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938 lines
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ReStructuredText
Abstracting a Chain of Trust
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============================
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.. section-numbering::
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:suffix: .
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.. contents::
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The aim of this document is to describe the authentication framework implemented
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in the Trusted Firmware. This framework fulfills the following requirements:
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#. It should be possible for a platform port to specify the Chain of Trust in
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terms of certificate hierarchy and the mechanisms used to verify a
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particular image/certificate.
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#. The framework should distinguish between:
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- The mechanism used to encode and transport information, e.g. DER encoded
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X.509v3 certificates to ferry Subject Public Keys, hashes and non-volatile
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counters.
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- The mechanism used to verify the transported information i.e. the
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cryptographic libraries.
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The framework has been designed following a modular approach illustrated in the
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next diagram:
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::
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+---------------+---------------+------------+
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| Trusted | Trusted | Trusted |
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| Firmware | Firmware | Firmware |
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| Generic | IO Framework | Platform |
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| Code i.e. | (IO) | Port |
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| BL1/BL2 (GEN) | | (PP) |
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+---------------+---------------+------------+
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^ ^ ^
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| | |
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v v v
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+-----------+ +-----------+ +-----------+
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| | | | | Image |
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| Crypto | | Auth | | Parser |
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| Module |<->| Module |<->| Module |
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| (CM) | | (AM) | | (IPM) |
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| | | | | |
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+-----------+ +-----------+ +-----------+
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^ ^
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| |
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v v
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+----------------+ +-----------------+
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| Cryptographic | | Image Parser |
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| Libraries (CL) | | Libraries (IPL) |
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+----------------+ +-----------------+
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| |
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v v
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+-----------------+
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| Misc. Libs e.g. |
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| ASN.1 decoder |
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+-----------------+
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DIAGRAM 1.
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This document describes the inner details of the authentication framework and
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the abstraction mechanisms available to specify a Chain of Trust.
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Framework design
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----------------
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This section describes some aspects of the framework design and the rationale
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behind them. These aspects are key to verify a Chain of Trust.
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Chain of Trust
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~~~~~~~~~~~~~~
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A CoT is basically a sequence of authentication images which usually starts with
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a root of trust and culminates in a single data image. The following diagram
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illustrates how this maps to a CoT for the BL31 image described in the
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TBBR-Client specification.
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::
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+------------------+ +-------------------+
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| ROTPK/ROTPK Hash |------>| Trusted Key |
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+------------------+ | Certificate |
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| (Auth Image) |
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/+-------------------+
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/ |
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/ |
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/ |
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/ |
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L v
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+------------------+ +-------------------+
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| Trusted World |------>| BL31 Key |
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| Public Key | | Certificate |
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+------------------+ | (Auth Image) |
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+-------------------+
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/ |
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/ |
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/ |
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/ |
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/ v
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+------------------+ L +-------------------+
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| BL31 Content |------>| BL31 Content |
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| Certificate PK | | Certificate |
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+------------------+ | (Auth Image) |
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+-------------------+
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/ |
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/ |
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/ |
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/ |
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/ v
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+------------------+ L +-------------------+
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| BL31 Hash |------>| BL31 Image |
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| | | (Data Image) |
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+------------------+ | |
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+-------------------+
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DIAGRAM 2.
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The root of trust is usually a public key (ROTPK) that has been burnt in the
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platform and cannot be modified.
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Image types
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~~~~~~~~~~~
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Images in a CoT are categorised as authentication and data images. An
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authentication image contains information to authenticate a data image or
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another authentication image. A data image is usually a boot loader binary, but
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it could be any other data that requires authentication.
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Component responsibilities
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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For every image in a Chain of Trust, the following high level operations are
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performed to verify it:
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#. Allocate memory for the image either statically or at runtime.
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#. Identify the image and load it in the allocated memory.
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#. Check the integrity of the image as per its type.
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#. Authenticate the image as per the cryptographic algorithms used.
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#. If the image is an authentication image, extract the information that will
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be used to authenticate the next image in the CoT.
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In Diagram 1, each component is responsible for one or more of these operations.
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The responsibilities are briefly described below.
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TF Generic code and IO framework (GEN/IO)
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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These components are responsible for initiating the authentication process for a
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particular image in BL1 or BL2. For each BL image that requires authentication,
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the Generic code asks recursively the Authentication module what is the parent
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image until either an authenticated image or the ROT is reached. Then the
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Generic code calls the IO framewotk to load the image and calls the
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Authentication module to authenticate it, following the CoT from ROT to Image.
