Electronic Design
The Many Faces of Embedded Security

The Many Faces of Embedded Security

While there is no such thing as absolute security, it is possible to make things much more difficult for a would-be attacker.

That new embedded system you are designing had better be secure or it might be hacked. Of course, how to prevent this will depend upon what you are going to protect. There are those that want to simply crash a system, while others would like to take it over to use it as a platform for nefarious purposes. There are others who simply want to what is inside because those algorithms can be worth a lot of money.

In the past, processors simply started running and security was something that was implemented in the program or operating system. It came down to who you trust and how well they did their job. Burroughs mainframes I worked on many years ago were protected by the compilers and a file system that prevented hacking by limiting the instructions that were emitted or preventing files from being marked as executable.

Initially microcontrollers and microprocessors took a similar approach, with features such as memory protection being added that allowed applications to be placed into a sandbox. Virtualization extended this further to the point where high-end systems virtualize the entire system. Securing the sandbox works, assuming the base software and hardware cannot be compromised.

That assumption is not always warranted. For example, most hardware has some form of software involved like the controllers on hard drives. One may have thought that the data on the hard drive would be maintained and it would be a matter of protecting the data alone, possibly using an encrypted hard drive. Unfortunately, we now know that this is not the case since modifying this firmware is one way a group has infiltrated some storage devices (see “Hacking Hard Drives and Other Nasties”). In this case, the controller firmware was modified so operating systems stored on the drive would be compromised. The malicious firmware hid itself and the modifications. This turns out to be easy because disks already do this type of data remapping to handle bad blocks.

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Secure boot and encrypted code are ways to prevent the initial attack from succeeding. One-time programmable (OTP) or ROM-based solutions are another, but these prevent field updates that are necessary for many applications. Some platforms also allow the debug or JTAG support to be disabled, often via OTP flags, so application code is no longer directly accessible—making reverse engineering more difficult. Some systems go to tamper-resistant extremes that erase the code and data if an attack is detected.

1. Texas Instruments’ MSP432 is based on an ARM Cortex-M4F core, but adds a software IP protection scheme that allows multiple blocks of flash to be execute-only. Even JTAG will not reveal the contents of the code.

Texas Instruments’ (TI) MSP432 has an interesting approach to protecting application code that has some useful implications.The MSP432 is based on an ARM Cortex-M4F core (Fig. 1). The protection scheme allows multiple blocks of flash to be execute-only, but it takes this a step further by allowing the code to access data within the same block. JTAG will not reveal the contents of the code or data.

There can be multiple blocks defined and encryption keys are used to verify and allow updates so a block cannot simply be removed and replaced by a malicious actor. The MSP432 does not have a security key store or OTP support, but these can be implemented using the software protection scheme.

The approach allows a vendor to incorporate runtime support on a chip and then provide it to a developer. The developer can call the support routines, but they will not have direct access to the code that could be disassembled. The scheme actually works for multiple vendors to provide services on the same chip.

Of course, it comes down to who and what you trust. With the MSP432 it is TI and its hardware implementation. There is no such thing as absolute security, but it is possible to make an attacker’s job a lot more difficult. 

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