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+Abstracting a Chain of Trust
+============================
+
+
+.. section-numbering::
+    :suffix: .
+
+.. contents::
+
+The aim of this document is to describe the authentication framework implemented
+in the Trusted Firmware. This framework fulfills the following requirements:
+
+#. It should be possible for a platform port to specify the Chain of Trust in
+   terms of certificate hierarchy and the mechanisms used to verify a
+   particular image/certificate.
+
+#. The framework should distinguish between:
+
+   -  The mechanism used to encode and transport information, e.g. DER encoded
+      X.509v3 certificates to ferry Subject Public Keys, hashes and non-volatile
+      counters.
+
+   -  The mechanism used to verify the transported information i.e. the
+      cryptographic libraries.
+
+The framework has been designed following a modular approach illustrated in the
+next diagram:
+
+::
+
+        +---------------+---------------+------------+
+        | Trusted       | Trusted       | Trusted    |
+        | Firmware      | Firmware      | Firmware   |
+        | Generic       | IO Framework  | Platform   |
+        | Code i.e.     | (IO)          | Port       |
+        | BL1/BL2 (GEN) |               | (PP)       |
+        +---------------+---------------+------------+
+               ^               ^               ^
+               |               |               |
+               v               v               v
+         +-----------+   +-----------+   +-----------+
+         |           |   |           |   | Image     |
+         | Crypto    |   | Auth      |   | Parser    |
+         | Module    |<->| Module    |<->| Module    |
+         | (CM)      |   | (AM)      |   | (IPM)     |
+         |           |   |           |   |           |
+         +-----------+   +-----------+   +-----------+
+               ^                               ^
+               |                               |
+               v                               v
+        +----------------+             +-----------------+
+        | Cryptographic  |             | Image Parser    |
+        | Libraries (CL) |             | Libraries (IPL) |
+        +----------------+             +-----------------+
+                      |                |
+                      |                |
+                      |                |
+                      v                v
+                     +-----------------+
+                     | Misc. Libs e.g. |
+                     | ASN.1 decoder   |
+                     |                 |
+                     +-----------------+
+
+        DIAGRAM 1.
+
+This document describes the inner details of the authentication framework and
+the abstraction mechanisms available to specify a Chain of Trust.
+
+Framework design
+----------------
+
+This section describes some aspects of the framework design and the rationale
+behind them. These aspects are key to verify a Chain of Trust.
+
+Chain of Trust
+~~~~~~~~~~~~~~
+
+A CoT is basically a sequence of authentication images which usually starts with
+a root of trust and culminates in a single data image. The following diagram
+illustrates how this maps to a CoT for the BL31 image described in the
+TBBR-Client specification.
+
+::
+
+        +------------------+       +-------------------+
+        | ROTPK/ROTPK Hash |------>| Trusted Key       |
+        +------------------+       | Certificate       |
+                                   | (Auth Image)      |
+                                  /+-------------------+
+                                 /            |
+                                /             |
+                               /              |
+                              /               |
+                             L                v
+        +------------------+       +-------------------+
+        | Trusted World    |------>| BL31 Key          |
+        | Public Key       |       | Certificate       |
+        +------------------+       | (Auth Image)      |
+                                   +-------------------+
+                                  /           |
+                                 /            |
+                                /             |
+                               /              |
+                              /               v
+        +------------------+ L     +-------------------+
+        | BL31 Content     |------>| BL31 Content      |
+        | Certificate PK   |       | Certificate       |
+        +------------------+       | (Auth Image)      |
+                                   +-------------------+
+                                  /           |
+                                 /            |
+                                /             |
+                               /              |
+                              /               v
+        +------------------+ L     +-------------------+
+        | BL31 Hash        |------>| BL31 Image        |
+        |                  |       | (Data Image)      |
+        +------------------+       |                   |
+                                   +-------------------+
+
+        DIAGRAM 2.
+
+The root of trust is usually a public key (ROTPK) that has been burnt in the
+platform and cannot be modified.
+
+Image types
+~~~~~~~~~~~
+
+Images in a CoT are categorised as authentication and data images. An
+authentication image contains information to authenticate a data image or
+another authentication image. A data image is usually a boot loader binary, but
+it could be any other data that requires authentication.
+
+Component responsibilities
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For every image in a Chain of Trust, the following high level operations are
+performed to verify it:
+
+#. Allocate memory for the image either statically or at runtime.