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TF Platform Port (PP)
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^^^^^^^^^^^^^^^^^^^^^
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The platform is responsible for:
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#. Specifying the CoT for each image that needs to be authenticated. Details of
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how a CoT can be specified by the platform are explained later. The platform
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also specifies the authentication methods and the parsing method used for
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each image.
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#. Statically allocating memory for each parameter in each image which is
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used for verifying the CoT, e.g. memory for public keys, hashes etc.
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#. Providing the ROTPK or a hash of it.
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#. Providing additional information to the IPM to enable it to identify and
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extract authentication parameters contained in an image, e.g. if the
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parameters are stored as X509v3 extensions, the corresponding OID must be
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provided.
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#. Fulfill any other memory requirements of the IPM and the CM (not currently
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described in this document).
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#. Export functions to verify an image which uses an authentication method that
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cannot be interpreted by the CM, e.g. if an image has to be verified using a
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NV counter, then the value of the counter to compare with can only be
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provided by the platform.
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#. Export a custom IPM if a proprietary image format is being used (described
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later).
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Authentication Module (AM)
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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It is responsible for:
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#. Providing the necessary abstraction mechanisms to describe a CoT. Amongst
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other things, the authentication and image parsing methods must be specified
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by the PP in the CoT.
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#. Verifying the CoT passed by GEN by utilising functionality exported by the
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PP, IPM and CM.
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#. Tracking which images have been verified. In case an image is a part of
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multiple CoTs then it should be verified only once e.g. the Trusted World
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Key Certificate in the TBBR-Client spec. contains information to verify
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SCP\_BL2, BL31, BL32 each of which have a separate CoT. (This
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responsibility has not been described in this document but should be
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trivial to implement).
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#. Reusing memory meant for a data image to verify authentication images e.g.
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in the CoT described in Diagram 2, each certificate can be loaded and
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verified in the memory reserved by the platform for the BL31 image. By the
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time BL31 (the data image) is loaded, all information to authenticate it
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will have been extracted from the parent image i.e. BL31 content
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certificate. It is assumed that the size of an authentication image will
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never exceed the size of a data image. It should be possible to verify this
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at build time using asserts.
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Cryptographic Module (CM)
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^^^^^^^^^^^^^^^^^^^^^^^^^
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The CM is responsible for providing an API to:
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#. Verify a digital signature.
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#. Verify a hash.
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The CM does not include any cryptography related code, but it relies on an
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external library to perform the cryptographic operations. A Crypto-Library (CL)
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linking the CM and the external library must be implemented. The following
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functions must be provided by the CL:
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.. code:: c
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void (*init)(void);
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int (*verify_signature)(void *data_ptr, unsigned int data_len,
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void *sig_ptr, unsigned int sig_len,
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void *sig_alg, unsigned int sig_alg_len,
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void *pk_ptr, unsigned int pk_len);
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int (*verify_hash)(void *data_ptr, unsigned int data_len,
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void *digest_info_ptr, unsigned int digest_info_len);
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These functions are registered in the CM using the macro:
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.. code:: c
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REGISTER_CRYPTO_LIB(_name, _init, _verify_signature, _verify_hash);
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``_name`` must be a string containing the name of the CL. This name is used for
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debugging purposes.
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Image Parser Module (IPM)
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^^^^^^^^^^^^^^^^^^^^^^^^^
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The IPM is responsible for:
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#. Checking the integrity of each image loaded by the IO framework.
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#. Extracting parameters used for authenticating an image based upon a
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description provided by the platform in the CoT descriptor.
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Images may have different formats (for example, authentication images could be
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x509v3 certificates, signed ELF files or any other platform specific format).
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The IPM allows to register an Image Parser Library (IPL) for every image format
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used in the CoT. This library must implement the specific methods to parse the
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image. The IPM obtains the image format from the CoT and calls the right IPL to
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check the image integrity and extract the authentication parameters.
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See Section "Describing the image parsing methods" for more details about the
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mechanism the IPM provides to define and register IPLs.
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Authentication methods
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~~~~~~~~~~~~~~~~~~~~~~
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The AM supports the following authentication methods:
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#. Hash
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#. Digital signature
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The platform may specify these methods in the CoT in case it decides to define
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a custom CoT instead of reusing a predefined one.
