% Introduction to Reverse Engineering
% Stefan Friese
% 02 November, 2023

---

# Topics

* Effective Reverse Engineering
* Reversing with Ghidra

---

## How Do You Reverse

Reverse Engineering demands a lot of knowledge in multiple fields.  

**Some topics are**

* Assembly Language
* ANSI C
* Other Languages
* Syscalls
* Cryptography

---

How do you reverse engineer without knowing little about these topics?

---

## Reversing is Work

Work is a product of power by time.  
`P` is your power to solve an issue.

`W = P x t`

The smarter you tackle work, the less time you need to solve an issue.

---

## Knowledge is a Map

You conventiently drive around the city using the underground.
That's how you get to know the main spots of the city.

<img src="./images/london_underground.jpg" alt="London Underground" width="50%" height="auto">

---

## Knowledge is a Map

Invest some time and explore deeper on foot.
That's how you get to know the back alleys.

<img src="./images/london_by_foot.jpg" alt="London by Foot" width="50%" height="auto">

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# Ghidra -- an Overview

---

![Main View of Ghidra](./images/Ghidra-Overview.png)

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## Watch Out for Low Hanging Fruits

---

* Data Segment
* Names of Functions
* Conditions & Comparisons
* Strings: Usernames, Passwords
* URLs, IP & Port Numbers

**Do not try to understand the whole code at once, it will only drive you mad.**

---

### Data Segments

<img src="./images/data-segments.png" alt="A look into the read only data segment" width="70%" height="auto">

A look into the read only data segment

---

### Name of Functions

![Functions contained in the binary a.k.a. Symbol Tree](./images/symbol-tree.png)

---

### Conditions & Comparisions

<img src="./images/decompiled-code.png" alt="Input is Compared to a Hard Coded String" width="50%" height="auto">

Input is compared to a hard coded string

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### Function Graph

<img src="./images/function-graph.png" alt="Take a Look at the Flow Graph of Functions" width="50%" height="auto">

Take a look at the flow graph of functions

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### Strings

<img src="./images/defined-strings-menu.png" alt="Open the Defined Strings Menu" width="50%" height="auto">

Strings can not only be located in data but also in other code segments, sometimes obfuscated

---

### Strings

![An old friend](./images/defined-strings.png)

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### Binary Patching

Bypass any undesireable condition via a `NOP` instruction.

<img src="./images/nop.jpg" alt="NOP, export your patched binary" width="30%" height="auto">

NOP, export your patched binary

---

### Do It Yourselves!

* [Download Ghidra](https://ghidra-sre.org/)
* [Download binaries at crackmes.one](https://crackmes.one)
* [Find more binaries on hackthebox](https://hackthebox.eu)
* [Or Find even more on tryhackme](https://tryhackme.com)
* Download firmware of your favorite IoT appliances

---

## A Word On Binary Obfuscation

Software Obfuscation was born in 1984 at the [International Obfuscated C Code Contest](https://ioccc.org/)

<img src="./images/donut.gif" alt="A donut as code compiles to a spinning donut by Jim Hague" width="50%" height="auto">

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### What Exactly might be Obfuscated in Your Code?

<section>
[Layered obfuscation: a taxonomy of software obfuscation techniques for layered security by Hui Xu et. al](https://cybersecurity.springeropen.com/track/pdf/10.1186/s42400-020-00049-3.pdf)
</section>

<section>

* Code Element Layers
    * Layout
    * Controls
    * Data
    * Methods
    * Classes

</section>

<section>
* Component
    * Library Calls
    * Used Resources

* Application Layer
    * DRM System
    * Neural Networks

</section>

---

## Techniques of Obfuscation

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### Splitting & Merging of Strings

```sh
a = "BABE"
b = "CAFFEE"
f"{b}{a}"
```

[String Deobfuscation with FLOSS](https://github.com/mandiant/flare-floss/)

---

## Packing 

Compress binary data

```sh
            ooooo     ooo  ooooooooo.  ooooooo  ooooo
            `888'     `8'  `888   `Y88. `8888    d8'
             888       8    888   .d88'   Y888..8P
             888       8    888ooo88P'     `8888'
             888       8    888           .8PY888.
             `88.    .8'    888          d8'  `888b
               `YbodP'     o888o       o888o  o88888o
```

[UPX Packer/Unpacker](https://upx.github.io/)

---

## Mangling

Library symbols in compiled code for data that have the same name
<section>
<pre><code data-trim data-noescape>
c++filt
_ZNSt7__cxx1114collate_bynameIcEC2ERKNS_12basic_stringIcSt11char_traitsIcESaIcEEEm
std::__cxx11::collate_byname<char>::collate_byname(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, unsigned long)
</code></pre></section>

<section><a href="https://demangler.io/" alt="demangler">Online Demangler</a></section>

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## Code Elements

* Adding Unnecessary Instructions
* Changing Control Flows
* Protecting Data

<img src="./images/spaghetti.jpg" alt="Convoluted Code" width="26%" height="auto%">

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### Deobfuscation Tools


* DotNet
    * [de4dot Deobfuscator and Unpacker](https://github.com/de4dot/de4dot)
    * [dnSpy Debugger and Assembly Editor](https://github.com/dnSpy/dnSpy)
    * [ILSpy Decompiler instead of Ghidra](https://github.com/icsharpcode/ILSpy)

---

# The End