Part 1, Topic 3: Clock Glitching to Dump Memory (MAIN)¶

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SUMMARY: In the previous lab, we learned how clock glitching can be used to get a microcontroller to skip a password check. This time, we'll look at a more practical example: getting an example bootloader to dump a large chunk of memory.

LEARNING OUTCOMES:

• Applying previous glitch settings to new firmware
• Checking for success and failure when glitching
• Understanding how compiler optimizations can cause devices to behave in strange ways

The Situation¶

Now that we've got our feet wet with glitching, we're going to try something a bit more realistic: an "encrypted" bootloader (it's actually just rot-13, but we'll pretend it's unbreakable encryption), where we make as few assumptions as possible. Our goal will be to get that bootloader to decrypt the data and send it back to us. Here's what we know about the bootloader:

1. The 'p' command is used to write encrypted firmware to the device. It takes in an encrypted ASCII-encoded string, terminated with a newline. Our first chunk of firmware is "516261276720736265747267206762206f686c207a76797821".
2. It does something to it (presumably unencrypts it, authenticates it, etc. and writes it to memory)
3. It sends back an error code of "r000000\n"

Of immediate interest is that error code. That's the only time the bootloader communicates back with us, so attacking there is a good place to start. One thing that we'll assume is that we've got a trigger right before the error code is sent back to us. This is just a simple trigger_high() call, but we could also trigger on an IO line (better with the CW1200 Pro) or with a SAD trigger on a power trace (CW1200 Pro only). We've got a place to start, but let's see if we can learn more about the bootloader first.

We recommend using SimpleSerial V2 for this as, though the firmware doesn't use the simpleserial protocol, the faster baud rate will help speed up glitching.

In [1]:

SCOPETYPE = 'OPENADC'
PLATFORM = 'CWLITEXMEGA'
SS_VER='SS_VER_2_1'

VERSION = 'HARDWARE'
CRYPTO_TARGET = 'TINYAES128C'
allowable_exceptions = None

SCOPETYPE = 'OPENADC' PLATFORM = 'CWLITEXMEGA' SS_VER='SS_VER_2_1' VERSION = 'HARDWARE' CRYPTO_TARGET = 'TINYAES128C' allowable_exceptions = None

In [2]:

%%bash -s "$PLATFORM" "$SS_VER"
cd ../../../firmware/mcu/bootloader-glitch
make PLATFORM=$1 CRYPTO_TARGET=NONE -j SS_VER=$2

%%bash -s "$PLATFORM" "$SS_VER" cd ../../../firmware/mcu/bootloader-glitch make PLATFORM=$1 CRYPTO_TARGET=NONE -j SS_VER=$2

SS_VER set to SS_VER_2_1

SS_VER set to SS_VER_2_1

avr-gcc (GCC) 5.4.0

Copyright (C) 2015 Free Software Foundation, Inc.

This is free software; see the source for copying conditions.  There is NO

warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

mkdir -p objdir-CWLITEXMEGA 

.

Welcome to another exciting ChipWhisperer target build!!

.

.

.

Compiling:

Compiling:

Compiling:

-en     bootloader.c ...

-en     decryption.c ...

-en     .././simpleserial/simpleserial.c ...

.

.

Compiling:

Compiling:

-en     .././hal//xmega/XMEGA_AES_driver.c ...

-e Done!

-en     .././hal//xmega/uart.c ...

.

.

-e Done!

Compiling:

Compiling:

-en     .././hal//xmega/usart_driver.c ...

-en     .././hal//xmega/xmega_hal.c ...

-e Done!

-e Done!

-e Done!

-e Done!

-e Done!

.

LINKING:

-en     bootloader-CWLITEXMEGA.elf ...

-e Done!

.

.

Creating load file for Flash: bootloader-CWLITEXMEGA.hex

.

avr-objcopy -O ihex -R .eeprom -R .fuse -R .lock -R .signature bootloader-CWLITEXMEGA.elf bootloader-CWLITEXMEGA.hex

Creating load file for Flash: bootloader-CWLITEXMEGA.bin

avr-objcopy -O binary -R .eeprom -R .fuse -R .lock -R .signature bootloader-CWLITEXMEGA.elf bootloader-CWLITEXMEGA.bin

Creating load file for EEPROM: bootloader-CWLITEXMEGA.eep

avr-objcopy -j .eeprom --set-section-flags=.eeprom="alloc,load" \

--change-section-lma .eeprom=0 --no-change-warnings -O ihex bootloader-CWLITEXMEGA.elf bootloader-CWLITEXMEGA.eep || exit 0

.