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If a data image uses multiple methods, then all the methods must be a part of
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the same CoT. The number and type of parameters are method specific. These
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parameters should be obtained from the parent image using the IPM.
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#. Hash
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Parameters:
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#. A pointer to data to hash
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#. Length of the data
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#. A pointer to the hash
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#. Length of the hash
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The hash will be represented by the DER encoding of the following ASN.1
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type:
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::
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DigestInfo ::= SEQUENCE {
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digestAlgorithm DigestAlgorithmIdentifier,
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digest Digest
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}
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This ASN.1 structure makes it possible to remove any assumption about the
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type of hash algorithm used as this information accompanies the hash. This
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should allow the Cryptography Library (CL) to support multiple hash
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algorithm implementations.
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#. Digital Signature
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Parameters:
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#. A pointer to data to sign
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#. Length of the data
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#. Public Key Algorithm
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#. Public Key value
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#. Digital Signature Algorithm
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#. Digital Signature value
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The Public Key parameters will be represented by the DER encoding of the
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following ASN.1 type:
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::
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SubjectPublicKeyInfo ::= SEQUENCE {
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algorithm AlgorithmIdentifier{PUBLIC-KEY,{PublicKeyAlgorithms}},
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subjectPublicKey BIT STRING }
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The Digital Signature Algorithm will be represented by the DER encoding of
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the following ASN.1 types.
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::
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AlgorithmIdentifier {ALGORITHM:IOSet } ::= SEQUENCE {
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algorithm ALGORITHM.&id({IOSet}),
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parameters ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL
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}
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The digital signature will be represented by:
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::
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signature ::= BIT STRING
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The authentication framework will use the image descriptor to extract all the
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information related to authentication.
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Specifying a Chain of Trust
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---------------------------
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A CoT can be described as a set of image descriptors linked together in a
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particular order. The order dictates the sequence in which they must be
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verified. Each image has a set of properties which allow the AM to verify it.
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These properties are described below.
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The PP is responsible for defining a single or multiple CoTs for a data image.
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Unless otherwise specified, the data structures described in the following
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sections are populated by the PP statically.
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Describing the image parsing methods
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The parsing method refers to the format of a particular image. For example, an
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authentication image that represents a certificate could be in the X.509v3
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format. A data image that represents a boot loader stage could be in raw binary
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or ELF format. The IPM supports three parsing methods. An image has to use one
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of the three methods described below. An IPL is responsible for interpreting a
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single parsing method. There has to be one IPL for every method used by the
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platform.
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#. Raw format: This format is effectively a nop as an image using this method
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is treated as being in raw binary format e.g. boot loader images used by ARM
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TF. This method should only be used by data images.
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#. X509V3 method: This method uses industry standards like X.509 to represent
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PKI certificates (authentication images). It is expected that open source
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libraries will be available which can be used to parse an image represented
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by this method. Such libraries can be used to write the corresponding IPL
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e.g. the X.509 parsing library code in mbed TLS.
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#. Platform defined method: This method caters for platform specific
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proprietary standards to represent authentication or data images. For
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example, The signature of a data image could be appended to the data image
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raw binary. A header could be prepended to the combined blob to specify the
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extents of each component. The platform will have to implement the
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corresponding IPL to interpret such a format.
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The following enum can be used to define these three methods.
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.. code:: c
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typedef enum img_type_enum {
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IMG_RAW, /* Binary image */
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IMG_PLAT, /* Platform specific format */
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IMG_CERT, /* X509v3 certificate */
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IMG_MAX_TYPES,
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} img_type_t;
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An IPL must provide functions with the following prototypes:
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.. code:: c
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void init(void);
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int check_integrity(void *img, unsigned int img_len);
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int get_auth_param(const auth_param_type_desc_t *type_desc,
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void *img, unsigned int img_len,
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void **param, unsigned int *param_len);
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An IPL for each type must be registered using the following macro:
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::
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REGISTER_IMG_PARSER_LIB(_type, _name, _init, _check_int, _get_param)
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- ``_type``: one of the types described above.
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- ``_name``: a string containing the IPL name for debugging purposes.
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- ``_init``: initialization function pointer.
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- ``_check_int``: check image integrity function pointer.
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- ``_get_param``: extract authentication parameter funcion pointer.
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The ``init()`` function will be used to initialize the IPL.
