Embedded Technology
Figure 3 - Adding authentication to message integrity using a PKI public key algorithm (source STMicro)
Another Secure Vault feature is the Anti-Rollback Prevention feature, which ensures that only new signed firmware updates operate on the device. This feature prevents an adversary, who might be aware of vulnerabilities in older firmware versions from uploading it to exploit and compromise the device.
Figure 4 - The feature set of the STM32Trust security framework from STMicro- electronics (source STMicroelectronics)
ST uses several system monitoring methods that offer Abnormal Situation Handling protection. These include advanced tamper detection of, for example, opening a product enclosure, abnormally low voltage supply rails, external clock disconnection, and differential power attacks. Monitoring of the device’s temperature also occurs. Together, these forms of detection highlight if an adversary is attempting to create a fault condition or force the IC into a mode that might make it vulnerable. Programmable response mechanisms to a potential tamper attack vector include interrupts, resets, or secret key deletion.
message integrity. A digest, a fixed length bitstream, is created from the message and sent to the recipient along with the message. Note, adversaries cannot recreate the message from the hash digest. Popular hashing algorithms include MD5 and SHA-1/2/3. Adding a signature, created using a public key algorithm, adds authentication to hashing’s integrity - see Figure 3.
Implementing embedded security To aid embedded developers to implement reliable and robust security functions in new designs, semiconductor vendors offer hardware-based security features and frameworks. Examples include Secure Vault from Silicon Labs and STM32Trust from STMicroelectronics (ST). Both SecureVault and
STM32Trust are certified to Platform Security Architecture (PSA) Level 3. PSA is an industry certification partnership, initially founded by Arm, but now a global collaboration of semiconductor companies, certification organisations, and embedded security evaluation labs.
The full features available in Secure Vault range from essential cryptographic functions to more advanced DPA countermeasures and secure key management functions. Providing a similar set of features is STM32Trust - see Figure 4. The features available on each vendor’s microcontrollers are device dependent.
The basic concepts of embedded security used in the two frameworks are essentially the same, although each vendor may implement a
feature in a specific manner.
For example, the Secure Vault, Secure boot with the root of trust feature, uses the initial bootable code from the device’s immutable gated ROM. Any adversary would not be able to change the boot code held in the IC die so that developers can trust this boot stage with a very high degree of confidence. The ROM code is deemed to be the root of trust and serves to validate the signature of the next section of code that is to be loaded; this is called the First Stage Bootloader – see Figure 5. The first stage then validates the signature of the second stage bootloader, first checking if any signed updates are available. The Second Stage Bootloader also checks for any signed over-the-air (OTA) updates for the application code before executing it.
Figure 5 - Secure Vault’s secure boot with a root of trust feature (source Silicon Labs)
STM32Trust and Secure Vault protect debug ports and other peripheral interfaces from attack. These ports and interfaces provide access to a device’s resources including, memory, CPU, and registers. Debugging any embedded system is essential to any design, but access to a debug port is often locked to maintain device security.
Protecting embedded SRAM memory is another critical feature of STM32Trust and provides features to erase SRAM automatically should the detection of an abnormal event occurs. Both STM32Trust and Secure Vault devices feature a NIST (National Institute of Standards and Technology) certified random number generator (RNG). An effective RNG is an essential requirement for any cryptographic process. A random number generator that is not genuinely random means that an adversary might exploit uncovering the random sequence, resulting in a security protocol that is vulnerable to attack.
Securing your embedded device Incorporating a high degree of security into an embedded system is vital. For most embedded developers, learning to accomplish this from scratch is a very daunting and time-consuming task. However, semiconductor vendors like Silicon Labs and ST have developed PSA-certified hardware and firmware-based security frameworks for their microcontrollers that greatly simplify the process. Implementing embedded security using one of these frameworks helps speed design cycles and allows developers to maintain their focus on the core application tasks.
www.mouser.com 18 July/August 2022 Components in Electronics
www.cieonline.co.uk
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