.

Creating Extended Listing: bootloader-CWLITEXMEGA.lss

avr-objdump -h -S -z bootloader-CWLITEXMEGA.elf > bootloader-CWLITEXMEGA.lss

Creating Symbol Table: bootloader-CWLITEXMEGA.sym

avr-nm -n bootloader-CWLITEXMEGA.elf > bootloader-CWLITEXMEGA.sym

Size after:

   text	   data	    bss	    dec	    hex	filename

   2414	    124	    202	   2740	    ab4	bootloader-CWLITEXMEGA.elf

+--------------------------------------------------------

+ Default target does full rebuild each time.

+ Specify buildtarget == allquick == to avoid full rebuild

+--------------------------------------------------------

+--------------------------------------------------------

+ Built for platform CW-Lite XMEGA with:

+ CRYPTO_TARGET = NONE

+ CRYPTO_OPTIONS = AES128C

+--------------------------------------------------------

In [3]:

#!/usr/bin/env python
# coding: utf-8

# In[ ]:


import chipwhisperer as cw

try:
    if not scope.connectStatus:
        scope.con()
except NameError:
    scope = cw.scope(hw_location=(5, 4))

try:
    if SS_VER == "SS_VER_2_1":
        target_type = cw.targets.SimpleSerial2
    elif SS_VER == "SS_VER_2_0":
        raise OSError("SS_VER_2_0 is deprecated. Use SS_VER_2_1")
    else:
        target_type = cw.targets.SimpleSerial
except:
    SS_VER="SS_VER_1_1"
    target_type = cw.targets.SimpleSerial

try:
    target = cw.target(scope, target_type)
except:
    print("INFO: Caught exception on reconnecting to target - attempting to reconnect to scope first.")
    print("INFO: This is a work-around when USB has died without Python knowing. Ignore errors above this line.")
    scope = cw.scope(hw_location=(5, 4))
    target = cw.target(scope, target_type)


print("INFO: Found ChipWhisperer😍")


# In[ ]:


if "STM" in PLATFORM or PLATFORM == "CWLITEARM" or PLATFORM == "CWNANO":
    prog = cw.programmers.STM32FProgrammer
elif PLATFORM == "CW303" or PLATFORM == "CWLITEXMEGA":
    prog = cw.programmers.XMEGAProgrammer
elif "neorv32" in PLATFORM.lower():
    prog = cw.programmers.NEORV32Programmer
elif PLATFORM == "CW308_SAM4S" or PLATFORM == "CWHUSKY":
    prog = cw.programmers.SAM4SProgrammer
else:
    prog = None


# In[ ]:


import time
time.sleep(0.05)
scope.default_setup()

def reset_target(scope):
    if PLATFORM == "CW303" or PLATFORM == "CWLITEXMEGA":
        scope.io.pdic = 'low'
        time.sleep(0.1)
        scope.io.pdic = 'high_z' #XMEGA doesn't like pdic driven high
        time.sleep(0.1) #xmega needs more startup time
    elif "neorv32" in PLATFORM.lower():
        raise IOError("Default iCE40 neorv32 build does not have external reset - reprogram device to reset")
    elif PLATFORM == "CW308_SAM4S" or PLATFORM == "CWHUSKY":
        scope.io.nrst = 'low'
        time.sleep(0.25)
        scope.io.nrst = 'high_z'
        time.sleep(0.25)
    else:  
        scope.io.nrst = 'low'
        time.sleep(0.05)
        scope.io.nrst = 'high_z'
        time.sleep(0.05)