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The ``check_integrity()`` function is passed a pointer to the memory where the
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image has been loaded by the IO framework and the image length. It should ensure
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that the image is in the format corresponding to the parsing method and has not
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been tampered with. For example, RFC-2459 describes a validation sequence for an
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X.509 certificate.
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The ``get_auth_param()`` function is passed a parameter descriptor containing
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information about the parameter (``type_desc`` and ``cookie``) to identify and
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extract the data corresponding to that parameter from an image. This data will
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be used to verify either the current or the next image in the CoT sequence.
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Each image in the CoT will specify the parsing method it uses. This information
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will be used by the IPM to find the right parser descriptor for the image.
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Describing the authentication method(s)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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As part of the CoT, each image has to specify one or more authentication methods
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which will be used to verify it. As described in the Section "Authentication
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methods", there are three methods supported by the AM.
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.. code:: c
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typedef enum {
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AUTH_METHOD_NONE,
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AUTH_METHOD_HASH,
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AUTH_METHOD_SIG,
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AUTH_METHOD_NUM
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} auth_method_type_t;
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The AM defines the type of each parameter used by an authentication method. It
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uses this information to:
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#. Specify to the ``get_auth_param()`` function exported by the IPM, which
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parameter should be extracted from an image.
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#. Correctly marshall the parameters while calling the verification function
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exported by the CM and PP.
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#. Extract authentication parameters from a parent image in order to verify a
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child image e.g. to verify the certificate image, the public key has to be
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obtained from the parent image.
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.. code:: c
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typedef enum {
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AUTH_PARAM_NONE,
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AUTH_PARAM_RAW_DATA, /* Raw image data */
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AUTH_PARAM_SIG, /* The image signature */
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AUTH_PARAM_SIG_ALG, /* The image signature algorithm */
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AUTH_PARAM_HASH, /* A hash (including the algorithm) */
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AUTH_PARAM_PUB_KEY, /* A public key */
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} auth_param_type_t;
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The AM defines the following structure to identify an authentication parameter
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required to verify an image.
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.. code:: c
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typedef struct auth_param_type_desc_s {
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auth_param_type_t type;
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void *cookie;
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} auth_param_type_desc_t;
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``cookie`` is used by the platform to specify additional information to the IPM
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which enables it to uniquely identify the parameter that should be extracted
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from an image. For example, the hash of a BL3x image in its corresponding
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content certificate is stored in an X509v3 custom extension field. An extension
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field can only be identified using an OID. In this case, the ``cookie`` could
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contain the pointer to the OID defined by the platform for the hash extension
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field while the ``type`` field could be set to ``AUTH_PARAM_HASH``. A value of 0 for
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the ``cookie`` field means that it is not used.
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For each method, the AM defines a structure with the parameters required to
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verify the image.
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.. code:: c
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/*
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* Parameters for authentication by hash matching
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*/
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typedef struct auth_method_param_hash_s {
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auth_param_type_desc_t *data; /* Data to hash */
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auth_param_type_desc_t *hash; /* Hash to match with */
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} auth_method_param_hash_t;
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/*
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* Parameters for authentication by signature
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*/
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typedef struct auth_method_param_sig_s {
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auth_param_type_desc_t *pk; /* Public key */
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auth_param_type_desc_t *sig; /* Signature to check */
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auth_param_type_desc_t *alg; /* Signature algorithm */
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auth_param_type_desc_t *tbs; /* Data signed */
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} auth_method_param_sig_t;
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The AM defines the following structure to describe an authentication method for
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verifying an image
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.. code:: c
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/*
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* Authentication method descriptor
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*/
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typedef struct auth_method_desc_s {
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auth_method_type_t type;
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union {
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auth_method_param_hash_t hash;
|
|
auth_method_param_sig_t sig;
|
|
} param;
|
|
} auth_method_desc_t;
|
|
|
|
Using the method type specified in the ``type`` field, the AM finds out what field
|
|
needs to access within the ``param`` union.
|
|
|
|
Storing Authentication parameters
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A parameter described by ``auth_param_type_desc_t`` to verify an image could be
|
|
obtained from either the image itself or its parent image. The memory allocated
|
|
for loading the parent image will be reused for loading the child image. Hence
|
|
parameters which are obtained from the parent for verifying a child image need
|
|
to have memory allocated for them separately where they can be stored. This
|
|
memory must be statically allocated by the platform port.
|
|
|
|
The AM defines the following structure to store the data corresponding to an
|
|
authentication parameter.