+
+#. Identify the image and load it in the allocated memory.
+
+#. Check the integrity of the image as per its type.
+
+#. Authenticate the image as per the cryptographic algorithms used.
+
+#. If the image is an authentication image, extract the information that will
+   be used to authenticate the next image in the CoT.
+
+In Diagram 1, each component is responsible for one or more of these operations.
+The responsibilities are briefly described below.
+
+TF Generic code and IO framework (GEN/IO)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+These components are responsible for initiating the authentication process for a
+particular image in BL1 or BL2. For each BL image that requires authentication,
+the Generic code asks recursively the Authentication module what is the parent
+image until either an authenticated image or the ROT is reached. Then the
+Generic code calls the IO framewotk to load the image and calls the
+Authentication module to authenticate it, following the CoT from ROT to Image.
+
+TF Platform Port (PP)
+^^^^^^^^^^^^^^^^^^^^^
+
+The platform is responsible for:
+
+#. Specifying the CoT for each image that needs to be authenticated. Details of
+   how a CoT can be specified by the platform are explained later. The platform
+   also specifies the authentication methods and the parsing method used for
+   each image.
+
+#. Statically allocating memory for each parameter in each image which is
+   used for verifying the CoT, e.g. memory for public keys, hashes etc.
+
+#. Providing the ROTPK or a hash of it.
+
+#. Providing additional information to the IPM to enable it to identify and
+   extract authentication parameters contained in an image, e.g. if the
+   parameters are stored as X509v3 extensions, the corresponding OID must be
+   provided.
+
+#. Fulfill any other memory requirements of the IPM and the CM (not currently
+   described in this document).
+
+#. Export functions to verify an image which uses an authentication method that
+   cannot be interpreted by the CM, e.g. if an image has to be verified using a
+   NV counter, then the value of the counter to compare with can only be
+   provided by the platform.
+
+#. Export a custom IPM if a proprietary image format is being used (described
+   later).
+
+Authentication Module (AM)
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+It is responsible for:
+
+#. Providing the necessary abstraction mechanisms to describe a CoT. Amongst
+   other things, the authentication and image parsing methods must be specified
+   by the PP in the CoT.
+
+#. Verifying the CoT passed by GEN by utilising functionality exported by the
+   PP, IPM and CM.
+
+#. Tracking which images have been verified. In case an image is a part of
+   multiple CoTs then it should be verified only once e.g. the Trusted World
+   Key Certificate in the TBBR-Client spec. contains information to verify
+   SCP\_BL2, BL31, BL32 each of which have a separate CoT. (This
+   responsibility has not been described in this document but should be
+   trivial to implement).
+
+#. Reusing memory meant for a data image to verify authentication images e.g.
+   in the CoT described in Diagram 2, each certificate can be loaded and
+   verified in the memory reserved by the platform for the BL31 image. By the
+   time BL31 (the data image) is loaded, all information to authenticate it
+   will have been extracted from the parent image i.e. BL31 content
+   certificate. It is assumed that the size of an authentication image will
+   never exceed the size of a data image. It should be possible to verify this
+   at build time using asserts.
+
+Cryptographic Module (CM)
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The CM is responsible for providing an API to:
+
+#. Verify a digital signature.
+#. Verify a hash.
+
+The CM does not include any cryptography related code, but it relies on an
+external library to perform the cryptographic operations. A Crypto-Library (CL)
+linking the CM and the external library must be implemented. The following
+functions must be provided by the CL:
+
+.. 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);
+
+These functions are registered in the CM using the macro:
+
+.. code:: c
+
+    REGISTER_CRYPTO_LIB(_name, _init, _verify_signature, _verify_hash);
+
+``_name`` must be a string containing the name of the CL. This name is used for
+debugging purposes.
+
+Image Parser Module (IPM)
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The IPM is responsible for:
+
+#. Checking the integrity of each image loaded by the IO framework.
+#. Extracting parameters used for authenticating an image based upon a
+   description provided by the platform in the CoT descriptor.
+
+Images may have different formats (for example, authentication images could be
+x509v3 certificates, signed ELF files or any other platform specific format).
+The IPM allows to register an Image Parser Library (IPL) for every image format
+used in the CoT. This library must implement the specific methods to parse the
+image. The IPM obtains the image format from the CoT and calls the right IPL to
+check the image integrity and extract the authentication parameters.
