1Trusted Board Boot 2================== 3 4The Trusted Board Boot (TBB) feature prevents malicious firmware from running on 5the platform by authenticating all firmware images up to and including the 6normal world bootloader. It does this by establishing a Chain of Trust using 7Public-Key-Cryptography Standards (PKCS). 8 9This document describes the design of Trusted Firmware-A (TF-A) TBB, which is an 10implementation of the `Trusted Board Boot Requirements (TBBR)`_ specification, 11Arm DEN0006D. It should be used in conjunction with the `Firmware Update`_ 12design document, which implements a specific aspect of the TBBR. 13 14Chain of Trust 15-------------- 16 17A Chain of Trust (CoT) starts with a set of implicitly trusted components. On 18the Arm development platforms, these components are: 19 20- A SHA-256 hash of the Root of Trust Public Key (ROTPK). It is stored in the 21 trusted root-key storage registers. 22 23- The BL1 image, on the assumption that it resides in ROM so cannot be 24 tampered with. 25 26The remaining components in the CoT are either certificates or boot loader 27images. The certificates follow the `X.509 v3`_ standard. This standard 28enables adding custom extensions to the certificates, which are used to store 29essential information to establish the CoT. 30 31In the TBB CoT all certificates are self-signed. There is no need for a 32Certificate Authority (CA) because the CoT is not established by verifying the 33validity of a certificate's issuer but by the content of the certificate 34extensions. To sign the certificates, the PKCS#1 SHA-256 with RSA Encryption 35signature scheme is used with a RSA key length of 2048 bits. Future version of 36TF-A will support additional cryptographic algorithms. 37 38The certificates are categorised as "Key" and "Content" certificates. Key 39certificates are used to verify public keys which have been used to sign content 40certificates. Content certificates are used to store the hash of a boot loader 41image. An image can be authenticated by calculating its hash and matching it 42with the hash extracted from the content certificate. The SHA-256 function is 43used to calculate all hashes. The public keys and hashes are included as 44non-standard extension fields in the `X.509 v3`_ certificates. 45 46The keys used to establish the CoT are: 47 48- **Root of trust key** 49 50 The private part of this key is used to sign the BL2 content certificate and 51 the trusted key certificate. The public part is the ROTPK. 52 53- **Trusted world key** 54 55 The private part is used to sign the key certificates corresponding to the 56 secure world images (SCP_BL2, BL31 and BL32). The public part is stored in 57 one of the extension fields in the trusted world certificate. 58 59- **Non-trusted world key** 60 61 The private part is used to sign the key certificate corresponding to the 62 non secure world image (BL33). The public part is stored in one of the 63 extension fields in the trusted world certificate. 64 65- **BL3-X keys** 66 67 For each of SCP_BL2, BL31, BL32 and BL33, the private part is used to 68 sign the content certificate for the BL3-X image. The public part is stored 69 in one of the extension fields in the corresponding key certificate. 70 71The following images are included in the CoT: 72 73- BL1 74- BL2 75- SCP_BL2 (optional) 76- BL31 77- BL33 78- BL32 (optional) 79 80The following certificates are used to authenticate the images. 81 82- **BL2 content certificate** 83 84 It is self-signed with the private part of the ROT key. It contains a hash 85 of the BL2 image. 86 87- **Trusted key certificate** 88 89 It is self-signed with the private part of the ROT key. It contains the 90 public part of the trusted world key and the public part of the non-trusted 91 world key. 92 93- **SCP_BL2 key certificate** 94 95 It is self-signed with the trusted world key. It contains the public part of 96 the SCP_BL2 key. 97 98- **SCP_BL2 content certificate** 99 100 It is self-signed with the SCP_BL2 key. It contains a hash of the SCP_BL2 101 image. 102 103- **BL31 key certificate** 104 105 It is self-signed with the trusted world key. It contains the public part of 106 the BL31 key. 107 108- **BL31 content certificate** 109 110 It is self-signed with the BL31 key. It contains a hash of the BL31 image. 111 112- **BL32 key certificate** 113 114 It is self-signed with the trusted world key. It contains the public part of 115 the BL32 key. 116 117- **BL32 content certificate** 118 119 It is self-signed with the BL32 key. It contains a hash of the BL32 image. 120 121- **BL33 key certificate** 122 123 It is self-signed with the non-trusted world key. It contains the public 124 part of the BL33 key. 125 126- **BL33 content certificate** 127 128 It is self-signed with the BL33 key. It contains a hash of the BL33 image. 129 130The SCP_BL2 and BL32 certificates are optional, but they must be present if the 131corresponding SCP_BL2 or BL32 images are present. 