#!/usr/bin/env python # coding: utf-8 # In[ ]: import chipwhisperer as cw try: if not scope.connectStatus: scope.con() except NameError: scope = cw.scope(hw_location=(5, 4)) try: if SS_VER == "SS_VER_2_1": target_type = cw.targets.SimpleSerial2 elif SS_VER == "SS_VER_2_0": raise OSError("SS_VER_2_0 is deprecated. Use SS_VER_2_1") else: target_type = cw.targets.SimpleSerial except: SS_VER="SS_VER_1_1" target_type = cw.targets.SimpleSerial try: target = cw.target(scope, target_type) except: print("INFO: Caught exception on reconnecting to target - attempting to reconnect to scope first.") print("INFO: This is a work-around when USB has died without Python knowing. Ignore errors above this line.") scope = cw.scope(hw_location=(5, 4)) target = cw.target(scope, target_type) print("INFO: Found ChipWhisperer😍") # In[ ]: if "STM" in PLATFORM or PLATFORM == "CWLITEARM" or PLATFORM == "CWNANO": prog = cw.programmers.STM32FProgrammer elif PLATFORM == "CW303" or PLATFORM == "CWLITEXMEGA": prog = cw.programmers.XMEGAProgrammer elif "neorv32" in PLATFORM.lower(): prog = cw.programmers.NEORV32Programmer elif PLATFORM == "CW308_SAM4S" or PLATFORM == "CWHUSKY": prog = cw.programmers.SAM4SProgrammer else: prog = None # In[ ]: import time time.sleep(0.05) scope.default_setup() def reset_target(scope): if PLATFORM == "CW303" or PLATFORM == "CWLITEXMEGA": scope.io.pdic = 'low' time.sleep(0.1) scope.io.pdic = 'high_z' #XMEGA doesn't like pdic driven high time.sleep(0.1) #xmega needs more startup time elif "neorv32" in PLATFORM.lower(): raise IOError("Default iCE40 neorv32 build does not have external reset - reprogram device to reset") elif PLATFORM == "CW308_SAM4S" or PLATFORM == "CWHUSKY": scope.io.nrst = 'low' time.sleep(0.25) scope.io.nrst = 'high_z' time.sleep(0.25) else: scope.io.nrst = 'low' time.sleep(0.05) scope.io.nrst = 'high_z' time.sleep(0.05)

INFO: Found ChipWhisperer😍

scope.gain.mode                          changed from low                       to high                     
scope.gain.gain                          changed from 0                         to 30                       
scope.gain.db                            changed from 5.5                       to 24.8359375               
scope.adc.basic_mode                     changed from low                       to rising_edge              
scope.adc.samples                        changed from 24400                     to 5000                     
scope.adc.trig_count                     changed from 10898586                  to 21921108                 
scope.clock.adc_src                      changed from clkgen_x1                 to clkgen_x4                
scope.clock.adc_freq                     changed from 20252357                  to 29906490                 
scope.clock.adc_rate                     changed from 20252357.0                to 29906490.0               
scope.clock.clkgen_div                   changed from 1                         to 26                       
scope.clock.clkgen_freq                  changed from 192000000.0               to 7384615.384615385        
scope.io.tio1                            changed from serial_tx                 to serial_rx                
scope.io.tio2                            changed from serial_rx                 to serial_tx                
scope.io.hs2                             changed from None                      to clkgen                   
scope.io.tio_states                      changed from (1, 0, 0, 0)              to (1, 1, 0, 0)             
scope.glitch.mmcm_locked                 changed from True                      to False                    

In [4]:

fw_path = "../../../firmware/mcu/bootloader-glitch/bootloader-{}.hex".format(PLATFORM)

fw_path = "../../../firmware/mcu/bootloader-glitch/bootloader-{}.hex".format(PLATFORM)

In [5]:

cw.program_target(scope, prog, fw_path)

cw.program_target(scope, prog, fw_path)

XMEGA Programming flash...

XMEGA Reading flash...

Verified flash OK, 2537 bytes

The first thing we'll do is some simple power analysis to see what the device is doing when it sends data back to us. Serial communication is pretty slow, so set the ChipWhisperer to capture around 24k samples with a "x1" ADC clock.

In [6]:

def reboot_flush():            
    reset_target(scope)
    #Flush garbage too
    target.flush()
scope.clock.adc_src = "clkgen_x1"
reboot_flush()
scope.adc.samples = 24000

def reboot_flush(): reset_target(scope) #Flush garbage too target.flush() scope.clock.adc_src = "clkgen_x1" reboot_flush() scope.adc.samples = 24000

Next, capture a power trace. The string "p516261276720736265747267206762206f686c207a76797821\n" will send the bootloader the first chunk of code and plot it. If you don't see the full serial message, you can increase scope.adc.decimate, which will throw out every nth ADC sample.

In [7]:

scope.adc.timeout = 3
scope.arm()
target.write("p516261276720736265747267206762206f686c207a76797821\n")
ret = scope.capture()
if ret:
    print("Timeout")
trace = scope.get_last_trace()


cw.plot(trace)

scope.adc.timeout = 3 scope.arm() target.write("p516261276720736265747267206762206f686c207a76797821\n") ret = scope.capture() if ret: print("Timeout") trace = scope.get_last_trace() cw.plot(trace)

Out[7]: