Simple Payload Encryption

This project demonstrates the use of the tiny-AES-C AES implementation to encrypt PE payloads.

AI had nothing to do with this project, everything is entirely human (me) made.

Table of Contents

• Overview
  • Project Structure
  • Code Structure
  • About the Key and IV
• Usage
• Padding
  • PKCS#7 Overview
  • Implementation
• Logging Format
• License

Overview

The aim of this project is to show in a simple way how to use tiny-AES-C so that anyone can use it for their own (friendly) purposes.

I am more than aware that my implementation is rather simple, and that it might lack more advanced techniques, but that was the purpose of this mini-project: to learn how to do it, and build from there.

You will also see some overkill stuff, like the ReadStdinAndFlush and GetInputData funcs, or the error handling... but as said, the purpose of this was purely educational, so why not learn?

The target OS is Windows. If you wish to add a Linux implementation, feel free :)

Project Structure

The repo is structured as follows:

.
├──include
│   └── aes.h   # Bundled with it's corresponding .c file
├──src
│   ├── main.c  # Contains all logic
│   └── aes.c   # tiny-AES-C implementation
├── .gitignore
├── LICENSE
├── Makefile    # Builds the example program
└── README.md   # This file ;)

Code Structure

The main.c file contains all the code and logic, and is organized into "sections" marked by // ======= <Section> ===... dividers.

There are 4 sections:

• Helpers
• Cipher: usage of the tiny-AES-C implementation
• Data Handling: various utils to get input data: size of plaintext and plaintext
• Entrypoint

About the Key and IV

I know that hardcoding the key and IV in the code is a no-no, as it can be easily retrieved by analyzing the (reversed) code. Will update that part once I learn a better way to do it :)

Usage

First, make sure the following requirements are installed or available in your Windows machine:

1. gcc compiler, see MSYS2 for install instructions
2. Make tool
3. Windows SDK

To build the code, simply run:

make

The spe.exe will be placed on the root directory. Run it like so:

./spe.exe

Padding

Padding means adding data to a message prior to encryption so that the plaintext meets specifics requirements set by the cipher.

On AES, input data must have a length that is multiple of the block size (16 bytes). If the plaintext doesn't follow this requirement, padding must be added to adjust its length.

The tiny-AES-C implementation doesn't add padding by default, so we need to implement it ourselves.

PKCS#7 Overview

We'll use the PKCS#7 algorithm, a simple yet elegant solution. How it works?

• If N is the number of bytes in a block and M bytes (N < M) are missing from the last block, it adds the character 0xM (hexadecimal) M times at the end.
• Important: PKCS#7 always add padding. If the plaintext is already multiple of 16 bytes, it adds the 0x10 character (16 in hex), 16 times.

Implementation

The PadBuffer and UnpadBuffer funcs implements the algorithm.

Logging Format

A set of symbols is used to indicate status:

• [*] info or progress
• [+] success
• [-] error
• [!] warning or smth unexpected
• [#] user input required

License

This project is licensed under the MIT License. See the LICENSE file for details.