As discussed in the earlier blog, it is becoming very important, in an embedded system, to ensure authenticity of the firmware before running it. Also, the system has to be made tamper proof against further hacking, especially for remotely managed internet connected IoT applications.

To prevent breach of security, the software can be strengthened with various techniques based on the underlying MCU and peripheral set. This blog discusses in particular how it can be done for iMx RT1020 based devices using the High Assurance Boot (HAB) mechanism as recommended by NXP.

Secure Boot Concepts

NXP’s HAB uses the mechanism of asymmetric encryption to protect its firmware. To give a quick introduction to asymmetric encryption, it is essentially creating a pair of keys in a way that one of the keys can encrypt the message and other can decrypt the message (and vice versa). It is mathematically impossible to use the same key used for encryption to decrypt the message. Also, with increased key sizes, it will be highly resource consuming to decrypt a message without the other pair.

Thus, with asymmetric encryption, it is enough to protect one of the keys (private key) and other can be shared (public key). The message encrypted by the private key can only be decrypted by the public key. Further if the public key (or at least its hash) is stored in a location that can not be modified such as On Time Programmable Flash, it will be impossible for any one to compromise the system. An attempt to modify the public key will be nullified because of the check with the OTP memory.

The high level of the above sequence can be capture in the below sequence diagram.

Secure Boot iMX RT 1020 HAB process

During the device provisioning process, the public and private key pairs are generated and private key is secured in the provisioning system. Hash for the public key is generated and stored in the device OTP area, which prevents further modification.

In the code signing sequence, the firmware image is hashed and encrypted using the private key. The final image generated is comprised of the firmware image, its encrypted image along with the public key and the same is programmed on to the boot memory.

During the bootup sequence, the HAB logic extracts the individual components of the signed image and validates to authenticity of the public key by comparing the computed hash and that stored in the OTP fuses. It is impossible to create public key such that the hash is same there by preventing any attempt of overriding the public key by external parties. Then it proceeds to calculate the hash of the firmware. It is compared with a hash generated by decrypting the encrypted hash using the public key. If it is a match, it proceeds to boot. If it fails in any of the place, the boot is aborted.

Code Signing for i.Mx RT1020

NXP provides all the tools necessary for generating public-private key pairs, code signing and blowing boot flashes such as MfgTool, elftosb, cst etc.

The device can be programmed using two methods: Device Boot and Secure Boot. The Device boot mode can be used during development purposes and secure boot for final programming. If the device is once programmed in Secure boot mode, it is not possible to revert back to Dev Boot mode and all further firmware has to be signed properly. The programming process is carried out by Flashloader tools such as- elftosb tool for boot image creation, Mfg tool for boot image programming.

Dev Boot Mode

To program the device, use the Mfgtool.

  • Create unsigned boot_image.sb using elftosb tool from SREC format of the application image (app.s19 file).
  • Make sure the file inside the Mfg tool is available in the name – cfg.ini
  • The content inside the file should be in the following format : chip → MXRT102X, name → MXRT102X-DevBoot
  • Import the boot_image.sb file to …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware from …/Tools/elftosb/linux/amd64/
  • After generating the boot_image.sb and placing it in the following directory …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware
  • Change the device’s boot mode into serial downloader mode and connect it to the host PC
  • Run the MfgTool and press the start button to program the target.
  • To exit MfgTool, click “Stop” and “Exit” in turn

Secure Boot Mode

To program the OTP flash once with hash of the public key, use the Mfgtool as follows

  • Check whether the device is in serial downloader mode
  • Generate the private/public keys using CST tool and create fuse.bin and fuse.table files.
  • Make sure the file inside the Mfg tool is available in the name – cfg.ini
  • The content inside the file should be in the following format : chip → MXRT102X, name → MXRT102X-Burnfuse
  • Create and import the enable_hab.sb file to the following directory …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware from the directory …/Flashloader_RT1020_1.0_GA/Tools/elftosb/linux/amd64/
  • After programming the above mentioned enable_hab.sb file successfully, the device will be ready for secure boot.