|
|
|
|
.. code:: c
|
|
|
|
typedef struct auth_param_data_desc_s {
|
|
void *auth_param_ptr;
|
|
unsigned int auth_param_len;
|
|
} auth_param_data_desc_t;
|
|
|
|
The ``auth_param_ptr`` field is initialized by the platform. The ``auth_param_len``
|
|
field is used to specify the length of the data in the memory.
|
|
|
|
For parameters that can be obtained from the child image itself, the IPM is
|
|
responsible for populating the ``auth_param_ptr`` and ``auth_param_len`` fields
|
|
while executing the ``img_get_auth_param()`` function.
|
|
|
|
The AM defines the following structure to enable an image to describe the
|
|
parameters that should be extracted from it and used to verify the next image
|
|
(child) in a CoT.
|
|
|
|
.. code:: c
|
|
|
|
typedef struct auth_param_desc_s {
|
|
auth_param_type_desc_t type_desc;
|
|
auth_param_data_desc_t data;
|
|
} auth_param_desc_t;
|
|
|
|
Describing an image in a CoT
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
An image in a CoT is a consolidation of the following aspects of a CoT described
|
|
above.
|
|
|
|
#. A unique identifier specified by the platform which allows the IO framework
|
|
to locate the image in a FIP and load it in the memory reserved for the data
|
|
image in the CoT.
|
|
|
|
#. A parsing method which is used by the AM to find the appropriate IPM.
|
|
|
|
#. Authentication methods and their parameters as described in the previous
|
|
section. These are used to verify the current image.
|
|
|
|
#. Parameters which are used to verify the next image in the current CoT. These
|
|
parameters are specified only by authentication images and can be extracted
|
|
from the current image once it has been verified.
|
|
|
|
The following data structure describes an image in a CoT.
|
|
|
|
.. code:: c
|
|
|
|
typedef struct auth_img_desc_s {
|
|
unsigned int img_id;
|
|
const struct auth_img_desc_s *parent;
|
|
img_type_t img_type;
|
|
auth_method_desc_t img_auth_methods[AUTH_METHOD_NUM];
|
|
auth_param_desc_t authenticated_data[COT_MAX_VERIFIED_PARAMS];
|
|
} auth_img_desc_t;
|
|
|
|
A CoT is defined as an array of ``auth_image_desc_t`` structures linked together
|
|
by the ``parent`` field. Those nodes with no parent must be authenticated using
|
|
the ROTPK stored in the platform.
|
|
|
|
Implementation example
|
|
----------------------
|
|
|
|
This section is a detailed guide explaining a trusted boot implementation using
|
|
the authentication framework. This example corresponds to the Applicative
|
|
Functional Mode (AFM) as specified in the TBBR-Client document. It is
|
|
recommended to read this guide along with the source code.
|
|
|
|
The TBBR CoT
|
|
~~~~~~~~~~~~
|
|
|
|
The CoT can be found in ``drivers/auth/tbbr/tbbr_cot.c``. This CoT consists of an
|
|
array of image descriptors and it is registered in the framework using the macro
|
|
``REGISTER_COT(cot_desc)``, where 'cot\_desc' must be the name of the array
|
|
(passing a pointer or any other type of indirection will cause the registration
|
|
process to fail).
|
|
|
|
The number of images participating in the boot process depends on the CoT. There
|
|
is, however, a minimum set of images that are mandatory in the Trusted Firmware
|
|
and thus all CoTs must present:
|
|
|
|
- ``BL2``
|
|
- ``SCP_BL2`` (platform specific)
|
|
- ``BL31``
|
|
- ``BL32`` (optional)
|
|
- ``BL33``
|
|
|
|
The TBBR specifies the additional certificates that must accompany these images
|
|
for a proper authentication. Details about the TBBR CoT may be found in the
|
|
`Trusted Board Boot`_ document.
|
|
|
|
Following the `Platform Porting Guide`_, a platform must provide unique
|
|
identifiers for all the images and certificates that will be loaded during the
|
|
boot process. If a platform is using the TBBR as a reference for trusted boot,
|
|
these identifiers can be obtained from ``include/common/tbbr/tbbr_img_def.h``.
|
|
ARM platforms include this file in ``include/plat/arm/common/arm_def.h``. Other
|
|
platforms may also include this file or provide their own identifiers.
|
|
|
|
**Important**: the authentication module uses these identifiers to index the
|
|
CoT array, so the descriptors location in the array must match the identifiers.
