# Writeup for beginners - BoF Vulnerability Lab (Syracuse University)

Visitors sometimes feel bored with our web blog because of too many boring stuffs which not often appear in their casual work/study. I just want to post such a simple tutorial for beginners and if you are experienced in CTF's pwn then just skip it. Enjoy!

# Basic concepts of Chinese Remainder Theorem with respect of RSA/AES

Chinese Remainder Theorem
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Suppose are positive integers and coprime in pair. For any sequence of integers , there exists an integer x solving the following system of congruence equations:

There exists an unique modulo solution of the system of simultaneous congruences above:

in which:

# Introduction to Threshold signature scheme

THRESHOLD SIGNATURE SCHEME

1. Introduction

Assuming there are 20 employees in a company, and if each employee has his or her own copy of the secret key then it is hard to assure on individuals due to compromise and machine break down. In the other hand, if there is a valid signature requires all 20 employees’ signature in the company then it will be very secure but not be easy to use. Therefore we can implement a scheme which requires only sign 5 or more out of 20 employees then it will be valid and that is exactly what a (5,20) threshold signature scheme tries to achieve. In addition, if a threat agent wants to compromise the system and obtain a message, he must compromise at least 5 people in the scheme and that is a harder thing to do compared to a traditional public scheme.

# Deciphering Ceasar basic concept

Deciphering

 Ciphertext: “VaqrprzoreoeratraWhyvhfraJnygreUbyynaqreqrgjrroebrefinaRqvguZnetbganne NzfgreqnzNaarjvytenntzrrxbzraznnezbrgabtrrarragvwqwrovwbzn oyvwiraBznmnyurgzbrvyvwxuroorabzNaarabtrracnnejrxraqnnegrubhqrafpuev wsgRqvguSenaxvarraoevrsnnaTregehqAnhznaauhaiebrtrerohhezrvfwrvaSenaxs hegnzZnva”

The given ciphertext has only letters without space, punctuation or separated key, there are two classic cipher systems such as substitution cipher and transposition cipher which are known to be easy to attack by using frequency analysis or bruteforce techniques. Continue reading Deciphering Ceasar basic concept