The above process of programming the fuse has to be executed only once. Further mode to program the device with signed image, use the Mfgtool as follows

  • Create signed boot_image.sb using elftosb tool from SREC format of the application image (app.s19 file).
  • Check whether the file inside the Mfg tool is available in the name – cfg.ini
  • The content inside the file should be in the following format : chip → MXRT102X, name → MXRT102X-SecureBoot
  • Import the signed boot_image.sb file to the following directory …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware from the directory …/Flashloader_RT1020_1.0_GA/Tools/elftosb/linux/amd64/

The details of the process can be obtained from NXP i.Mx1020 product page. Once secured, it will be impossible to run unauthorized software.

Same concepts can be extended to OTA updates so that the new firmware can be authenticated even before programming.

About Embien :

Embien has been actively developing IoT devices that form important part of a larger network with huge ramifications on security. We have been using advanced tools and techniques to prevent unauthorized access and tampering of the device. Get in touch with us to get your design unprecedented security.

The number of IoT devices connected to the internet are increasing rapidly and is expected to reach 75.44 billion by the end of 2025, which clearly states a fivefold increase within a decade. IoT devices are expected to become the major target for malware attacks. Even in 2019, of the overall cyber-attacks, it is estimated that IoT related breaches is about 26%.

Ranging from automobiles, medical implants to casinos – IoT related attacks have become prominent and it is essential to incorporate security features within IoT devices to help lower the increasing menace.

This blog covers a few steps that can be employed in securing Embedded Firmware for IoT Applications at a high level. While these guidelines help you understand the overall picture, it is also necessary to analyze the threats that are specific to your product and integrate them with due diligence. Some of the considerations discussed here include

  • Securing Boot Firmware
  • Secure firmware update
  • Secure data storage
  • IoT Product Physical Security

Securing Boot Firmware

The First and foremost step in securing the firmware is to protect the processor which is the heart of the system. Almost all the modern variants of the MCUs/MPUs etc., are equipped with features that are specific to security such as HAB (high assurance boo) and TPM (Trusted Platform Module). In some cases, the processor offers a Secure Boot mechanism in which it validates the sanity of the boot code before it starts executing it.

Some silicon vendors provide mechanisms where the hash of the boot code can be stored in a One Time Programmable (OTP) memory. Once the boot code is prepared, the hash is created and stored in these flashes. Upon bootup, the processor reads the complete boot code, calculates the hash and compares it with the pre-programmed value. Only if there is a match, the code will be executed.

On a more powerful system, it is possible to encrypt the hashes with a private key. The public keys can be programmed along with the boot code. The system calculates the hash and compares it with the hash that was decrypted with the public key. To authenticate the veracity of the public key, its hash is stored in the OTP flashes.

On simpler devices, when the reprogramming of boot code is envisaged, microcontrollers offer a mechanism that tend to blow fuses that will permanently lockout the code area in flash and prevent it from further writes and reads by external sources. Such mechanisms lock out the binaries and increase the security as it is difficult to reverse engineer the firmware and identify potential weaknesses.

Once the boot code is authenticated by the system, it validates the next level of firmware/applications by maintaining the chain of trust.

Secure firmware update

In many cases, during the life cycle of a product, it is required to update the firmware for specific reasons like: adding new features, fixing defects in the present release etc., Such products offer secure firmware update feature where by the new firware can be sent to the device either remotely Over The Air (OTA) or physically using memory cards. Before usage, the firmware is must be authenticated and updated in a fail-safe manner.

Modern firmware typically take advantage of asymmetric cryptographic algorithms with the private keys stored securely at the vendor facility. The public key is stored usually in a secured location within the end-product. The firmware to be updated will be signed by the private key and shared over the internet in a secured manner. The device validates the downloaded image with the public key that is available with itself.

While the above mechanism only validates the authenticity of the image, sometimes it is also essential to protect the firmware against reverse engineering. In such scenarios, the whole image is encrypted with the help of the private key. Only the end device will be able to decrypt it and program itself.