|
|
|
|
Each image descriptor must specify:
|
|
|
|
- ``img_id``: the corresponding image unique identifier defined by the platform.
|
|
- ``img_type``: the image parser module uses the image type to call the proper
|
|
parsing library to check the image integrity and extract the required
|
|
authentication parameters. Three types of images are currently supported:
|
|
|
|
- ``IMG_RAW``: image is a raw binary. No parsing functions are available,
|
|
other than reading the whole image.
|
|
- ``IMG_PLAT``: image format is platform specific. The platform may use this
|
|
type for custom images not directly supported by the authentication
|
|
framework.
|
|
- ``IMG_CERT``: image is an x509v3 certificate.
|
|
|
|
- ``parent``: pointer to the parent image descriptor. The parent will contain
|
|
the information required to authenticate the current image. If the parent
|
|
is NULL, the authentication parameters will be obtained from the platform
|
|
(i.e. the BL2 and Trusted Key certificates are signed with the ROT private
|
|
key, whose public part is stored in the platform).
|
|
- ``img_auth_methods``: this array defines the authentication methods that must
|
|
be checked to consider an image authenticated. Each method consists of a
|
|
type and a list of parameter descriptors. A parameter descriptor consists of
|
|
a type and a cookie which will point to specific information required to
|
|
extract that parameter from the image (i.e. if the parameter is stored in an
|
|
x509v3 extension, the cookie will point to the extension OID). Depending on
|
|
the method type, a different number of parameters must be specified.
|
|
Supported methods are:
|
|
|
|
- ``AUTH_METHOD_HASH``: the hash of the image must match the hash extracted
|
|
from the parent image. The following parameter descriptors must be
|
|
specified:
|
|
|
|
- ``data``: data to be hashed (obtained from current image)
|
|
- ``hash``: reference hash (obtained from parent image)
|
|
|
|
- ``AUTH_METHOD_SIG``: the image (usually a certificate) must be signed with
|
|
the private key whose public part is extracted from the parent image (or
|
|
the platform if the parent is NULL). The following parameter descriptors
|
|
must be specified:
|
|
|
|
- ``pk``: the public key (obtained from parent image)
|
|
- ``sig``: the digital signature (obtained from current image)
|
|
- ``alg``: the signature algorithm used (obtained from current image)
|
|
- ``data``: the data to be signed (obtained from current image)
|
|
|
|
- ``authenticated_data``: this array indicates what authentication parameters
|
|
must be extracted from an image once it has been authenticated. Each
|
|
parameter consists of a parameter descriptor and the buffer address/size
|
|
to store the parameter. The CoT is responsible for allocating the required
|
|
memory to store the parameters.
|
|
|
|
In the ``tbbr_cot.c`` file, a set of buffers are allocated to store the parameters
|
|
extracted from the certificates. In the case of the TBBR CoT, these parameters
|
|
are hashes and public keys. In DER format, an RSA-2048 public key requires 294
|
|
bytes, and a hash requires 51 bytes. Depending on the CoT and the authentication
|
|
process, some of the buffers may be reused at different stages during the boot.
|
|
|
|
Next in that file, the parameter descriptors are defined. These descriptors will
|
|
be used to extract the parameter data from the corresponding image.
