Trusted Board Boot Design Guide

Contents :

Introduction
Chain of Trust
Trusted Board Boot Sequence
Authentication Module
Certificate Generation Tool

1. Introduction

The Trusted Board Boot (TBB) feature prevents malicious firmware from running on the platform by authenticating all firmware images up to and including the normal world bootloader. It does this by establishing a Chain of Trust using Public-Key-Cryptography Standards (PKCS).

This document describes the design of the ARM Trusted Firmware TBB implementation. The current implementation is a proof of concept; future versions will provide stronger architectural interfaces and implement the missing functionality required in a production TBB-enabled system.

2. Chain of Trust

A Chain of Trust (CoT) starts with a set of implicitly trusted components. On the ARM development platforms, these components are:

A SHA-256 hash of the Root of Trust Public Key (ROTPK). It is stored in the trusted root-key storage registers.
The BL1 image, on the assumption that it resides in ROM so cannot be tampered with.

The remaining components in the CoT are either certificates or boot loader images. The certificates follow the [X.509 v3] standard. This standard enables adding custom extensions to the certificates, which are used to store essential information to establish the CoT.

In the TBB CoT all certificates are self-signed. There is no need for a Certificate Authority (CA) because the CoT is not established by verifying the validity of a certificate's issuer but by the content of the certificate extensions. To sign the certificates, the PKCS#1 SHA-1 with RSA Encryption signature scheme is used with a RSA key length of 2048 bits. Future version of Trusted Firmware will replace SHA-1 usage with SHA-256 and support additional cryptographic algorithms.

The certificates are categorised as "Key" and "Content" certificates. Key certificates are used to verify public keys which have been used to sign content certificates. Content certificates are used to store the hash of a boot loader image. An image can be authenticated by calculating its hash and matching it with the hash extracted from the content certificate. The SHA-256 function is used to calculate all hashes. The public keys and hashes are included as non-standard extension fields in the [X.509 v3] certificates.

The keys used to establish the CoT are:

Root of trust key

The private part of this key is used to sign the BL2 content certificate and the trusted key certificate. The public part is the ROTPK.
Trusted world key

The private part is used to sign the key certificates corresponding to the secure world images (BL3-0, BL3-1 and BL3-2). The public part is stored in one of the extension fields in the trusted world certificate.
Non-trusted world key

The private part is used to sign the key certificate corresponding to the non secure world image (BL3-3). The public part is stored in one of the extension fields in the trusted world certificate.
BL3-X keys

For each of BL3-0, BL3-1, BL3-2 and BL3-3, the private part is used to sign the content certificate for the BL3-X image. The public part is stored in one of the extension fields in the corresponding key certificate.

The following images are included in the CoT:

BL1
BL2
BL3-0 (optional)
BL3-1
BL3-3
BL3-2 (optional)

The following certificates are used to authenticate the images.

BL2 content certificate

It is self-signed with the private part of the ROT key. It contains a hash of the BL2 image.
Trusted key certificate

It is self-signed with the private part of the ROT key. It contains the public part of the trusted world key and the public part of the non-trusted world key.
BL3-0 key certificate

It is self-signed with the trusted world key. It contains the public part of the BL3-0 key.
BL3-0 content certificate

It is self-signed with the BL3-0 key. It contains a hash of the BL3-0 image.
BL3-1 key certificate

It is self-signed with the trusted world key. It contains the public part of the BL3-1 key.
BL3-1 content certificate

It is self-signed with the BL3-1 key. It contains a hash of the BL3-1 image.
BL3-2 key certificate

It is self-signed with the trusted world key. It contains the public part of the BL3-2 key.
BL3-2 content certificate

It is self-signed with the BL3-2 key. It contains a hash of the BL3-2 image.
BL3-3 key certificate

It is self-signed with the non-trusted world key. It contains the public part of the BL3-3 key.
BL3-3 content certificate

It is self-signed with the BL3-3 key. It contains a hash of the BL3-3 image.