When such designs tend to improve security, it is of prime necessity to consider the flexibility of the update process as well. For example, if there is a security breach at the server, these designs should be able to revoke the present keys and replace them with another set.

Secure data storage

This section emphasizes on the importance of safe guarding the data content of the end device. These devices might be collecting sensitive information that should be protected against un-authorized access. In such cases, mechanisms such as encrypting the disk drives can be used to render the disk content unusable, unless it is decrypted by the same set of keys. The key storage should be in a safe place that is protected against cyber-attacks. Cryptographic chips are also available for storing the keys in a secured manner, that even when probing at wafer level, the data cannot be retrieved. Though the cost might go up by a few tens of cents, the amount of security these devices offer is really worth it.

Across product lines, the OEM must ensure that different keys are used for each of them. In this case, even if one of the products is compromised, it will lose effect on the others.

Also, communication with external devices, if possible, must be encrypted to prevent hacking of the system easily.

IoT Product Physical Security

Securing the IoT device physically is also a major consideration that is supposed to be taken into account while improving the security outlook. They include:

Closing Debug Ports – Debug serial ports, USB drivers etc., are used during the course of development. These features should be removed in the final release to reduce the surface of attack. In boot loaders such as Linux Uboot, break-on-user-input feature must be disabled to avoid manipulation of boot images.

JTAG/SWD connectors – Debugging tools such as debuggers and emulators that come in handy during board bring up must also be prevented from access in the field.

Firewall and closing ports – Network enabled products must run at least a minimal version of firewall. Also, closing of TCP/IP/UDP ports that are not in use is highly recommended. Using non-standard port numbers add a small level of security than that of using the standard ports.

Passwords – Many Linux systems have ubiquitous passwords such as root, admin, password, root123. These login credentials must be hardened and validated before product launch.

Mechanicals – Physical product designs can also include mechanisms that prevent easy device access as well as tamper detection mechanisms.

As an English phrase states – “a chain is as strong as its weakest link”, it is essential to ensure whether all the steps in the device operation is carefully reviewed and scrutinized. Security audits can be performed to profile the effectiveness of the design. Standards such as ISO/IEC 27001 can be a base to start with. But as mentioned earlier, these are all just outlines that aid in Securing Embedded Firmware for IoT Applications. Other device specific and domain related improvements must be built over it.

About Embien:

Embien is in the field of embedded product design since a decade and has helped customers realize products with all the features needed for today’s world.

For developing a secured embedded system for your business or to perform a security audit of your present design, get in touch with our team.

With deployment of IoT is spreading across various domains and applications, the requirements of the underlying communication mechanism varies. There is no one-size-fill-all solution as the needs are different in case of throughput, range, power consumption etc. There are many wireless communication technologies, such as Short-range wireless, Cellular, LPWAN etc.

LPWAN stands for Low Power Wide Area Network, designed for sending small data packages over long distances. While short range technologies like Bluetooth, Wi-Fi, Zigbee are cheap, it is limited by distance, cellular technologies like 3G, 4G and 5G have more transmission rate and range but are more costly and high power consuming. LPWAN has overcome the cons of existing wireless technology by compromising on the data rate and featuring the long-range data transmission, low power consumption and being economical. Some of the technologies that comes under LPWAN includes Narrowband IoT (NB-IoT), Sigfox, LoRa and others.

Heterogeneous Wireless communication Technologies

Of these LPWAN, LoRa has a significant market share and finds application across use cases.

Following are key features of LoRa Technology,

  • It has very wide coverage range about 5 km in urban areas and 15 km in suburban areas
  • Battery lifetime up to 15 years
  • One LoRa gateway takes care of thousands of nodes.
  • Easy to deploy and low cost.
  • Enhanced the secure data transmission by embedded end-to-end AES128 encryption

In this blog, we will cover the underlying technology behind LoRa and its network topology.