|
|
|
|
Example: the BL31 Chain of Trust
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Four image descriptors form the BL31 Chain of Trust:
|
|
|
|
.. code:: asm
|
|
|
|
[TRUSTED_KEY_CERT_ID] = {
|
|
.img_id = TRUSTED_KEY_CERT_ID,
|
|
.img_type = IMG_CERT,
|
|
.parent = NULL,
|
|
.img_auth_methods = {
|
|
[0] = {
|
|
.type = AUTH_METHOD_SIG,
|
|
.param.sig = {
|
|
.pk = &subject_pk,
|
|
.sig = &sig,
|
|
.alg = &sig_alg,
|
|
.data = &raw_data,
|
|
}
|
|
}
|
|
},
|
|
.authenticated_data = {
|
|
[0] = {
|
|
.type_desc = &trusted_world_pk,
|
|
.data = {
|
|
.ptr = (void *)trusted_world_pk_buf,
|
|
.len = (unsigned int)PK_DER_LEN
|
|
}
|
|
},
|
|
[1] = {
|
|
.type_desc = &non_trusted_world_pk,
|
|
.data = {
|
|
.ptr = (void *)non_trusted_world_pk_buf,
|
|
.len = (unsigned int)PK_DER_LEN
|
|
}
|
|
}
|
|
}
|
|
},
|
|
[SOC_FW_KEY_CERT_ID] = {
|
|
.img_id = SOC_FW_KEY_CERT_ID,
|
|
.img_type = IMG_CERT,
|
|
.parent = &cot_desc[TRUSTED_KEY_CERT_ID],
|
|
.img_auth_methods = {
|
|
[0] = {
|
|
.type = AUTH_METHOD_SIG,
|
|
.param.sig = {
|
|
.pk = &trusted_world_pk,
|
|
.sig = &sig,
|
|
.alg = &sig_alg,
|
|
.data = &raw_data,
|
|
}
|
|
}
|
|
},
|
|
.authenticated_data = {
|
|
[0] = {
|
|
.type_desc = &soc_fw_content_pk,
|
|
.data = {
|
|
.ptr = (void *)content_pk_buf,
|
|
.len = (unsigned int)PK_DER_LEN
|
|
}
|
|
}
|
|
}
|
|
},
|
|
[SOC_FW_CONTENT_CERT_ID] = {
|
|
.img_id = SOC_FW_CONTENT_CERT_ID,
|
|
.img_type = IMG_CERT,
|
|
.parent = &cot_desc[SOC_FW_KEY_CERT_ID],
|
|
.img_auth_methods = {
|
|
[0] = {
|
|
.type = AUTH_METHOD_SIG,
|
|
.param.sig = {
|
|
.pk = &soc_fw_content_pk,
|
|
.sig = &sig,
|
|
.alg = &sig_alg,
|
|
.data = &raw_data,
|
|
}
|
|
}
|
|
},
|
|
.authenticated_data = {
|
|
[0] = {
|
|
.type_desc = &soc_fw_hash,
|
|
.data = {
|
|
.ptr = (void *)soc_fw_hash_buf,
|
|
.len = (unsigned int)HASH_DER_LEN
|
|
}
|
|
}
|
|
}
|
|
},
|
|
[BL31_IMAGE_ID] = {
|
|
.img_id = BL31_IMAGE_ID,
|
|
.img_type = IMG_RAW,
|
|
.parent = &cot_desc[SOC_FW_CONTENT_CERT_ID],
|
|
.img_auth_methods = {
|
|
[0] = {
|
|
.type = AUTH_METHOD_HASH,
|
|
.param.hash = {
|
|
.data = &raw_data,
|
|
.hash = &soc_fw_hash,
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
The **Trusted Key certificate** is signed with the ROT private key and contains
|
|
the Trusted World public key and the Non-Trusted World public key as x509v3
|
|
extensions. This must be specified in the image descriptor using the
|
|
``img_auth_methods`` and ``authenticated_data`` arrays, respectively.
|
|
|
|
The Trusted Key certificate is authenticated by checking its digital signature
|
|
using the ROTPK. Four parameters are required to check a signature: the public
|
|
key, the algorithm, the signature and the data that has been signed. Therefore,
|
|
four parameter descriptors must be specified with the authentication method:
|
|
|
|
- ``subject_pk``: parameter descriptor of type ``AUTH_PARAM_PUB_KEY``. This type
|
|
is used to extract a public key from the parent image. If the cookie is an
|
|
OID, the key is extracted from the corresponding x509v3 extension. If the
|
|
cookie is NULL, the subject public key is retrieved. In this case, because
|
|
the parent image is NULL, the public key is obtained from the platform
|
|
(this key will be the ROTPK).
|
|
- ``sig``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to extract
|
|
the signature from the certificate.
|
|
- ``sig_alg``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to
|
|
extract the signature algorithm from the certificate.
|
|
- ``raw_data``: parameter descriptor of type ``AUTH_PARAM_RAW_DATA``. It is used
|
|
to extract the data to be signed from the certificate.
|
|
|
|
Once the signature has been checked and the certificate authenticated, the
|
|
Trusted World public key needs to be extracted from the certificate. A new entry
|
|
is created in the ``authenticated_data`` array for that purpose. In that entry,
|
|
the corresponding parameter descriptor must be specified along with the buffer
|
|
address to store the parameter value. In this case, the ``tz_world_pk`` descriptor
|
|
is used to extract the public key from an x509v3 extension with OID
|
|
``TRUSTED_WORLD_PK_OID``. The BL31 key certificate will use this descriptor as
|
|
parameter in the signature authentication method. The key is stored in the
|
|
``plat_tz_world_pk_buf`` buffer.
