Background

In the world of microcontrollers, a bootloader is a special piece of firmware that is made to let the user upload new programs into memory. This is especially useful for devices with complex code that may need to be patched or otherwise updated in the future - a bootloader makes it possible for the user to upload a patched version of the firmware onto the micro. The bootloader receives information from a communication line (a USB port, serial port, ethernet port, WiFi connection, etc...) and stores this data into program memory. Once the full firmware has been received, the micro can happily run its updated code.

There is one big security issue to worry about with bootloaders. A company may want to stop their customers from writing their own firmware and uploading it onto the micro. For example, this might be for protection reasons - hackers might be able to access parts of the device that weren't meant to be accessed. One way of stopping this is to add encryption. The company can add their own secret signature to the firmware code and encrypt it with a secret key. Then, the bootloader can decrypt the incoming firmware and confirm that the incoming firmware is correctly signed. Users will not know the secret key or the signature tied to the firmware, so they won't be able to "fake" their own.

This tutorial will work with a simple AES-256 bootloader. The victim will receive data through a serial connection, decrypt the command, and confirm that the included signature is correct. Then, it will only save the code into memory if the signature check succeeded. To make this system more robust against attacks, the bootloader will use cipher-block chaining (CBC mode). Our goal is to find the secret key and the CBC initialization vector so that we could successfully fake our own firmware.

       |<-------- Encrypted block (16 bytes) ---------->|
       |                                                |
+------+------+------+------+------+------+ .... +------+------+------+
| 0x00 |    Signature (4 Bytes)    |  Data (12 Bytes)   |   CRC-16    |
+------+------+------+------+------+------+ .... +------+------+------+

            +------+
CRC-OK:     | 0xA1 |
            +------+

            +------+
CRC Failed: | 0xA4 |
            +------+

Setting up the Hardware

This tutorial uses the CW1173 ChipWhisperer-Lite hardware. This hardware does not require any special setup - it should be ready to go out-of-the-box.

Note that you don't need hardware to complete the tutorial. Instead, you can download example traces from the ChipWhisperer Site. Just look for the traces titled AVR: AES256 Bootloader (ChipWhisperer Tutorial #A5).

Capturing the Traces

Once the hardware is ready, we can capture some traces for our attack using the ChipWhisperer Capture software. If you somehow got to the 5th Advanced Tutorial without getting this software ready, you can follow the helpful guide at Installing ChipWhisperer.

The first thing we need to do is add a new target to the ChipWhisperer system. (None of the existing ones know about the bootloader's data format, nor do they recognize the CRC responses that are sent back to us.) The code for this target is included in #Appendix A: Target Code. Copy/paste this into a Python file (call it whatever you want) and save it in a place where ChipWhisperer will look for it. There are two folders that you can use:

• Your computer should have a folder called chipwhisperer_projects - if you don't know where this is, the File > Preferences window will tell you. The system looks in the folder chipwhisperer_projects\chipwhisperer\capture\targets for new targets, so you can save your file here.
• Alternatively, all of the normal targets are stored in chipwhisperer\software\chipwhisperer\capture\targets, so you can also save the file here. Note that this may not be possible if you don't have access to these folders (ex: your account doesn't have admin access).

Next is the capture script. In some of the previous tutorials, we entered all of the capture settings by hand. Since we are civilized humans armed with technology, we can use a script to do all of this setup for us. A pre-written Python script is provided at #Appendix B: Capture Script. Take a look at this code and notice what it does:

• it fills in the scope, target, and trace format that we'll use;
• it connects to the hardware; and
• it loads all of the hardware parameters for us. Nice!

Copy this script into a .py file somewhere convenient. Then, perform the following steps to finish the capture:

1. Run the capture script, which will open a ChipWhisperer Capture window with everything connected for us.
2. Open the terminal (Tools > Terminal) and connect to the board. While the terminal is open, press the Capture 1 button. A single byte of data should appear in the terminal. This byte will either be a1 (CRC failed) or a4 (CRC OK). If you see any other responses, something is wrong.

   

3. Once you're happy with this, open the General Settings tab and set the Number of Traces. You should need around 100 traces to break AES.
4. Press the Capture Many button to record the 100 traces. You'll see the new traces plotted on-screen.
5. Once the program is finished capturing the traces, save the project. Put it somewhere memorable and give it a nice name.