LoRa Technology

LoRa is a long range, low power, inexpensive technology for Internet of Things (IoT) developed by a company called Cycleo, France in 2009, later acquired by Semtech in 2012. The LoRa radio and modulation part is patented and its source is closed. Semtech has licensed its LoRa intellectual property to other chip manufacturers. The LoRa Alliance, an open, non-profit association has been formed to promote the adoption of this technology and has grown to more than 500 members since its inception in March 2015.

The most important aspect of the LoRa is that it uses license-free sub-gigahertz radio frequency ISM bands in the deployed region such as 868 MHz in Europe and 915MHz in North America. Thus, there is no need for a separate licensing for using LoRa in any country.

Usually in digital communication, there are three types of basic modulation techniques such as

Amplitude Shift Keying, Frequency Shift Keying and Phase Shift Keying, in which either amplitude or frequency or phase of the carrier varies according to the digital signal changes. The short coming with these approaches is that since the bandwidth is quiet limited the signal is quiet prone to interference and could be easily jammed. To over come this, spread spectrum techniques are being used where by the signal is modulated such that it is spread across the entire bandwidth. There are many spread spectrum techniques such as DSSS, FHSS, THSS, CSS etc.

Chirp Spread Spectrum

LoRa is a proprietary spread spectrum modulation scheme that is based on Chirp Spread Spectrum modulation (CSS). Chirp Spread Spectrum is a spread spectrum technique that uses wideband linear frequency modulated chirp pulses to encode information.A chirp is a sinusoidal signal whose frequency increases(up chirp) or decreases(down chirp) over time across the entire bandwidth. This signal is used as the carrier and is modulated according to the data to be transmitted.

LoRa uses three bandwidths: 125kHz, 250kHz and 500kHz. The chirp uses the entire bandwidth and the spreading factors are – in short – the duration of the chirp. LoRa operates with spread factors from 7 to 12. This delivers orthogonal transmissions at different data rates. Moreover it provides processing gain and hence transmitter output power can be reduced with same RF link budget and hence will increase battery life.

LoRa WAN

While LoRa is the underlying physical part, LoRaWAN is the network on which that LoRa operates. It is a media access control (MAC) in the data link layer that is maintained by the LoRa Alliance. LoRaWAN defines a set of rules and software that ensures data arrives with an acknowledgement and does not have duplicate packets. It is a network architecture that is deployed in a star topology and so the communication between the end node and gateway is bidirectional.

LoRaWAN defines role of end points and gateway. End points or End nodes are the remote nodes typically housing the sensors/actuators. Gateways or Concentrators forms the heart of the star topology, to which the end points communicate to.

Lora WAN Network Architecture

When an end node transmits data to the gateway, it is called an uplink. When the gateway transmits data to the end node, it is called a downlink. The gateways forward this packet to the network server. The network server collects the messages from all gateways and filters out the duplicate data and determines the gateway that has the best reception. The network server forwards the packet to the correct application server where the end user can process the sensor data. Optionally the application server can send a response back to the end node. When a response is sent, the network server receives the response and determines which gateway to use to broadcast the response back to the end node.

The LoRaWAN protocol defines the Adaptive Data Rate (ADR) scheme to control the uplink transmission parameters of LoRa devices. Whether the ADR functionality will be used is requested by the end nodes by setting the ADR flag in the uplink message. If the ADR flag is set, the network server can control the end node’s transmission parameters. ADR should only be used in stable Radio Frequency (RF) situations where end nodes do not move. Mobile end nodes which are stationary for longer times can enable ADR during those times.

This blog introduced the basics behind LoRa technology including the underlying communication techniques and network topology. In the next blog, we will cover the communication model in more detail including the classes, bands and also the typical configuration available in a gateway.

About Embien: Embien Technologies is a proven enabler in adoption of IoT. We have been working with different communication technologies such as ZigBee, BLE, SigFox, LoRa, NB-IoT and have designed gateways to inter-operate between them. Our services include end device development, gateways design, cloud application development and analytics.