Tutorial A7 Glitch Buffer Attacks

This tutorial discusses a specific type of glitch attack. It shows how a simple printing loop can be abused, causing a target to print some otherwise private information. This attack will be used to recover a plaintext without any knowledge of the encryption scheme being used.

Background

This section introduces the attack concept by showing some real world examples of vulnerable firmware. Then, it describes the victim firmware that will be used in this tutorial.

Real Firmware

Typically, one of the slowest parts of an embedded system is its communication lines. It's pretty common to see a processor running in the MHz range with a serial connection of 96k baud. To make these two different speeds work together, embedded firmware usually fills up a buffer with data and lets a serial driver print on its own time. This setup means we can expect to see code like

for(int i = 0; i < number_of_bytes_to_print; i++)
{
    print_one_byte_to_serial(buffer[i]);
}

This is a pretty vulnerable piece of C. Imagine that we could sneak into the source code and change it to

for(int i = 0; i < really_big_number; i++)
{
    print_one_byte_to_serial(buffer[i]);
}

C compilers don't care that buffer[] has a limited size - this loop will happily print every byte it comes across, which could include other variables, registers, and even source code. Although we probably don't have a good way of changing the source code on the fly, we do have glitches: a well-timed clock or power glitch could let us skip the i < number_of_bytes_to_print check, which would have the same result.

How could this be applied? Imagine that we have an encrypted firmware image that we're going to transmit to a bootloader. A typical communication process might look like:

1. We send the encrypted image ciphertexts over a serial connection
2. The bootloader decrypts the ciphertexts and stores the result somewhere in memory
3. The bootloader sends back a response over the serial port

We have a pretty straightforward attack for this type of bootloader. During the last step, we'll apply a glitch at precisely the right time, causing the bootloader to print all kinds of things to the serial connection. With some luck, we'll be able to find the decrypted plaintext somewhere in this memory dump.

Bootloader Setup

For this tutorial, a very simple bootloader using the SimpleSerial protocol has been set up. The source for this bootloader can be found in chipwhisperer/hardware/victims/firmware/bootloader-glitch. The following commands are used:

• pABCD\n: Send an encrypted ciphertext to the bootloader. For example, this message is made up of the two bytes AB and CD.
• r0\n: The reply from the bootloader. Acknowledges that a message was received. No other responses are used.
• x: Clear the bootloader's received buffer.
• k: See x.

The bootloader uses triple-ROT-13 encryption to encrypt/decrypt the messages. To help you send messages to the target, the script private/encrypt.py prints the SimpleSerial command for a given fixed string. For example, the ciphertext for the string Don't forget to buy milk! is

p516261276720736265747267206762206f686c207a76797821\n

The Attack Plan

The Sensitive Code

Disassembly

Attack Script & Results

Ideas

• Change hex file to use BRLT
• Use volatile loop variables

Tutorials

See Tutorial Map for important information & suggested order of completion.

Theory

Correlation Power Analysis

Template Attacks

Extending AES-128 Attacks to AES-256

Basic Tutorials

Tutorial B1 Building a SimpleSerial Project

Tutorial B2 Viewing Instruction Power Differences

Tutorial B3-1 Timing Analysis with Power for Password Bypass

Tutorial B5 Breaking AES (Straightforward)

Tutorial B6 Breaking AES (Manual CPA Attack)

Tutorial B7 Profiling Attacks (with HW Assumption)

Tutorial B11 Breaking RSA

Advanced Tutorials

Tutorial A2 Introduction to Glitch Attacks (including Glitch Explorer)

Tutorial A3 VCC Glitch Attacks

Tutorial A5 Breaking AES-256 Bootloader

Tutorial A5-Bonus Breaking AES-256 Bootloader

Tutorial A7 Glitch Buffer Attacks

Tutorial A8 32bit AES

Hardware-Specific Tutorials

Tutorial P1 Stream Mode and Advanced Triggering (CW Pro)

Tutorial CW305-1 Building a Project

Tutorial CW305-2 Breaking AES on FPGA

Tutorial CW305-3 Clock Glitching

Tutorial CW305-4 Voltage Glitching with Crowbars

Archived Tutorials (v4/v3 only)

Tutorial B3-2 Timing Analysis with Power for Attacking TSB

Tutorial B4 Testing Two Partitions of Traces

Tutorial B5-2 Breaking DES (Straightforward)

Tutorial B8 Profiling Attacks (Manual Template Attack)

Tutorial B9 The Preprocessing Modules

Tutorial B10 Using with SASEBO-W CardOS or DPA Contest v4.2

Tutorial A1 Synchronization to Communication Lines

Tutorial A4 SAD Trigger for SCA and Glitch

Tutorial A6 Replication of Ilya Kizhvatov's XMEGA® Attack

Tutorial A9 Bypassing LPC1114 Read Protect