1Authentication Framework & Chain of Trust 2========================================= 3 4The aim of this document is to describe the authentication framework 5implemented in Trusted Firmware-A (TF-A). This framework fulfills the 6following requirements: 7 8#. It should be possible for a platform port to specify the Chain of Trust in 9 terms of certificate hierarchy and the mechanisms used to verify a 10 particular image/certificate. 11 12#. The framework should distinguish between: 13 14 - The mechanism used to encode and transport information, e.g. DER encoded 15 X.509v3 certificates to ferry Subject Public Keys, hashes and non-volatile 16 counters. 17 18 - The mechanism used to verify the transported information i.e. the 19 cryptographic libraries. 20 21The framework has been designed following a modular approach illustrated in the 22next diagram: 23 24:: 25 26 +---------------+---------------+------------+ 27 | Trusted | Trusted | Trusted | 28 | Firmware | Firmware | Firmware | 29 | Generic | IO Framework | Platform | 30 | Code i.e. | (IO) | Port | 31 | BL1/BL2 (GEN) | | (PP) | 32 +---------------+---------------+------------+ 33 ^ ^ ^ 34 | | | 35 v v v 36 +-----------+ +-----------+ +-----------+ 37 | | | | | Image | 38 | Crypto | | Auth | | Parser | 39 | Module |<->| Module |<->| Module | 40 | (CM) | | (AM) | | (IPM) | 41 | | | | | | 42 +-----------+ +-----------+ +-----------+ 43 ^ ^ 44 | | 45 v v 46 +----------------+ +-----------------+ 47 | Cryptographic | | Image Parser | 48 | Libraries (CL) | | Libraries (IPL) | 49 +----------------+ +-----------------+ 50 | | 51 | | 52 | | 53 v v 54 +-----------------+ 55 | Misc. Libs e.g. | 56 | ASN.1 decoder | 57 | | 58 +-----------------+ 59 60 DIAGRAM 1. 61 62This document describes the inner details of the authentication framework and 63the abstraction mechanisms available to specify a Chain of Trust. 64 65Framework design 66---------------- 67 68This section describes some aspects of the framework design and the rationale 69behind them. These aspects are key to verify a Chain of Trust. 70 71Chain of Trust 72~~~~~~~~~~~~~~ 73 74A CoT is basically a sequence of authentication images which usually starts with 75a root of trust and culminates in a single data image. The following diagram 76illustrates how this maps to a CoT for the BL31 image described in the 77`TBBR-Client specification`_. 78 79:: 80 81 +------------------+ +-------------------+ 82 | ROTPK/ROTPK Hash |------>| Trusted Key | 83 +------------------+ | Certificate | 84 | (Auth Image) | 85 /+-------------------+ 86 / | 87 / | 88 / | 89 / | 90 L v 91 +------------------+ +-------------------+ 92 | Trusted World |------>| BL31 Key | 93 | Public Key | | Certificate | 94 +------------------+ | (Auth Image) | 95 +-------------------+ 96 / | 97 / | 98 / | 99 / | 100 / v 101 +------------------+ L +-------------------+ 102 | BL31 Content |------>| BL31 Content | 103 | Certificate PK | | Certificate | 104 +------------------+ | (Auth Image) | 105 +-------------------+ 106 / | 107 / | 108 / | 109 / | 110 / v 111 +------------------+ L +-------------------+ 112 | BL31 Hash |------>| BL31 Image | 113 | | | (Data Image) | 114 +------------------+ | | 115 +-------------------+ 116 117 DIAGRAM 2. 118 119The root of trust is usually a public key (ROTPK) that has been burnt in the 120platform and cannot be modified. 121 122Image types 123~~~~~~~~~~~ 124 125Images in a CoT are categorised as authentication and data images. An 126authentication image contains information to authenticate a data image or 127another authentication image. A data image is usually a boot loader binary, but 128it could be any other data that requires authentication. 129 130Component responsibilities 131~~~~~~~~~~~~~~~~~~~~~~~~~~ 132 133For every image in a Chain of Trust, the following high level operations are 134performed to verify it: 135 136#. Allocate memory for the image either statically or at runtime. 137 138#. Identify the image and load it in the allocated memory. 139 140#. Check the integrity of the image as per its type. 141 142#. Authenticate the image as per the cryptographic algorithms used. 143 144#. If the image is an authentication image, extract the information that will 145 be used to authenticate the next image in the CoT. 146 147In Diagram 1, each component is responsible for one or more of these operations. 148The responsibilities are briefly described below. 149 150TF-A Generic code and IO framework (GEN/IO) 151^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 152 153These components are responsible for initiating the authentication process for a 154particular image in BL1 or BL2. For each BL image that requires authentication, 155the Generic code asks recursively the Authentication module what is the parent 156image until either an authenticated image or the ROT is reached. Then the 157Generic code calls the IO framework to load the image and calls the 158Authentication module to authenticate it, following the CoT from ROT to Image. 