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IoT Security - Physical and hardware security

Perry Lea

April 23, 2019

Perry LeaApril 23, 2019

Editor's Note: Securing the Internet of Things is critical not only for the integrity of data streams and software within each IoT application, but also for the integrity of the enterprise resources tied into those applications. IoT security is a complex problem, requiring a systematic approach for understanding possible threats and corresponding mitigation methods.

 

Adapted from Internet of Things for Architects, by Perry Lea.


Chapter 12. IoT Security (Continued)
By Perry Lea

Physical and hardware security

Many IoT deployments will be in remote and isolated areas leaving sensors and edge routers vulnerable to physical attack. Additionally, the hardware itself needs modern protection mechanisms common in processors and the circuitry of mobile devices and personal electronics.

Root of Trust

The first layer of hardware security is the establishment of a Root of Trust. The Root of Trust (RoT) is a hardware-validated boot process that ensures the first executable opcode starts from an immutable source. This is the anchor of the boot process that subsequently plays a role in bootstrapping the rest of the system from BIOS to the operating system to the application. A RoT is a baseline defense against a rootkit.

Each phase validates the next phase in the boot process and builds a Chain of Trust. An RoT can have different starting methods such as:

  • Boot from ROM or a non-writable memory to store the image and root key

  • One-time programmable memory using fuse bits for root key storage

  • Boot from a protected memory region that loads code into a protected memory store

An RoT also needs to validate each new phase of boot. Each phase of boot maintains a set of cryptographically signed keys that are used to verify the next phase of boot:

click for larger image

Establishing a Root of Trust. Here is a five-phase boot building up a Chain of Trust and starting with a Boot Loader in immutable read-only memory. Each phase maintains a public key that is used to verify the authenticity of the next component loaded.

Processors that support an RoT are architecturally unique. Intel and ARM support the following:

  • ARM TrustZone: ARM sells a security silicon IP block for SOC manufacturers that provides a hardware Root of Trust as well as other security services. TrustZone divides hardware into secure and non-secure "worlds". TrustZone is a separated microprocessor from the non-secure core. IT runs a Trusted OS specially designed for security that has a well-defined interface to the non-secure world. Protected assets and functions reside in the trusted core and should be lightweight by design. The switching between worlds is done through hardware context switching, eliminating the need for secure monitor software. Other uses for TrustZone are to manage system keys, credit card transactions, and Digital Rights Management. TrustZone is available for A "application" and M "microcontroller" CPUs. This form of secure CPU, Trusted OS, and RoT is called a Trusted Execution Environment (TEE).

  • Intel Boot Guard: This is a hardware-based mechanism that provides a verified boot which cryptographically verifies the initial boot block or uses a measuring process for validation. Boot Guard requires a manufacture to generate a 2048-bit key for verifying the initial block. The key is split into a private and public portion. The public key is imprinted by programmatically "blowing" fuse-bits during manufacturing. These are one-time fuses and immutable. The private portion generates the signature of the subsequently verified portion of the boot phase.

Key management and trusted platform modules

Public and private keys are critical to ensuring a secure system. The keys themselves need proper management to ensure their safety. There are hardware standards for key security and one particularly popular mechanism is the Trusted Platform Module (TPM). The specification for TPM was written by the Trusted Computing Group and is an ISO and IEC standard. The current specification is TPM 2.0 released in September of 2016. Computer assets sold to the DoD require TPM 1.2.

A TPM is a discrete hardware component with a secret RSA key burned into the device at manufacturing.

Generally, a TPM is used to hold, secure, and manage other keys for services such as disk encryption, Root of Trust booting, verifying the authenticity of hardware (as well as software), and password management. A TPM can create a hash of the litany of software and hardware in a "known good" configuration that can be used to verify tampering at runtime. Additional services include assisting in SHA-1 and SHA-256 hashing, AES encryption blocks, asymmetric encryption, and random number generation. Several vendors produce TPM devices such as Broadcom, Nation Semiconductor, and Texas Instruments.

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