+
+See Section "Describing the image parsing methods" for more details about the
+mechanism the IPM provides to define and register IPLs.
+
+Authentication methods
+~~~~~~~~~~~~~~~~~~~~~~
+
+The AM supports the following authentication methods:
+
+#. Hash
+#. Digital signature
+
+The platform may specify these methods in the CoT in case it decides to define
+a custom CoT instead of reusing a predefined one.
+
+If a data image uses multiple methods, then all the methods must be a part of
+the same CoT. The number and type of parameters are method specific. These
+parameters should be obtained from the parent image using the IPM.
+
+#. Hash
+
+   Parameters:
+
+   #. A pointer to data to hash
+   #. Length of the data
+   #. A pointer to the hash
+   #. Length of the hash
+
+   The hash will be represented by the DER encoding of the following ASN.1
+   type:
+
+   ::
+
+       DigestInfo ::= SEQUENCE {
+           digestAlgorithm  DigestAlgorithmIdentifier,
+           digest           Digest
+       }
+
+   This ASN.1 structure makes it possible to remove any assumption about the
+   type of hash algorithm used as this information accompanies the hash. This
+   should allow the Cryptography Library (CL) to support multiple hash
+   algorithm implementations.
+
+#. Digital Signature
+
+   Parameters:
+
+   #. A pointer to data to sign
+   #. Length of the data
+   #. Public Key Algorithm
+   #. Public Key value
+   #. Digital Signature Algorithm
+   #. Digital Signature value
+
+   The Public Key parameters will be represented by the DER encoding of the
+   following ASN.1 type:
+
+   ::
+
+       SubjectPublicKeyInfo  ::=  SEQUENCE  {
+           algorithm         AlgorithmIdentifier{PUBLIC-KEY,{PublicKeyAlgorithms}},
+           subjectPublicKey  BIT STRING  }
+
+   The Digital Signature Algorithm will be represented by the DER encoding of
+   the following ASN.1 types.
+
+   ::
+
+       AlgorithmIdentifier {ALGORITHM:IOSet } ::= SEQUENCE {
+           algorithm         ALGORITHM.&id({IOSet}),
+           parameters        ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL
+       }
+
+   The digital signature will be represented by:
+
+   ::
+
+       signature  ::=  BIT STRING
+
+The authentication framework will use the image descriptor to extract all the
+information related to authentication.
+
+Specifying a Chain of Trust
+---------------------------
+
+A CoT can be described as a set of image descriptors linked together in a
+particular order. The order dictates the sequence in which they must be
+verified. Each image has a set of properties which allow the AM to verify it.
+These properties are described below.
+
+The PP is responsible for defining a single or multiple CoTs for a data image.
+Unless otherwise specified, the data structures described in the following
+sections are populated by the PP statically.
+
+Describing the image parsing methods
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The parsing method refers to the format of a particular image. For example, an
+authentication image that represents a certificate could be in the X.509v3
+format. A data image that represents a boot loader stage could be in raw binary
+or ELF format. The IPM supports three parsing methods. An image has to use one
+of the three methods described below. An IPL is responsible for interpreting a
+single parsing method. There has to be one IPL for every method used by the
+platform.
+
+#. Raw format: This format is effectively a nop as an image using this method
+   is treated as being in raw binary format e.g. boot loader images used by ARM
+   TF. This method should only be used by data images.
+
+#. X509V3 method: This method uses industry standards like X.509 to represent
+   PKI certificates (authentication images). It is expected that open source
+   libraries will be available which can be used to parse an image represented
+   by this method. Such libraries can be used to write the corresponding IPL
+   e.g. the X.509 parsing library code in mbed TLS.
+
+#. Platform defined method: This method caters for platform specific
+   proprietary standards to represent authentication or data images. For
+   example, The signature of a data image could be appended to the data image
+   raw binary. A header could be prepended to the combined blob to specify the
+   extents of each component. The platform will have to implement the
+   corresponding IPL to interpret such a format.
+
+The following enum can be used to define these three methods.
+
+.. code:: c
+
+    typedef enum img_type_enum {
+        IMG_RAW,            /* Binary image */
+        IMG_PLAT,           /* Platform specific format */
+        IMG_CERT,           /* X509v3 certificate */
+        IMG_MAX_TYPES,
+    } img_type_t;
+
+An IPL must provide functions with the following prototypes:
+
+.. 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);
+
+An IPL for each type must be registered using the following macro:
+
+::
+
+    REGISTER_IMG_PARSER_LIB(_type, _name, _init, _check_int, _get_param)
+
+-  ``_type``: one of the types described above.
