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1Trusted Board Boot Design Guide
2===============================
3
4
5.. section-numbering::
6    :suffix: .
7
8.. contents::
9
10The Trusted Board Boot (TBB) feature prevents malicious firmware from running on
11the platform by authenticating all firmware images up to and including the
12normal world bootloader. It does this by establishing a Chain of Trust using
13Public-Key-Cryptography Standards (PKCS).
14
15This document describes the design of ARM Trusted Firmware TBB, which is an
16implementation of the Trusted Board Boot Requirements (TBBR) specification,
17ARM DEN0006C-1. It should be used in conjunction with the `Firmware Update`_
18design document, which implements a specific aspect of the TBBR.
19
20Chain of Trust
21--------------
22
23A Chain of Trust (CoT) starts with a set of implicitly trusted components. On
24the ARM development platforms, these components are:
25
26-  A SHA-256 hash of the Root of Trust Public Key (ROTPK). It is stored in the
27   trusted root-key storage registers.
28
29-  The BL1 image, on the assumption that it resides in ROM so cannot be
30   tampered with.
31
32The remaining components in the CoT are either certificates or boot loader
33images. The certificates follow the `X.509 v3`_ standard. This standard
34enables adding custom extensions to the certificates, which are used to store
35essential information to establish the CoT.
36
37In the TBB CoT all certificates are self-signed. There is no need for a
38Certificate Authority (CA) because the CoT is not established by verifying the
39validity of a certificate's issuer but by the content of the certificate
40extensions. To sign the certificates, the PKCS#1 SHA-256 with RSA Encryption
41signature scheme is used with a RSA key length of 2048 bits. Future version of
42Trusted Firmware will support additional cryptographic algorithms.
43
44The certificates are categorised as "Key" and "Content" certificates. Key
45certificates are used to verify public keys which have been used to sign content
46certificates. Content certificates are used to store the hash of a boot loader
47image. An image can be authenticated by calculating its hash and matching it
48with the hash extracted from the content certificate. The SHA-256 function is
49used to calculate all hashes. The public keys and hashes are included as
50non-standard extension fields in the `X.509 v3`_ certificates.
51
52The keys used to establish the CoT are:
53
54-  **Root of trust key**
55
56   The private part of this key is used to sign the BL2 content certificate and
57   the trusted key certificate. The public part is the ROTPK.
58
59-  **Trusted world key**
60
61   The private part is used to sign the key certificates corresponding to the
62   secure world images (SCP\_BL2, BL31 and BL32). The public part is stored in
63   one of the extension fields in the trusted world certificate.
64
65-  **Non-trusted world key**
66
67   The private part is used to sign the key certificate corresponding to the
68   non secure world image (BL33). The public part is stored in one of the
69   extension fields in the trusted world certificate.
70
71-  **BL3-X keys**
72
73   For each of SCP\_BL2, BL31, BL32 and BL33, the private part is used to
74   sign the content certificate for the BL3-X image. The public part is stored
75   in one of the extension fields in the corresponding key certificate.
76
77The following images are included in the CoT:
78
79-  BL1
80-  BL2
81-  SCP\_BL2 (optional)
82-  BL31
83-  BL33
84-  BL32 (optional)
85
86The following certificates are used to authenticate the images.
87
88-  **BL2 content certificate**
89
90   It is self-signed with the private part of the ROT key. It contains a hash
91   of the BL2 image.
92
93-  **Trusted key certificate**
94
95   It is self-signed with the private part of the ROT key. It contains the
96   public part of the trusted world key and the public part of the non-trusted
97   world key.
98
99-  **SCP\_BL2 key certificate**
100
101   It is self-signed with the trusted world key. It contains the public part of
102   the SCP\_BL2 key.
103
104-  **SCP\_BL2 content certificate**
105
106   It is self-signed with the SCP\_BL2 key. It contains a hash of the SCP\_BL2
107   image.
108
109-  **BL31 key certificate**
110
111   It is self-signed with the trusted world key. It contains the public part of
112   the BL31 key.
113
114-  **BL31 content certificate**
115
116   It is self-signed with the BL31 key. It contains a hash of the BL31 image.
117
118-  **BL32 key certificate**
119
120   It is self-signed with the trusted world key. It contains the public part of
121   the BL32 key.
122
123-  **BL32 content certificate**
124
125   It is self-signed with the BL32 key. It contains a hash of the BL32 image.
126
127-  **BL33 key certificate**
128
129   It is self-signed with the non-trusted world key. It contains the public
130   part of the BL33 key.
