INTRODUCTION

This tutorial is part of the SecurityTube Linux Assembly Expert certification.

The goal of this assignment is to encrypt a given Linux x86 shellcode using a custom crypter.

The tutorial will contain example source with comments. Listed source code may have formatting issues so best place to obtain copies is from the project’s Github repo. This assignment will build from the previous assignments and so will not be reiterating explanations already covered there.

Methodology

1. Write encryption and decryption algorithm using high level language
2. Test encryption algorithm on shellcode
3. Verify decryption algorithm on encrypted shellcode

Background

For this assignment I shall implement a simple XOR algorithm to encrypt my shellcode.

Wikipedia quote https://en.wikipedia.org/wiki/XOR_cipher

“The XOR operator is extremely common as a component in more complex ciphers. By itself, using a constant repeating key, a simple XOR cipher can trivially be broken using frequency analysis. If the content of any message can…