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Trusted Board Boot Design Guide

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 Trusted Firmware-A (TF-A) TBB, which is an implementation of the Trusted Board Boot Requirements (TBBR) specification, Arm DEN0006C-1. It should be used in conjunction with the Firmware Update design document, which implements a specific aspect of the TBBR.

1.   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-256 with RSA Encryption signature scheme is used with a RSA key length of 2048 bits. Future version of TF-A will 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 (SCP_BL2, BL31 and BL32). 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 (BL33). The public part is stored in one of the extension fields in the trusted world certificate.

  • BL3-X keys

    For each of SCP_BL2, BL31, BL32 and BL33, 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
  • SCP_BL2 (optional)
  • BL31
  • BL33
  • BL32 (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.

  • SCP_BL2 key certificate

    It is self-signed with the trusted world key. It contains the public part of the SCP_BL2 key.

  • SCP_BL2 content certificate

    It is self-signed with the SCP_BL2 key. It contains a hash of the SCP_BL2 image.

  • BL31 key certificate

    It is self-signed with the trusted world key. It contains the public part of the BL31 key.

  • BL31 content certificate

    It is self-signed with the BL31 key. It contains a hash of the BL31 image.

  • BL32 key certificate

    It is self-signed with the trusted world key. It contains the public part of the BL32 key.

  • BL32 content certificate

    It is self-signed with the BL32 key. It contains a hash of the BL32 image.

  • BL33 key certificate

    It is self-signed with the non-trusted world key. It contains the public part of the BL33 key.

  • BL33 content certificate

    It is self-signed with the BL33 key. It contains a hash of the BL33 image.

The SCP_BL2 and BL32 certificates are optional, but they must be present if the corresponding SCP_BL2 or BL32 images are present.

2.   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 SCP_BL2, BL31 & BL32 images. The steps for the optional SCP_BL2 and BL32 images are skipped if these images are not present.

  • BL2 loads and verifies the BL3x key certificate. The certificate signature is verified using the trusted world public key. If the signature verification succeeds, BL2 reads and saves the BL3x public key from the certificate.
  • BL2 loads and verifies the BL3x content certificate. The signature is verified using the BL3x public key. If the signature verification succeeds, BL2 reads and saves the BL3x image hash from the certificate.

The next two steps are executed only for the BL33 image.

  • BL2 loads and verifies the BL33 key certificate. If the signature verification succeeds, BL2 reads and saves the BL33 public key from the certificate.
  • BL2 loads and verifies the BL33 content certificate. If the signature verification succeeds, BL2 reads and saves the BL33 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 TF-A.

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

3.   Authentication Framework

The authentication framework included in TF-A provides support to implement the desired trusted boot sequence. Arm platforms use this framework to implement the boot requirements specified in the TBBR-client document.

More information about the authentication framework can be found in the Auth Framework document.

4.   Certificate Generation Tool

The cert_create tool is built and runs on the host machine as part of the TF-A 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 fiptool utility 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.


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