Category: Hardware Hacking

On April 1st, 2022 I gave a “workshop” at New England Hardware Security Day. This blog post is a quick summary of some of the links to recreate my demos from that talk. Here is a copy of the slides if you’d like them:

DFA on Raspberry Pi with PicoEMP

This demo is pretty simple – it recreates the classic DFA attack on RSA (I find David’s description great here, or you can see my Hardware Hacking Handbook which includes another derivation of it using a different method).

You can see the full code source in my repo from Hackaday Remoticon 2021. That repo just includes the R-Pi Python side (it also makes reference to voltage glitching, which I showed in the talk as another way to perform the demo).

You’ll need to install a specific version of pycryptodome along with a library that performs the analysis afterwards:

pip install pycryptodome==3.1
sudo apt-get install python3-gmpy2

From there you can run rsaglitch.py – if you run it as-is it will sit there waiting for a fault. If you don’t have a FI generator or just want to see it work, simply uncomment the lines that include some example fault output.

The actual fault injection in my demo was done with the PicoEMP. This is a low-cost/open-source EMFI tool. Critically it doesn’t require dangerous exposure to high voltage that some other open-source tools inherently present.

Watch out with this demo – it can be annoying as you crash the R-Pi a lot while dialing it in! And it can take a while to boot, but I gaurantee you it will work!

RISC-V Soft-Core

This demo was based on one of the targets that will come with the ChipWhisperer-Husky, an iCE40 based FPGA target.

The soft-core in question is the excellent NEORV32 RISC-V core. I find that core’s got great documentation. You don’t need to build the core to use the existing design, as the ChipWhisperer repo has a pre-built binary of the FPGA image. So you can compile software for that image. But it’s fun to build your own core!

• Checkout the neorv32-setups repository.
• Follow instructions to configure the required FPGA toolchain (the ‘osflow’ toolchain).
• Run make BOARD=iCE40CW312 MinimalBoot

Hardware ECC Attacks

This demo uses the ChipWhisperer CW305 board (here in the A35 variant), which has a series of nice ECC jupyter notebook tutorials . These tutorials will walk you through how the entire attack works:

The ECC core is based on the excellent and open-source CrypTech project.

We have a detailed write-up of the attack that can also be helpful to see!

At CHES 2019 [rump session], I presented my revolutionary talk on Time Travel Resistant Cryptography (TTRC). This is a hugely important area of research that has been widely ignored in academic work, and it’s time to finally make this right.

Why is this so critical? While Post Quantum Cryptography (PQC) gets NIST contests, and invested companies, nobody is considering TTRC. The general thought-process of PQC is that the existence of sufficiently powerful quantum computers is an open problem with no clear solution. BUT – if someone solves that problem (that is unclear is even physically possible to solve), it’s going to be hell on Earth for crypto implementations. Better safe than sorry.

That sounds a hell of a lot like some other problems to me.

And that problem even have had multiple movies made about it:

Lots of open questions exist. But note that many of them are not so unreasonable. For example – what if time travel requires us to create a Closed Timelike Curve (CTC), and time travel is only possible from the point that curve is created and onward.

This would mean that from the point the CTC is created crypto would immediately be broken, but any point before that (i.e., now) is safe. Thus we must create TTRC implementations since we cannot know when CTCs could be created.

I discuss many of these problems in my CHES 2019 Rump Presentation, you can see the slides below. When video is posted I’ll update this blog post with such material.

This blog post covers several topics that I should have made independent posts about… but anyway. Here we are. It’s September and I should have done this months ago.

Trezor / USB Hacking Updates (Black Hat + WOOT)

I had an earlier blog post with details of the Trezor attack. It turns out this is more generic type of attack than I realized, so I extended this work into a WOOT paper as well. Quickly I thought I should update on that…

To begin with – you can check out my Black Hat USA slides on the web at http://i.blackhat.com/USA-19/Wednesday/us-19-OFlynn-MINimum-Failure-Stealing-Bitcoins-With-EMFI.pdf .

Probably you want more details though, so luckily I included them in a paper presented at WOOT ’19 (see https://www.usenix.org/system/files/woot19-paper_oflynn_0.pdf).

This paper includes some additional details. One major thing is that the USB attack I used in the Trezor applies to many other devices. Basically almost everything has something like the following chunk of code:

if (∗length > setup−>wLength) {
   ∗length = setup−>wLength;
}

The problem comes about because the wLength field ends up coming from the computer (host). Using fault injection we can always cause that code-path to be taken, meaning we can read out data directly from the target device. This applies in only certain circumstances… here is a quick flow-chart of when you should care:

PhyWhisperer-USB

As part of this project, I also started a new open-source USB trigger logic device. You can check it out on
https://github.com/newaetech/phywhispererusb

We’re also doing a CrowdSupply for the initial run – see
https://crowdsupply.com/newae/phywhisperer-usb

At the FICHSA Conference (
https://fichsa.sise.bgu.ac.il ) I will be running a short workshop on ChipWhisperer using the ChipWhisperer-Nano.

A direct link to a Google Doc with the most up to date information is available here: https://docs.google.com/document/d/1IgDeGZ6d0FEYJbaF4a-KsBhdIHlMZg04-wQYUSZgnks/edit?usp=sharing

If you want to fully play along, please bring a laptop with the following installed and setup:

1. VirtualBox 5.x (5.2.28 is latest supported). You CANNOT use VirtualBox 6 due to some unknown incompatibility.
2. VirtualBox Extension pack for version you installed (direct link to 5.2.28, does not depend on the host OS).

I will be (hopefully) posting a VirtualBox image once one is fully updated.

If you don’t need hardware and still want to see how the system works, please have a Google account – some of the work can be done on Google Colab.

See the Google Doc link above for more updated details (more detailed install / setup instructions etc)

As mentioned on the Trezor blog post, their latest security patch fixes a flaw I disclosed to them in Jan 2019. This flaw meant an attacker with physical access to the wallet can find the recovery seed stored in FLASH, and leave no evidence of tampering.

This work was heavily inspired by the wallet.fail disclosure – I’m directly dumping FLASH instead of forcing the flash erase then dumping from SRAM, so the results are the same but with a different path. It also has the same limitations – if you used a password protected recovery seed you can’t dump that.

Practically, it has the advantage of not requiring you to modify/tamper with the enclosure at all either to do the fault injection or read data off . Wallet.fail uses the physical JTAG connection, whereas I’m using the USB connection. But by comparison the wallet.fail attack is more robust and easier to apply – my EMFI attack can be fiddly to setup, and there is some inherent timing jitter due to the USB stack processing. Without further effort to speed up my attack, it’s likely the process of opening the enclosure & applying the wallet.fail method would be a faster in practice.

Please do not contact me if you have a “forgotten password” or something similar with your Trezor. I’m not willing to perform this work under any circumstances.

Summary

USB requests that read from memory (such as a USB descriptor read) accept a maximum packet size that is MIN(requested_size, size_of_data_structure). The code as written defaults to accepting the requested size, and overwriting it with a smaller “valid” value.

Using fault injection (here demo’d with EMFI), this check can be skipped. The device will accept up to 0xFFFF as a size argument, giving an attacker access to read large chunks of either SRAM or FLASH (depending where the descriptor is stored).

Due to the FLASH memory layout, the sensitive metadata lies roughly AFTER the descriptors stored in bootloader flash space. Thus an attacker can read out the metadata structure from memory (seed etc). Notably this does not require one to open the enclosure due to use of EMFI.

Using voltage glitching would almost certainly work as well, but likely requires opening the enclosure making tampering more evident (possibly it would work via USB +5V input even which wouldn’t require opening the enclosure).

Details

I’ve chosen to demo this with WinUSB descriptor requests, as they have a very simple structure. In addition the WinUSB has one descriptor in FLASH and one in SRAM, giving you the ability to chose which memory segment to dump. Other USB requests should work too, but I haven’t tested them.

Consider the following function, where the critical calls are the len = MIN(len,…) calls (two of them can be found):

static int winusb_control_vendor_request(usbd_device *usbd_dev,
					struct usb_setup_data *req,
					uint8_t **buf, uint16_t *len,
					usbd_control_complete_callback* complete) {
	(void)complete;
	(void)usbd_dev;

	if (req->bRequest != WINUSB_MS_VENDOR_CODE) {
		return USBD_REQ_NEXT_CALLBACK;
	}

	int status = USBD_REQ_NOTSUPP;
	if (((req->bmRequestType & USB_REQ_TYPE_RECIPIENT) == USB_REQ_TYPE_DEVICE) &&
		(req->wIndex == WINUSB_REQ_GET_COMPATIBLE_ID_FEATURE_DESCRIPTOR)) {
		*buf = (uint8_t*)(&winusb_wcid);
		*len = MIN(*len, winusb_wcid.header.dwLength);
		status = USBD_REQ_HANDLED;

	} else if (((req->bmRequestType & USB_REQ_TYPE_RECIPIENT) == USB_REQ_TYPE_INTERFACE) &&
		(req->wIndex == WINUSB_REQ_GET_EXTENDED_PROPERTIES_OS_FEATURE_DESCRIPTOR) &&
		(usb_descriptor_index(req->wValue) == winusb_wcid.functions[0].bInterfaceNumber)) {

		*buf = (uint8_t*)(&guid);
		*len = MIN(*len, guid.header.dwLength);
		status = USBD_REQ_HANDLED;

	} else {
		status = USBD_REQ_NOTSUPP;
	}

	return status;
} 

We can see from disassembly this is a simple comparison to the maximum size. I’m mostly going to be looking at the guid structure since it’s located in FLASH below the metadata. Glitching the code in the red box below would create our required vulnerability:

The glitch is inserted with a ChipSHOUTER EMFI tool. The Trezor is held in bootloader mode with two spacers on the buttons, and the EMFI tool is positioned above the case, as shown below:

The entire setup is shown below, which has the ChipSHOUTER (bottom left), a ChipWhisperer-Lite (top left) for trigger delay, a Beagle 480 for USB sniffing/triggering (top center) and a switchable USB hub to power-cycle the Trezor when the EMFI triggers the various hard fault/stack smashing/memory corruption errors (top right).

The forced bootloader entry mode simplifies the glitch, as we can freely power cycle the device and try many glitches.

A glitch is timed based on a Total Phase USB analyzer. The USB request is sent from a host computer as shown below:

def get_winusb(dev, scope):
    """WinUSB Request is most useful for glitch attack"""  
    scope.io.glitch_lp = True #Enable glitch (actual trigger comes from Total Phase USB Analyzer)
    scope.arm()    
    resp = dev.ctrl_transfer(int('11000001', 2), ord('!'), 0x0, 0x05, 0xFFFF, timeout=1)
    resp = list(resp)    
    scope.io.glitch_lp = False #Disable glitch
    return resp

This corresponds to reading the “guid”. The Beagle480 is setup to generate a trigger on the first two bytes of this request:

To confirm the timing, I also compiled my own bootloader, and saw the sensitive operation was typically occurring in the 4.2 to 5.7uS after the Beagle480 trigger. There was some jitter due to the USB stack processing.

A ChipWhisperer-Lite is used to then generate a 4.4uS offset (empirically a good “waiting” place), after which an EMFI pulse is triggered. Should the 4.4uS offset happen to line up with the length checking/limiting, we can get a successful glitch.

A successful glitch will return much more data than expected – possibly up to the full 0xFFFF, but because things are getting corrupted it sometimes seems to be less. Also some OSes limited the maximum size you can request -you can find some references to that on the libusb mailing list. As a first step you might just try 0x1FF or something (anything larger than the correct 0x92 response). But here is a screen-shot showing three correct transfers (146 bytes transferred) followed by an incorrect one, which dumped 24800 bytes:

Here is an example metadata from another dump (this doesn’t correspond to the screen-shot above), note I never set a PIN as I had damaged the screen since the first device I tested with was being used for development as well:

Simulating the Glitch

Note you can “simulate” this glitch by simply commenting out the length checking operation in the WinUSB request. This allows you to confirm that should that operation be skipped, you can dump large chunks of memory including the metadata.