Finding the Encryption Key

Now that we have our traces, we can go ahead and perform the attack. As described in the background theory, we'll have to do two attacks - one to get the 14th round key, and another (using the first result) to get the 13th round key. Then, we'll do some post-processing to finally get the 256 bit encryption key.

# Imports for AES256 Attack
from chipwhisperer.analyzer.attacks.models.base import ModelsBase
from chipwhisperer.analyzer.attacks.models.AES128_8bit import AESLeakageHelper, AES128_8bit

class AES256_Model(AESLeakageHelper):
    name = 'Our model'
    def leakage(self, pt, ct, guess, bnum):
        knownkey = <PUT YOUR 14TH ROUND KEY YOU RECOVERED HERE>
        #For example: knownkey = [0xea, 0x79, 0x79, 0x20, 0xc8, 0x71, 0x44, 0x7d, 0x46, 0x62, 0x5f, 0x51, 0x85, 0xc1, 0x3b, 0xcb]
        xored = [knownkey[i] ^ pt[i] for i in range(0, 16)]
        block = xored
        block = self.inv_shiftrows(block)
        block = self.inv_subbytes(block)
        block = self.inv_mixcolumns(block)
        block = self.inv_shiftrows(block)
        result = block
        return self.inv_sbox((result[bnum] ^ guess[bnum]))

>>> from chipwhisperer.analyzer.attacks.models.aes.funcs import shiftrows,mixcolumns
>>> knownkey = [0xC6, 0xBD, 0x4E, 0x50, 0xAB, 0xCA, 0x75, 0x77, 0x79, 0x87, 0x96, 0xCA, 0x1C, 0x7F, 0xC5, 0x82]
>>> key = shiftrows(knownkey)
>>> key = mixcolumns(key)
>>> print " ".join(["%02x" % i for i in key])
c6 6a a6 12 4a ba 4d 04 4a 22 03 54 5b 28 0e 63

c6 6a a6 12 4a ba 4d 04 4a 22 03 54 5b 28 0e 63 ea 79 79 20 c8 71 44 7d 46 62 5f 51 85 c1 3b cb

94 28 5d 4d 6d cf ec 08 d8 ac dd f6 be 25 a4 99 c4 d9 d0 1e c3 40 7e d7 d5 28 d4 09 e9 f0 88 a1

Next Steps

If you want to go further with this tutorial, Tutorial A5-Bonus Breaking AES-256 Bootloader continues working with the same firmware to find the remaining secrets in the bootloader (the IV and the signature).

Appendix A: Target Code

#!/usr/bin/python
# -*- coding: utf-8 -*-
#
# Copyright (c) 2013-2016, NewAE Technology Inc
# All rights reserved.
#
# Authors: Colin O'Flynn, Greg d'Eon
#
# Find this and more at newae.com - this file is part of the chipwhisperer
# project, http://www.assembla.com/spaces/chipwhisperer
#
#    This file is part of chipwhisperer.
#
#    chipwhisperer is free software: you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation, either version 3 of the License, or
#    (at your option) any later version.
#
#    chipwhisperer is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU Lesser General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with chipwhisperer.  If not, see <http://www.gnu.org/licenses/>.
#=================================================

import sys
import time
import chipwhisperer.capture.ui.CWCaptureGUI as cwc
from chipwhisperer.common.api.CWCoreAPI import CWCoreAPI
from chipwhisperer.capture.targets.SimpleSerial import SimpleSerial
from chipwhisperer.common.scripts.base import UserScriptBase
from chipwhisperer.capture.targets._base import TargetTemplate
from chipwhisperer.common.utils import pluginmanager
from chipwhisperer.capture.targets.simpleserial_readers.cwlite import SimpleSerial_ChipWhispererLite
from chipwhisperer.common.utils.parameter import setupSetParam

# Class Crc
#############################################################
# These CRC routines are copy-pasted from pycrc, which are:
# Copyright (c) 2006-2013 Thomas Pircher <tehpeh@gmx.net>
#
class Crc(object):
    """
    A base class for CRC routines.
    """

    def __init__(self, width, poly):
        """The Crc constructor.

        The parameters are as follows:
            width
            poly
            reflect_in
            xor_in
            reflect_out
            xor_out
        """
        self.Width = width
        self.Poly = poly


        self.MSB_Mask = 0x1 << (self.Width - 1)
        self.Mask = ((self.MSB_Mask - 1) << 1) | 1

        self.XorIn = 0x0000
        self.XorOut = 0x0000

        self.DirectInit = self.XorIn
        self.NonDirectInit = self.__get_nondirect_init(self.XorIn)
        if self.Width < 8:
            self.CrcShift = 8 - self.Width
        else:
            self.CrcShift = 0

    def __get_nondirect_init(self, init):
        """
        return the non-direct init if the direct algorithm has been selected.
        """
        crc = init
        for i in range(self.Width):
            bit = crc & 0x01
            if bit:
                crc ^= self.Poly
            crc >>= 1
            if bit:
                crc |= self.MSB_Mask
        return crc & self.Mask


    def bit_by_bit(self, in_data):
        """
        Classic simple and slow CRC implementation.  This function iterates bit
        by bit over the augmented input message and returns the calculated CRC
        value at the end.
        """
        # If the input data is a string, convert to bytes.
        if isinstance(in_data, str):
            in_data = [ord(c) for c in in_data]

        register = self.NonDirectInit
        for octet in in_data:
            for i in range(8):
                topbit = register & self.MSB_Mask
                register = ((register << 1) & self.Mask) | ((octet >> (7 - i)) & 0x01)
                if topbit:
                    register ^= self.Poly

        for i in range(self.Width):
            topbit = register & self.MSB_Mask
            register = ((register << 1) & self.Mask)
            if topbit:
                register ^= self.Poly

        return register ^ self.XorOut

        
class BootloaderTarget(TargetTemplate):
    _name = 'AES Bootloader'

    def __init__(self):
        TargetTemplate.__init__(self)

        ser_cons = pluginmanager.getPluginsInDictFromPackage("chipwhisperer.capture.targets.simpleserial_readers", True, False)
        self.ser = ser_cons[SimpleSerial_ChipWhispererLite._name]

        self.keylength = 16
        self.input = ""
        self.crc = Crc(width=16, poly=0x1021)
        self.setConnection(self.ser)

    def setKeyLen(self, klen):
        """ Set key length in BITS """
        self.keylength = klen / 8        
 
    def keyLen(self):
        """ Return key length in BYTES """
        return self.keylength

    def getConnection(self):
        return self.ser

    def setConnection(self, con):
        self.ser = con
        self.params.append(self.ser.getParams())
        self.ser.connectStatus.connect(self.connectStatus.emit)
        self.ser.selectionChanged()

    def con(self, scope=None):
        if not scope or not hasattr(scope, "qtadc"): Warning(
            "You need a scope with OpenADC connected to use this Target")

        self.ser.con(scope)
        # 'x' flushes everything & sets system back to idle
        self.ser.write("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
        self.ser.flush()
        self.connectStatus.setValue(True)

    def close(self):
        if self.ser != None:
            self.ser.close()
        return

    def init(self):
        pass

    def setModeEncrypt(self):
        return

    def setModeDecrypt(self):
        return

    def convertVarToString(self, var):
        if isinstance(var, str):
            return var

        sep = ""
        s = sep.join(["%c" % b for b in var])
        return s

    def loadEncryptionKey(self, key):
        pass

    def loadInput(self, inputtext):
        self.input = inputtext

    def readOutput(self):
        # No actual output
        return [0] * 16

    def isDone(self):
        return True

    def checkEncryptionKey(self, kin):
        return kin

    def go(self):
        # Starting byte is 0x00
        message = [0x00]

        # Append 16 bytes of data
        message.extend(self.input)

        # Append 2 bytes of CRC for input only (not including 0x00)
        crcdata = self.crc.bit_by_bit(self.input)

        message.append(crcdata >> 8)
        message.append(crcdata & 0xff)

        # Write message
        message = self.convertVarToString(message)
        for i in range(0, 5):
            self.ser.flush()
            self.ser.write(message)
            time.sleep(0.1)
            data = self.ser.read(1)

            if len(data) > 0:
                resp = ord(data[0])

                if resp == 0xA4:
                    # Encryption run OK
                    break

                if resp != 0xA1:
                    raise IOError("Bad Response %x" % resp)

        if len(data) > 0:
            if resp != 0xA4:
                raise IOError("Failed to communicate, last response: %x" % resp)
        else:
            raise IOError("Failed to communicate, no response")