The BL3-0 and BL3-2 certificates are optional, but they must be present if the corresponding BL3-0 or BL3-2 images are present.

3. Trusted Board Boot Sequence

The CoT is verified through the following sequence of steps. The system panics if any of the steps fail.

BL1 loads and verifies the BL2 content certificate. The issuer public key is read from the verified certificate. A hash of that key is calculated and compared with the hash of the ROTPK read from the trusted root-key storage registers. If they match, the BL2 hash is read from the certificate.

Note: the matching operation is platform specific and is currently unimplemented on the ARM development platforms.
BL1 loads the BL2 image. Its hash is calculated and compared with the hash read from the certificate. Control is transferred to the BL2 image if all the comparisons succeed.
BL2 loads and verifies the trusted key certificate. The issuer public key is read from the verified certificate. A hash of that key is calculated and compared with the hash of the ROTPK read from the trusted root-key storage registers. If the comparison succeeds, BL2 reads and saves the trusted and non-trusted world public keys from the verified certificate.

The next two steps are executed for each of the BL3-0, BL3-1 & BL3-2 images. The steps for the optional BL3-0 and BL3-2 images are skipped if these images are not present.

BL2 loads and verifies the BL3-x key certificate. The certificate signature is verified using the trusted world public key. If the signature verification succeeds, BL2 reads and saves the BL3-x public key from the certificate.
BL2 loads and verifies the BL3-x content certificate. The signature is verified using the BL3-x public key. If the signature verification succeeds, BL2 reads and saves the BL3-x image hash from the certificate.

The next two steps are executed only for the BL3-3 image.

BL2 loads and verifies the BL3-3 key certificate. If the signature verification succeeds, BL2 reads and saves the BL3-3 public key from the certificate.
BL2 loads and verifies the BL3-3 content certificate. If the signature verification succeeds, BL2 reads and saves the BL3-3 image hash from the certificate.

The next step is executed for all the boot loader images.

BL2 calculates the hash of each image. It compares it with the hash obtained from the corresponding content certificate. The image authentication succeeds if the hashes match.

The Trusted Board Boot implementation spans both generic and platform-specific BL1 and BL2 code, and in tool code on the host build machine. The feature is enabled through use of specific build flags as described in the [User Guide].

On the host machine, a tool generates the certificates, which are included in the FIP along with the boot loader images. These certificates are loaded in Trusted SRAM using the IO storage framework. They are then verified by an Authentication module included in the Trusted Firmware.

The mechanism used for generating the FIP and the Authentication module are described in the following sections.

4. Authentication Module

The authentication module implements the required support to authenticate the corresponding certificates or images at each step in the Trusted Board Boot sequence. The module relies on the PolarSSL library (v1.3.9) to perform the following operations:

Parsing X.509 certificates and verifying them using SHA-1 with RSA Encryption.
Extracting public keys and hashes from the certificates.
Generating hashes (SHA-256) of boot loader images

At each step, the module is responsible for allocating memory to store the public keys or hashes that will be used in later steps. The step identifier is used to determine what information must be saved, according to the CoT model detailed in the previous sections.

The authentication module resides in the common/auth/polarssl directory. Instructions for including the necessary modules of the PolarSSL SSL library and building the authentication module can be found in the [User Guide].

5. Certificate Generation Tool

The cert_create tool is built and runs on the host machine as part of the Trusted Firmware build process when GENERATE_COT=1. It takes the boot loader images and keys as inputs (keys must be in PEM format) and generates the certificates (in DER format) required to establish the CoT. New keys can be generated by the tool in case they are not provided. The certificates are then passed as inputs to the fip_create tool for creating the FIP.

The certificates are also stored individually in the in the output build directory.

The tool resides in the tools/cert_create directory. It uses OpenSSL SSL library version 1.0.1 or later to generate the X.509 certificates. Instructions for building and using the tool can be found in the [User Guide].