This simulation could be useful in validating some of the proposed fixes as well (I would highly suggest doing such testing as part of regular building).

Reliability Notes

Note due to the timing jitter the glitch is very unreliable (<0.1% success rate), however this typically translates to taking a few hours due to how quickly the search can be done. So it remains extremely practical since it seems reasonable for an attacker to have access to a wallet for a few hours.

I suspect this could be improved by doing the USB request from an embedded system (something like a GreatFET), which would allow me to synchronize a device to the internal state of the Trezor. This would mean the operation would be:

1. Send a USB request (get descriptor etc).
2. After the USB request comes back, send our WinUSB/glitch target request at a very defined amount of time after we received the USB request.
3. Insert the glitch.

Simple Fixes / Countermeasures

1) The low-level USB functions should accept some absolute maximum size. For example, if you only ever do 64-byte transfers, never allow a larger transfer to occur by masking upper bits to force them to zero. Do this in multiple spots to make glitching past this more difficult.

2) The metadata needs to be guarded by invalid memory segments. Doing a read from a valid segment to the metadata should hit an invalid segment, causing an exception.

3) Move descriptor /metadata arrangement (less useful than #2 in practice).

Conclusions

EMFI is a simple way of abusing the USB stack in the Trezor. You’ll find a similar vulnerability in a number of USB stacks, meaning this type of attack should be something you validate against in your own devices.

More details of this example will be available in the book Jasper & I are working on for (eventual?) release.

At Embedded World I gave a talk on embedded security. There was also an associated paper, and I’m now making those available. I’ve also duplicated the paper contents in this blog post for your ease of access.

Download Slides (PPTX):

ABSTRACT: As interconnected devices proliferate, security of those devices becomes more important. Two critical attacks can bypass many standard security mechanisms. These attacks are broadly known as side-channel attacks & fault injection attacks. This paper will introduce side-channel power analysis and fault injection attacks and their relevance to embedded systems. Examples of attacks are given, along with a short discussion of countermeasures and effectiveness. The use of open-source tools is highlighted, allowing the reader the chance to better understand these attacks with hands-on examples.

Introduction

Side-channel attacks are the broad class given to attacks which rely on “additional information” that is accidentally leaked. A variety of such side-channels exist, such as the time an algorithm takes to execute can betray information about the code paths taken in the algorithm. Of great interest to embedded developers is side channel power analysis, first introduced by Kocher et al. in 1999 [1]. This attack takes advantage of a small piece of information – the data being processed by a system affects the power consumption of that system. This allows us to break advanced cryptography systems such as recovering an AES-256 key in a matter of minutes. These attacks do not rely on substantial resources – they can be performed with commodity hardware and for less than $50. A second class of attack will be known as fault injection attacks. They allow us to modify the program flow of code, which can cause a variety of security flaws to be exploited. This paper will briefly introduce those two methods of attacks and discuss how engineers can understand them to develop effective countermeasures.

Power Analysis for Algorithm Flow

The most basic form of power analysis attack is simple power analysis. In this method we will extract information about the algorithm flow. This could be used to directly extract key bits where changes in program flows reveal key information (such as with RSA), but can also be used to extract information such as timing that simplifies further attacks. Observe a simple password check which checks a byte of the password at a time. The execution time through this algorithm would reveal which password byte was incorrect, allowing an attacker the ability to quickly brute-force the algorithm. A password of ADBC would entail only the guess sequence “A/A..B..C..D/A..B../A..B…C” to find the correct password, as once the correct letter is found for one digit the guess algorithm can move onto the next digit.

Such an attack could be performed from the communications protocol. But many systems will add a random delay before returning the results. With power analysis we could see the unique signatures in the power trace, as in Figure 1. Fig. 1. A simple password check shows how many iterations through the loop we took.

These power measurement examples are taken with the ChipWhisperer project. Here the power measurements are done by inserting a shunt resistor into the device power pin, and using the fact that a change in current will cause a change in voltage across the resistor. The decoupling capacitors are removed in this example to provide a clean signal. This is shown in Figure 2.

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