|
|
|
|
The **BL31 Key certificate** is authenticated by checking its digital signature
|
|
using the Trusted World public key obtained previously from the Trusted Key
|
|
certificate. In the image descriptor, we specify a single authentication method
|
|
by signature whose public key is the ``tz_world_pk``. Once this certificate has
|
|
been authenticated, we have to extract the BL31 public key, stored in the
|
|
extension specified by ``bl31_content_pk``. This key will be copied to the
|
|
``plat_content_pk`` buffer.
|
|
|
|
The **BL31 certificate** is authenticated by checking its digital signature
|
|
using the BL31 public key obtained previously from the BL31 Key certificate.
|
|
We specify the authentication method using ``bl31_content_pk`` as public key.
|
|
After authentication, we need to extract the BL31 hash, stored in the extension
|
|
specified by ``bl31_hash``. This hash will be copied to the ``plat_bl31_hash_buf``
|
|
buffer.
|
|
|
|
The **BL31 image** is authenticated by calculating its hash and matching it
|
|
with the hash obtained from the BL31 certificate. The image descriptor contains
|
|
a single authentication method by hash. The parameters to the hash method are
|
|
the reference hash, ``bl31_hash``, and the data to be hashed. In this case, it is
|
|
the whole image, so we specify ``raw_data``.
|
|
|
|
The image parser library
|
|
~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The image parser module relies on libraries to check the image integrity and
|
|
extract the authentication parameters. The number and type of parser libraries
|
|
depend on the images used in the CoT. Raw images do not need a library, so
|
|
only an x509v3 library is required for the TBBR CoT.
|
|
|
|
ARM platforms will use an x509v3 library based on mbed TLS. This library may be
|
|
found in ``drivers/auth/mbedtls/mbedtls_x509_parser.c``. It exports three
|
|
functions:
|
|
|
|
.. code:: c
|
|
|
|
void init(void);
|
|
int check_integrity(void *img, unsigned int img_len);
|
|
int get_auth_param(const auth_param_type_desc_t *type_desc,
|
|
void *img, unsigned int img_len,
|
|
void **param, unsigned int *param_len);
|
|
|
|
The library is registered in the framework using the macro
|
|
``REGISTER_IMG_PARSER_LIB()``. Each time the image parser module needs to access
|
|
an image of type ``IMG_CERT``, it will call the corresponding function exported
|
|
in this file.
|
|
|
|
The build system must be updated to include the corresponding library and
|
|
mbed TLS sources. ARM platforms use the ``arm_common.mk`` file to pull the
|
|
sources.
|
|
|
|
The cryptographic library
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The cryptographic module relies on a library to perform the required operations,
|
|
i.e. verify a hash or a digital signature. ARM platforms will use a library
|
|
based on mbed TLS, which can be found in
|
|
``drivers/auth/mbedtls/mbedtls_crypto.c``. This library is registered in the
|
|
authentication framework using the macro ``REGISTER_CRYPTO_LIB()`` and exports
|
|
three functions:
|
|
|
|
.. code:: c
|
|
|
|
void init(void);
|
|
int verify_signature(void *data_ptr, unsigned int data_len,
|
|
void *sig_ptr, unsigned int sig_len,
|
|
void *sig_alg, unsigned int sig_alg_len,
|
|
void *pk_ptr, unsigned int pk_len);
|
|
int verify_hash(void *data_ptr, unsigned int data_len,
|
|
void *digest_info_ptr, unsigned int digest_info_len);
|
|
|
|
The key algorithm (rsa, ecdsa) must be specified in the build system using the
|
|
``TF_MBEDTLS_KEY_ALG`` variable, so the Makefile can include the corresponding
|
|
sources in the build.
|
|
|
|
Note: If code size is a concern, the build option ``MBEDTLS_SHA256_SMALLER`` can
|
|
be defined in the platform Makefile. It will make mbed TLS use an implementation
|
|
of SHA-256 with smaller memory footprint (~1.5 KB less) but slower (~30%).
|
|
|
|
--------------
|
|
|
|
*Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.*
|
|
|
|
.. _Trusted Board Boot: ./trusted-board-boot.rst
|
|
.. _Platform Porting Guide: ./porting-guide.rst
|