Appendix B: Capture Script

Note you need to manually CONNECT to the CW-Lite & AES Bootloader target before running this. To do this:

1. Set the 'Scope Module' as 'ChipWhisperer/OpenADC'
2. Set the 'Target Module' as 'AES Bootloader' (you need to have that target on your system)

"""Setup script for CWLite/1200 with XMEGA (CW303/CW308-XMEGA/CWLite target)
specifically for Tutorial A5: the AES-256 bootloader attack
"""

try:
    scope = self.scope
except NameError:
    pass
    
scope.gain.gain = 45
scope.adc.samples = 11000
scope.adc.offset = 0
scope.adc.basic_mode = "rising_edge"
scope.clock.clkgen_freq = 7370000
scope.clock.adc_src = "clkgen_x4"
scope.trigger.triggers = "tio4"
scope.io.tio1 = "serial_rx"
scope.io.tio2 = "serial_tx"
scope.io.hs2 = "clkgen"

Appendix C: AES-256 14th Round Key Script

Full attack script, copy/paste into a file then add as active attack script:

# AES-256 14th Round Key Attack
from chipwhisperer.common.scripts.base import UserScriptBase
# Imports from Preprocessing
import chipwhisperer.analyzer.preprocessing as preprocessing
# Imports from Attack
from chipwhisperer.analyzer.attacks.cpa import CPA
from chipwhisperer.analyzer.attacks.cpa_algorithms.progressive import CPAProgressive
import chipwhisperer.analyzer.attacks.models.AES128_8bit
# Imports from utilList

class UserScript(UserScriptBase):
    name = "Auto-generated"
    description = "Auto-generated Attack Script"
    def __init__(self, api):
        UserScriptBase.__init__(self, api)
        self.initProject()
        self.initPreprocessing()
        self.initAnalysis()
        self.initReporting()

    def initProject(self):
        pass

    def initPreprocessing(self):
        self.traces = self.api.project().traceManager()

    def initAnalysis(self):
        self.attack = CPA()
        self.attack.setTraceSource(self.traces, blockSignal=True)
        self.attack.setAnalysisAlgorithm(CPAProgressive,chipwhisperer.analyzer.attacks.models.AES128_8bit.AES128_8bit,chipwhisperer.analyzer.attacks.models.AES128_8bit.AES128_8bit.LEAK_HW_INVSBOXOUT_FIRSTROUND)
        self.attack.setTraceStart(0)
        self.attack.setTracesPerAttack(200)
        self.attack.setIterations(1)
        self.attack.setReportingInterval(10)
        self.attack.setTargetSubkeys([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])
        self.attack.setPointRange((0,10991))

    def initReporting(self):
        # Configures the attack observers (usually a set of GUI widgets)
        self.api.getResults("Attack Settings").setAnalysisSource(self.attack)
        self.api.getResults("Correlation vs Traces in Attack").setAnalysisSource(self.attack)
        self.api.getResults("Output vs Point Plot").setAnalysisSource(self.attack)
        self.api.getResults("PGE vs Trace Plot").setAnalysisSource(self.attack)
        self.api.getResults("Results Table").setAnalysisSource(self.attack)
        self.api.getResults("Save to Files").setAnalysisSource(self.attack)
        self.api.getResults("Trace Output Plot").setTraceSource(self.traces)
        self.api.getResults("Trace Recorder").setTraceSource(self.traces)

    def run(self):
        self.attack.processTraces()

if __name__ == '__main__':
    from chipwhisperer.common.api.CWCoreAPI import CWCoreAPI
    import chipwhisperer.analyzer.ui.CWAnalyzerGUI as cwa
    from chipwhisperer.common.utils.parameter import Parameter
    app = cwa.makeApplication()     # Comment if you don't need the GUI
    Parameter.usePyQtGraph = True   # Comment if you don't need the GUI
    api = CWCoreAPI()               # Instantiate the API
    gui = cwa.CWAnalyzerGUI(api)    # Comment if you don't need the GUI
    gui.show()                      # Comment if you don't need the GUI
    api.runScriptClass(UserScript)  # Run UserScript through the API
    app.exec_()                     # Comment if you don't need the GUI

