ATmega644 Smart-Card running AES decryption

Table of Contents

• Introduction
• Project Overview
• Build Configurations
  • Prerequisites
  • Building the Project
  • Debug Mode
  • Countermeasures
• Credits

Introduction

This repository contains an 128-bit AES decryption algorithm, that runs on a Smart-Card with an AVR ATmega644 microcontroller on it. The code was implemented in the scope of a "Smart-Card Laboratory" at the Technical University of Munich, with the purpose to decrypt chunks of a video stream while communicating with a Smart-Card reader or Terminal. The communication between the Smart-Card an the Terminal occurs over the Smart-Card's ISO7816 I/O contact and follows the T=0 protocol specified in ISO7816. This markdown page provides some information on build configurations & gives an overview of the code.

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Project Overview

The code for the clone consists of these main classes:

• The Communication class manages the T=0 protocol to communicate with the Terminal.
• The AES class contains all the functionality required for the 128-bit AES decryption running on the processor.
• The AESMath class contains some math helper functions for the decryption.
• The Hiding class implements countermeasures Shuffling & Dummy-Ops.
• The Masking class implements the Masking countermeasure.
• The RNG class implements a small, lightweight Random-Number-Generator.
• The Logger class can be used to log message to a serial console over USART & USB. Note that this functionality is only available in debug mode.

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Build Configurations

Prerequisites

• This project uses CMake and requires Version 3.5 or higher.
• If you want to flash the executable onto an ATmega644 yourself, make sure to install avrdude (e.g. on Debian: sudo apt-get install avrdude)

Building the Project

The repository is built using CMake. To build the default .hex file that can be flashed on the ATmega644 using an AVR programmer, do the following:

• Create a build directory: $ mkdir build/
• Change into the directory: $ cd build/
• Run CMake: $ cmake ..
• If you only want to compile the project, run: $ make
• If you also want to flash it onto an ATmega644, run: $ make flash

There are a couple of CMake options that can be turned on/off. To see a complete list of them, run: $ cmake -L .. from you build folder.

Debug Mode

The Smart-Card used in this laboratory came with an USART to USB converter chip. This makes it possible to log messages to a serial console on a PC over the ATmega644's USART peripheral. If you are compiling this code for your own Smart-Card, you might run into issues with USART. To enable logging of debug messages, the project can be compiled in debug mode. Note that this greatly increases the executable size & that some UART message might interfere with the Terminal communication. If these message are unwanted, the project can also be compiled in release mode.

• Run $ cmake -DDebug=ON .. to build the project in debug mode.
• Run $ cmake -DDebug=OFF .. to build the project in release mode.
• The default value is OFF.

Countermeasures

The AES implementation contains a number of DPA countermeasures, including Masking, Shuffling the S-Box access & inserting Dummy NOPs. You can enable/disable these countermeasures with CMake flags:

• Masking: When masking is enabled, the current 16-byte AES state & all round-keys, will be masked with 6 randomly generated masks. These masks will be applied once before the first round, re-applied after every inverse MixColumns operation & removed after the last round. To enable/disable masking, do the following:
  • Run $ cmake -DMasking=ON .. to enable masking.
  • Run $ cmake -DMasking=OFF .. to disable masking.
  • The default value is OFF.
• Shuffling: Another countermeasure that was added is the shuffling of S-Box accesses. When reading values from the S-Box look-up table, these accesses are not performed in a specific order, but a random. The goal of this is to randomize the chips power consumption during decryption. To enable/disable Shuffling, do the following:
  • Run $ cmake -DShuffling=ON to enable Shuffling.
  • Run $ cmake -DShuffling=OFF to disable Shuffling.
  • The default value is OFF.
• Dummy-Ops: The last countermeasure that was implemented are Dummy NOPs. These are inserted before every AES operation (AddRoundKey, InvMixCol, InvShiftRows, InvSubBytes) to also randomize the power consumption during decryption. To enable/disable Dummy-Ops do the following:
  • Run $ cmake -DDummyOps=ON to enable Dummy-Ops.
  • Run $ cmake -DDummyOps=OFF to disable Dummy-Ops.
  • The default value is OFF.

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Credits

The Doxygen custom CSS template used in this project can be found here.