854416e525e11506f485206554e51```
Write-Ups

# EasyCTF IV – Soupstitution

## Description

We had a flag, but lost it in a mess of alphabet soup! Can you help us find it?

Connect to the server via `nc c1.easyctf.com 12484`. Continue reading

Write-Ups

# EasyCTF IV – RSA V

## Description

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…

```Here are the 5 public keys that Bob used, each in the format of (N, e):
(9247606623523847772698953161616455664821867183571218056970099751301682205123115716089486799837447397925308887976775994817175994945760278197527909621793469, 11)
(9247606623523847772698953161616455664821867183571218056970099751301682205123115716089486799837447397925308887976775994817175994945760278197527909621793469, 41)
(9247606623523847772698953161616455664821867183571218056970099751301682205123115716089486799837447397925308887976775994817175994945760278197527909621793469, 67623079903)
(9247606623523847772698953161616455664821867183571218056970099751301682205123115716089486799837447397925308887976775994817175994945760278197527909621793469, 5161910578063)
(9247606623523847772698953161616455664821867183571218056970099751301682205123115716089486799837447397925308887976775994817175994945760278197527909621793469, 175238643578591220695210061216092361657427152135258210375005373467710731238260448371371798471959129039441888531548193154205671)

Here is his encrypted message:
7117565509436551004326380884878672285722722211683863300406979545670706419248965442464045826652880670654603049188012705474321735863639519103720255725251120```
Posts

# Nullcom Hackim 2018 – Recap

## Description

The Hackim 2018 CTF took place on Fri. 09/02/2018, 23:00 until Sun. 11/02/2018, 11:00. Over 1800 people took part in the event but only the top 40 were eligible to win a free conference pass for the Nullcom Goa 2018. Continue reading

Write-Ups

# Hackim18-What did he said ?

## Description

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… Continue reading