Appendix D: AES-256 13th Round Key Script

# AES-256 13th Round Key Script
from chipwhisperer.common.scripts.base import UserScriptBase
# Imports from Preprocessing
import chipwhisperer.analyzer.preprocessing as preprocessing
# Imports from Attack
from chipwhisperer.analyzer.attacks.cpa import CPA
from chipwhisperer.analyzer.attacks.cpa_algorithms.progressive import CPAProgressive
import chipwhisperer.analyzer.attacks.models.AES128_8bit
# Imports from utilList

# Imports for AES256 Attack
from chipwhisperer.analyzer.attacks.models.base import ModelsBase
from chipwhisperer.analyzer.attacks.models.AES128_8bit import AESLeakageHelper, AES128_8bit

class AES256_Model(AESLeakageHelper):
    name = 'Our model'
    def leakage(self, pt, ct, guess, bnum):
        knownkey = [0xea, 0x79, 0x79, 0x20, 0xc8, 0x71, 0x44, 0x7d, 0x46, 0x62, 0x5f, 0x51, 0x85, 0xc1, 0x3b, 0xcb]
        xored = [knownkey[i] ^ pt[i] for i in range(0, 16)]
        block = xored
        block = self.inv_shiftrows(block)
        block = self.inv_subbytes(block)
        block = self.inv_mixcolumns(block)
        block = self.inv_shiftrows(block)
        result = block
        return self.inv_sbox((result[bnum] ^ guess[bnum]))

class UserScript(UserScriptBase):
    _name = "Auto-generated"
    _description = "Auto-generated Attack Script"
    def __init__(self, api):
        UserScriptBase.__init__(self, api)
        self.initProject()
        self.initPreprocessing()
        self.initAnalysis()
        self.initReporting()

    def initProject(self):
        pass

    def initPreprocessing(self):
        ppMod0 = preprocessing.resync_sad.ResyncSAD(self.api.project().traceManager())
        ppMod0.setEnabled(True)
        ppMod0.setReference(rtraceno=0, refpoints=(9100,9300), inputwindow=(8900,9500))
        ppMod0.init()
        self.traces = ppMod0

    def initAnalysis(self):
        self.attack = CPA()
        self.attack.setProject(self.api.project())
        self.attack.setTraceSource(self.traces, blockSignal=True)
        leakage_object = AES128_8bit(AES256_Model)
        self.attack.setAnalysisAlgorithm(chipwhisperer.analyzer.attacks.cpa_algorithms.progressive.CPAProgressive,leakage_object)
        self.attack.setTargetSubkeys([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])
        self.attack.setTraceStart(0)
        self.attack.setTracesPerAttack(150)
        self.attack.setIterations(1)
        self.attack.setReportingInterval(10)
        self.attack.setPointRange((0,10991))

    def initReporting(self):
        # Configures the attack observers (usually a set of GUI widgets)
        self.api.getResults("Attack Settings").setAnalysisSource(self.attack)
        self.api.getResults("Correlation vs Traces in Attack").setAnalysisSource(self.attack)
        self.api.getResults("Output vs Point Plot").setAnalysisSource(self.attack)
        self.api.getResults("PGE vs Trace Plot").setAnalysisSource(self.attack)
        self.api.getResults("Results Table").setAnalysisSource(self.attack)
        self.api.getResults("Save to Files").setAnalysisSource(self.attack)
        self.api.getResults("Trace Output Plot").setTraceSource(self.traces)
        self.api.getResults("Trace Recorder").setTraceSource(self.traces)

    def run(self):
        self.attack.processTraces()

if __name__ == '__main__':
    import chipwhisperer.analyzer.ui.CWAnalyzerGUI as cwa
    from chipwhisperer.common.utils.parameter import Parameter
    Parameter.usePyQtGraph = True            # Comment if you don't need the GUI
    api = CWCoreAPI()                        # Instantiate the API
    app = cwa.makeApplication("Analyzer")    # Comment if you don't need the GUI
    gui = cwa.CWAnalyzerGUI(api)             # Comment if you don't need the GUI
    api.runScriptClass(UserScript)           # Run UserScript through the API
    app.exec_()                              # Comment if you don't need the GUI