131
132-  **BL33 content certificate**
133
134   It is self-signed with the BL33 key. It contains a hash of the BL33 image.
135
136The SCP\_BL2 and BL32 certificates are optional, but they must be present if the
137corresponding SCP\_BL2 or BL32 images are present.
138
139Trusted Board Boot Sequence
140---------------------------
141
142The CoT is verified through the following sequence of steps. The system panics
143if any of the steps fail.
144
145-  BL1 loads and verifies the BL2 content certificate. The issuer public key is
146   read from the verified certificate. A hash of that key is calculated and
147   compared with the hash of the ROTPK read from the trusted root-key storage
148   registers. If they match, the BL2 hash is read from the certificate.
149
150   Note: the matching operation is platform specific and is currently
151   unimplemented on the ARM development platforms.
152
153-  BL1 loads the BL2 image. Its hash is calculated and compared with the hash
154   read from the certificate. Control is transferred to the BL2 image if all
155   the comparisons succeed.
156
157-  BL2 loads and verifies the trusted key certificate. The issuer public key is
158   read from the verified certificate. A hash of that key is calculated and
159   compared with the hash of the ROTPK read from the trusted root-key storage
160   registers. If the comparison succeeds, BL2 reads and saves the trusted and
161   non-trusted world public keys from the verified certificate.
162
163The next two steps are executed for each of the SCP\_BL2, BL31 & BL32 images.
164The steps for the optional SCP\_BL2 and BL32 images are skipped if these images
165are not present.
166
167-  BL2 loads and verifies the BL3x key certificate. The certificate signature
168   is verified using the trusted world public key. If the signature
169   verification succeeds, BL2 reads and saves the BL3x public key from the
170   certificate.
171
172-  BL2 loads and verifies the BL3x content certificate. The signature is
173   verified using the BL3x public key. If the signature verification succeeds,
174   BL2 reads and saves the BL3x image hash from the certificate.
175
176The next two steps are executed only for the BL33 image.
177
178-  BL2 loads and verifies the BL33 key certificate. If the signature
179   verification succeeds, BL2 reads and saves the BL33 public key from the
180   certificate.
181
182-  BL2 loads and verifies the BL33 content certificate. If the signature
183   verification succeeds, BL2 reads and saves the BL33 image hash from the
184   certificate.
185
186The next step is executed for all the boot loader images.
187
188-  BL2 calculates the hash of each image. It compares it with the hash obtained
189   from the corresponding content certificate. The image authentication succeeds
190   if the hashes match.
191
192The Trusted Board Boot implementation spans both generic and platform-specific
193BL1 and BL2 code, and in tool code on the host build machine. The feature is
194enabled through use of specific build flags as described in the `User Guide`_.
195
196On the host machine, a tool generates the certificates, which are included in
197the FIP along with the boot loader images. These certificates are loaded in
198Trusted SRAM using the IO storage framework. They are then verified by an
199Authentication module included in the Trusted Firmware.
200
201The mechanism used for generating the FIP and the Authentication module are
202described in the following sections.
203
204Authentication Framework
205------------------------
206
207The authentication framework included in the Trusted Firmware provides support
208to implement the desired trusted boot sequence. ARM platforms use this framework
209to implement the boot requirements specified in the TBBR-client document.
210
211More information about the authentication framework can be found in the
212`Auth Framework`_ document.
213
214Certificate Generation Tool
215---------------------------
216
217The ``cert_create`` tool is built and runs on the host machine as part of the
218Trusted Firmware build process when ``GENERATE_COT=1``. It takes the boot loader
219images and keys as inputs (keys must be in PEM format) and generates the
220certificates (in DER format) required to establish the CoT. New keys can be
221generated by the tool in case they are not provided. The certificates are then
222passed as inputs to the ``fiptool`` utility for creating the FIP.
223
224The certificates are also stored individually in the in the output build
225directory.
226
227The tool resides in the ``tools/cert_create`` directory. It uses OpenSSL SSL
228library version 1.0.1 or later to generate the X.509 certificates. Instructions
229for building and using the tool can be found in the `User Guide`_.
230
231--------------
232
233*Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.*
234
235.. _Firmware Update: firmware-update.rst
236.. _X.509 v3: http://www.ietf.org/rfc/rfc5280.txt
237.. _User Guide: user-guide.rst
238.. _Auth Framework: auth-framework.rst
239