+-  ``_name``: a string containing the IPL name for debugging purposes.
+-  ``_init``: initialization function pointer.
+-  ``_check_int``: check image integrity function pointer.
+-  ``_get_param``: extract authentication parameter funcion pointer.
+
+The ``init()`` function will be used to initialize the IPL.
+
+The ``check_integrity()`` function is passed a pointer to the memory where the
+image has been loaded by the IO framework and the image length. It should ensure
+that the image is in the format corresponding to the parsing method and has not
+been tampered with. For example, RFC-2459 describes a validation sequence for an
+X.509 certificate.
+
+The ``get_auth_param()`` function is passed a parameter descriptor containing
+information about the parameter (``type_desc`` and ``cookie``) to identify and
+extract the data corresponding to that parameter from an image. This data will
+be used to verify either the current or the next image in the CoT sequence.
+
+Each image in the CoT will specify the parsing method it uses. This information
+will be used by the IPM to find the right parser descriptor for the image.
+
+Describing the authentication method(s)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As part of the CoT, each image has to specify one or more authentication methods
+which will be used to verify it. As described in the Section "Authentication
+methods", there are three methods supported by the AM.
+
+.. code:: c
+
+    typedef enum {
+        AUTH_METHOD_NONE,
+        AUTH_METHOD_HASH,
+        AUTH_METHOD_SIG,
+        AUTH_METHOD_NUM
+    } auth_method_type_t;
+
+The AM defines the type of each parameter used by an authentication method. It
+uses this information to:
+
+#. Specify to the ``get_auth_param()`` function exported by the IPM, which
+   parameter should be extracted from an image.
+
+#. Correctly marshall the parameters while calling the verification function
+   exported by the CM and PP.
+
+#. Extract authentication parameters from a parent image in order to verify a
+   child image e.g. to verify the certificate image, the public key has to be
+   obtained from the parent image.
+
+.. code:: c
+
+    typedef enum {
+        AUTH_PARAM_NONE,
+        AUTH_PARAM_RAW_DATA,        /* Raw image data */
+        AUTH_PARAM_SIG,         /* The image signature */
+        AUTH_PARAM_SIG_ALG,     /* The image signature algorithm */
+        AUTH_PARAM_HASH,        /* A hash (including the algorithm) */
+        AUTH_PARAM_PUB_KEY,     /* A public key */
+    } auth_param_type_t;
+
+The AM defines the following structure to identify an authentication parameter
+required to verify an image.
+
+.. code:: c
+
+    typedef struct auth_param_type_desc_s {
+        auth_param_type_t type;
+        void *cookie;
+    } auth_param_type_desc_t;
+
+``cookie`` is used by the platform to specify additional information to the IPM
+which enables it to uniquely identify the parameter that should be extracted
+from an image. For example, the hash of a BL3x image in its corresponding
+content certificate is stored in an X509v3 custom extension field. An extension
+field can only be identified using an OID. In this case, the ``cookie`` could
+contain the pointer to the OID defined by the platform for the hash extension
+field while the ``type`` field could be set to ``AUTH_PARAM_HASH``. A value of 0 for
+the ``cookie`` field means that it is not used.
+
+For each method, the AM defines a structure with the parameters required to
+verify the image.
+
+.. code:: c
+
+    /*
+     * Parameters for authentication by hash matching
+     */
+    typedef struct auth_method_param_hash_s {
+        auth_param_type_desc_t *data;   /* Data to hash */
+        auth_param_type_desc_t *hash;   /* Hash to match with */
+    } auth_method_param_hash_t;
+
+    /*
+     * Parameters for authentication by signature
+     */
+    typedef struct auth_method_param_sig_s {
+        auth_param_type_desc_t *pk; /* Public key */
+        auth_param_type_desc_t *sig;    /* Signature to check */
+        auth_param_type_desc_t *alg;    /* Signature algorithm */
+        auth_param_type_desc_t *tbs;    /* Data signed */
+    } auth_method_param_sig_t;
+
+The AM defines the following structure to describe an authentication method for
+verifying an image
+
+.. code:: c
+
+    /*
+     * Authentication method descriptor
+     */
+    typedef struct auth_method_desc_s {
+        auth_method_type_t type;
+        union {
+            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