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+Trusted Board Boot Design Guide
+===============================
+
+
+
+
+.. contents::
+
+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 DEN0006D. It should be used in conjunction with the `Firmware Update`_
+design document, which implements a specific aspect of the TBBR.
+
+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.
+
+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.
+
+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.
+
+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`_.
+
+--------------
+
+*Copyright (c) 2015-2019, Arm Limited and Contributors. All rights reserved.*
+
+.. _Firmware Update: firmware-update.rst
+.. _X.509 v3: https://tools.ietf.org/rfc/rfc5280.txt
+.. _User Guide: ../getting_started/user-guide.rst
+.. _Auth Framework: auth-framework.rst
+.. _TBBR-client: https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a
+.. _Trusted Board Boot Requirements (TBBR): `TBBR-client`_