5.2.2. Secure boot

A secure boot scheme adds cryptographic checks to each stage of the Secure world boot process. This process aims to assert the integrity of all of the Secure world software images that are executed, preventing any unauthorized or maliciously modified software from running.

Cryptographic signature protocol

The most logical cryptographic protocol to apply is one based on a public-key signature algorithm, such as RSA-PSS (Rivest, Shamir and Adleman - Probabilistic Signature Scheme).

In these protocols a trusted vendor uses their Private Key (PrK) to generate a signature of the code that they want to deploy, and pushes this to the device alongside the software binary. The device contains the Public Key (PuK) of the vendor, which can be used to verify that the binary has not been modified and that it was provided by the trusted vendor in question.

The PuK does not need to be kept confidential, but it does need to be stored within the device in a manner which means it cannot be replaced by a PuK that belongs to an attacker.

Chain of trust

The secure boot process implements a chain of trust. Starting with an implicitly trusted component, every other component can be authenticated before being executed. The ownership of the chain can change at each stage - a PuK belonging to the device OEM might be used to authenticate the first bootloader, but the Secure world OS binary might include a secondary PuK that is used to authenticate the applications that it loads.

Unless a design can discount hardware shack attacks the foundations of the secure boot process, known as the root of trust, must be located in the on-SoC ROM. The SoC ROM is the only component in the system that cannot be trivially modified or replaced by simple reprogramming attacks.

Storage of the PuK for the root of trust can be problematic; embedding it in the on-SoC ROM implies that all devices use the same PuK. This makes them vulnerable to class-break attacks if the PrK is stolen or successfully reverse-engineered. On-SoC One-Time-Programmable (OTP) hardware, such as poly-silicon fuses, can be used to store unique values in each SoC during device manufacture. This enables a number of different PuK values to be stored in a single class of devices, reducing risk of class break attacks.

Note

OTP memory can consume considerable silicon area, so the number of bits are that available is typically limited. A RSA PuK is over 1024-bits long, which is typically too large to fit in the available OTP storage. However, as the PuK is not confidential it can be stored in off-SoC storage, provided that a cryptographic hash of the PuK is stored on-SoC in the OTP. The hash is much smaller than the PuK itself (256-bits for a SHA256 hash), and can be used to authenticate the value of the PuK at run-time.

On-SoC Secure world or Off-SoC Secure world

The simplest defense against shack attacks is to keep any Secure world resource execution located in on-SoC memory locations. If the code and data is never exposed outside of the SoC package it becomes significantly more difficult to snoop or modify data values; a physical attack on the SoC package is much harder than connecting a logic probe to a PCB track or a package pin.

The secure boot code is generally responsible for loading code into the on-SoC memory, and it is critical to correctly order the authentication to avoid introducing a window of opportunity for an attacker. Assuming the running code and required cryptographic hashes are already in safe on-SoC memory, the binary or PuK being verified should be copied to a secure location before being authenticated using cryptographic methods. A design that authenticates an image, and then copies it into the safe memory location risks attack. The attacker can modify the image in the short window between the check completing and the copy taking place.

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