MindShaRE: Decapping Chips for Electromagnetic Fault Injection (EMFI)

Recently, the automotive VR team has undertaken an effort to reproduce the software extraction attack against one of the target devices used during the Automotive Pwn2Own 2024 held in Tokyo, Japan. The electromagnetic fault injection (EMFI) approach was chosen to attempt an attack against the existing readout protection mechanisms. This blog post details preparatory steps to speed up the attack, hopefully considerably.

Electromagnetic fault injection

In general, fault injection attacks against hardware attempt to produce some sort of gain for an attacker by injecting faults into a device under attack by manipulating clock pulses, supply voltages, temperature, electromagnetic fields around the device, and aiming short light pulses at certain locations on the device. Of these vectors, EMFI stands out as probably the only attack approach that requires close to no modifications of the device under attack, with all action being conducted at a quite short distance. The attack then proceeds by moving an EM probe above the device in very small increments and triggering an EM pulse. With any luck, this would disturb the normal operation of the device under attack in just the right way to cause the desired effect.

Practically speaking, some sort of an EM pulse tool is required to conduct the attack. In this case, the PicoEMP was chosen for that purpose, which has been mounted on a modified 3D printer carriage.

However, the device in question (a GD32F407Z by GigaDevice) is physically rather large, with the package measuring 20mm by either side. Considering how long each individual attempt runs, the fact that the attempt needs to be retried multiple times to collect meaningful outcome statistics, and rather small increments used to move the probe, it would make sense to narrow down the search area as much as possible. Injecting EM faults into the epoxy encapsulation would not bring much of an effect.

Decapping

Unfortunately, the encapsulation is not transparent and does not allow for easy visual identification of the die in the package. This means that some way of getting the die out is required to measure it, or better yet, leave the die in the package so it will be possible to measure both the die dimensions and the position of the die within the package.

There are multiple approaches to decapsulation, or decapping for short:

·      Mechanical: sanding or milling the package, cracking the encapsulation when heated
·      Chemical: applying acid to dissolve encapsulation
·      Thermal: placing the package in a furnace to burn encapsulation away
·      Optical: using a laser to burn encapsulation away in a precise manner

Of these, many require specialized equipment (mill, laser, furnace, fume hood for nitric acid), are time-consuming (sanding), or do not preserve important information (cracking the package). The choice was thus limited to what was available: hot sulfuric acid.

DANGER: Hot sulfuric acid is extremely corrosive; avoid spilling and wear proper PPE at all times.

DANGER: Sulfuric acid vapors are extremely corrosive; avoid inhaling and work in a well-ventilated area (fume hood or outside).

NOTE: Study relevant safety information including but not limited to materials handling, spill containment, and clean-up procedures before working with any hazardous chemicals.

NOTE: This blog post was written purely for educational purposes; any attempts to replicate the work are at your own risk.

Decapping process

As my home lab is, sadly, not equipped with a fume hood, all work was conducted outside.

The following tools were used:

·      Sulfuric acid, 96%
·      A heat source in the form of a hot air station
·      A crocodile clip “helping hands”
·      A squirt bottle with acetone
·      A PE pipette
·      A waste container.

To begin, the device under attack is fixed in the clip, and a small drop of acid was applied with the pipette in the package center.

The device was then heated using the hot air station set to 200°C  and a moderate air flow of around 40%. The aim of this process is to slowly dissolve the packaging epoxy. The device was heated until some fuming was observed from the drop and stopped before any bubbling would occur. If the acid gets hot enough to produce bubbles, the material will form a hard carbonized “cake” which will be problematic to remove. Unfortunately, this has been a problem before.

After the acid visibly darkened, which should take around 1 minute +- 50%, the heating was stopped, and the device was allowed to cool down somewhat. Then, the acid was washed off with acetone into the waste container. The device then was dried off with hot air to remove moisture.

The process was then repeated multiple times, with each iteration removing a bit of the packaging material. This was captured in the following series of images (more steps were taken than is presented here):

A stack of dice slowly emerged from the package: the larger one is the microcontroller itself, and the smaller one is the serial Flash memory holding all the programmed code and data. Unfortunately, the current process does not preserve the bond wires, rendering the device inoperable. Its operation was not required in our case. This could possibly be mitigated by using a 98% acid and anhydrous acetone – something to attempt in the future.

Measurements

The end result of the decapping process is pictured below.

Using a graphics editor, it is possible to take measurements in pixels of the package, the die, and the die positioning. This came out to be the following:

·      Package size 1835x1835 pixels (measured) = 20x20 mm (known from the datasheet)
·      Pixels per mm: 91.75
·      Die size 366x366 pixels (measured) = 4x4mm (computed)
·      Die offset from bottom left: 745x745 pixels (measured) = 8.12x8.12mm (computed)

The obtained numbers are immediately useful to program the EM probe motion restricted to the die area only. To find out how much experiment time this could save, let’s compute the areas: 4x4 = 16 mm2 for the die itself, and 20x20 = 400 mm2 for the whole package. This is 25 times decrease in the area and thus the experiment time.

Another approach that could avoid the decapping process is moving the probe in a spiral fashion, starting from the package center and moving outwards. This is of course possible to implement. However, the challenge here is the possibility of the two dice getting packaged side-to-side instead of being stacked like in this example – this would severely decrease the gain from this approach. Given the decapping only takes no more than 1-2 hours including cleanup, this was deemed well worth the information gained – and the die pictures obtained.

Conclusion

I hope you enjoyed this brief demonstration. Again, please take caution when using sulfuric acid or any other corrosive agents. Please dispose of waste materials responsibly. The world of hardware hacking offers many opportunities for discovery. We’ll continue to post guides and methodologies in future posts. Until then, you can follow the team on Twitter, Mastodon, LinkedIn, or Instagram for the latest in exploit techniques and security patches.