132 133Trusted Board Boot Sequence 134--------------------------- 135 136The CoT is verified through the following sequence of steps. The system panics 137if any of the steps fail. 138 139- BL1 loads and verifies the BL2 content certificate. The issuer public key is 140 read from the verified certificate. A hash of that key is calculated and 141 compared with the hash of the ROTPK read from the trusted root-key storage 142 registers. If they match, the BL2 hash is read from the certificate. 143 144 .. note:: 145 The matching operation is platform specific and is currently 146 unimplemented on the Arm development platforms. 147 148- BL1 loads the BL2 image. Its hash is calculated and compared with the hash 149 read from the certificate. Control is transferred to the BL2 image if all 150 the comparisons succeed. 151 152- BL2 loads and verifies the trusted key certificate. The issuer public key is 153 read from the verified certificate. A hash of that key is calculated and 154 compared with the hash of the ROTPK read from the trusted root-key storage 155 registers. If the comparison succeeds, BL2 reads and saves the trusted and 156 non-trusted world public keys from the verified certificate. 157 158The next two steps are executed for each of the SCP_BL2, BL31 & BL32 images. 159The steps for the optional SCP_BL2 and BL32 images are skipped if these images 160are not present. 161 162- BL2 loads and verifies the BL3x key certificate. The certificate signature 163 is verified using the trusted world public key. If the signature 164 verification succeeds, BL2 reads and saves the BL3x public key from the 165 certificate. 166 167- BL2 loads and verifies the BL3x content certificate. The signature is 168 verified using the BL3x public key. If the signature verification succeeds, 169 BL2 reads and saves the BL3x image hash from the certificate. 170 171The next two steps are executed only for the BL33 image. 172 173- BL2 loads and verifies the BL33 key certificate. If the signature 174 verification succeeds, BL2 reads and saves the BL33 public key from the 175 certificate. 176 177- BL2 loads and verifies the BL33 content certificate. If the signature 178 verification succeeds, BL2 reads and saves the BL33 image hash from the 179 certificate. 180 181The next step is executed for all the boot loader images. 182 183- BL2 calculates the hash of each image. It compares it with the hash obtained 184 from the corresponding content certificate. The image authentication succeeds 185 if the hashes match. 186 187The Trusted Board Boot implementation spans both generic and platform-specific 188BL1 and BL2 code, and in tool code on the host build machine. The feature is 189enabled through use of specific build flags as described in the `User Guide`_. 190 191On the host machine, a tool generates the certificates, which are included in 192the FIP along with the boot loader images. These certificates are loaded in 193Trusted SRAM using the IO storage framework. They are then verified by an 194Authentication module included in TF-A. 195 196The mechanism used for generating the FIP and the Authentication module are 197described in the following sections. 198 199Authentication Framework 200------------------------ 201 202The authentication framework included in TF-A provides support to implement 203the desired trusted boot sequence. Arm platforms use this framework to 204implement the boot requirements specified in the `TBBR-client`_ document. 205 206More information about the authentication framework can be found in the 207`Auth Framework`_ document. 208 209Certificate Generation Tool 210--------------------------- 211 212The ``cert_create`` tool is built and runs on the host machine as part of the 213TF-A build process when ``GENERATE_COT=1``. It takes the boot loader images 214and keys as inputs (keys must be in PEM format) and generates the 215certificates (in DER format) required to establish the CoT. New keys can be 216generated by the tool in case they are not provided. The certificates are then 217passed as inputs to the ``fiptool`` utility for creating the FIP. 218 219The certificates are also stored individually in the in the output build 220directory. 221 222The tool resides in the ``tools/cert_create`` directory. It uses OpenSSL SSL 223library version 1.0.1 or later to generate the X.509 certificates. Instructions 224for building and using the tool can be found in the `User Guide`_. 225 226-------------- 227 228*Copyright (c) 2015-2019, Arm Limited and Contributors. All rights reserved.* 229 230.. _Firmware Update: firmware-update.rst 231.. _X.509 v3: https://tools.ietf.org/rfc/rfc5280.txt 232.. _User Guide: ../getting_started/user-guide.rst 233.. _Auth Framework: auth-framework.rst 234.. _TBBR-client: https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a 235.. _Trusted Board Boot Requirements (TBBR): `TBBR-client`_ 236