159 160TF-A Platform Port (PP) 161^^^^^^^^^^^^^^^^^^^^^^^ 162 163The platform is responsible for: 164 165#. Specifying the CoT for each image that needs to be authenticated. Details of 166 how a CoT can be specified by the platform are explained later. The platform 167 also specifies the authentication methods and the parsing method used for 168 each image. 169 170#. Statically allocating memory for each parameter in each image which is 171 used for verifying the CoT, e.g. memory for public keys, hashes etc. 172 173#. Providing the ROTPK or a hash of it. 174 175#. Providing additional information to the IPM to enable it to identify and 176 extract authentication parameters contained in an image, e.g. if the 177 parameters are stored as X509v3 extensions, the corresponding OID must be 178 provided. 179 180#. Fulfill any other memory requirements of the IPM and the CM (not currently 181 described in this document). 182 183#. Export functions to verify an image which uses an authentication method that 184 cannot be interpreted by the CM, e.g. if an image has to be verified using a 185 NV counter, then the value of the counter to compare with can only be 186 provided by the platform. 187 188#. Export a custom IPM if a proprietary image format is being used (described 189 later). 190 191Authentication Module (AM) 192^^^^^^^^^^^^^^^^^^^^^^^^^^ 193 194It is responsible for: 195 196#. Providing the necessary abstraction mechanisms to describe a CoT. Amongst 197 other things, the authentication and image parsing methods must be specified 198 by the PP in the CoT. 199 200#. Verifying the CoT passed by GEN by utilising functionality exported by the 201 PP, IPM and CM. 202 203#. Tracking which images have been verified. In case an image is a part of 204 multiple CoTs then it should be verified only once e.g. the Trusted World 205 Key Certificate in the TBBR-Client spec. contains information to verify 206 SCP_BL2, BL31, BL32 each of which have a separate CoT. (This 207 responsibility has not been described in this document but should be 208 trivial to implement). 209 210#. Reusing memory meant for a data image to verify authentication images e.g. 211 in the CoT described in Diagram 2, each certificate can be loaded and 212 verified in the memory reserved by the platform for the BL31 image. By the 213 time BL31 (the data image) is loaded, all information to authenticate it 214 will have been extracted from the parent image i.e. BL31 content 215 certificate. It is assumed that the size of an authentication image will 216 never exceed the size of a data image. It should be possible to verify this 217 at build time using asserts. 218 219Cryptographic Module (CM) 220^^^^^^^^^^^^^^^^^^^^^^^^^ 221 222The CM is responsible for providing an API to: 223 224#. Verify a digital signature. 225#. Verify a hash. 226 227The CM does not include any cryptography related code, but it relies on an 228external library to perform the cryptographic operations. A Crypto-Library (CL) 229linking the CM and the external library must be implemented. The following 230functions must be provided by the CL: 231 232.. code:: c 233 234 void (*init)(void); 235 int (*verify_signature)(void *data_ptr, unsigned int data_len, 236 void *sig_ptr, unsigned int sig_len, 237 void *sig_alg, unsigned int sig_alg_len, 238 void *pk_ptr, unsigned int pk_len); 239 int (*verify_hash)(void *data_ptr, unsigned int data_len, 240 void *digest_info_ptr, unsigned int digest_info_len); 241 242These functions are registered in the CM using the macro: 243 244.. code:: c 245 246 REGISTER_CRYPTO_LIB(_name, _init, _verify_signature, _verify_hash); 247 248``_name`` must be a string containing the name of the CL. This name is used for 249debugging purposes. 250 251Image Parser Module (IPM) 252^^^^^^^^^^^^^^^^^^^^^^^^^ 253 254The IPM is responsible for: 255 256#. Checking the integrity of each image loaded by the IO framework. 257#. Extracting parameters used for authenticating an image based upon a 258 description provided by the platform in the CoT descriptor. 259 260Images may have different formats (for example, authentication images could be 261x509v3 certificates, signed ELF files or any other platform specific format). 262The IPM allows to register an Image Parser Library (IPL) for every image format 263used in the CoT. This library must implement the specific methods to parse the 264image. The IPM obtains the image format from the CoT and calls the right IPL to 265check the image integrity and extract the authentication parameters. 266 267See Section "Describing the image parsing methods" for more details about the 268mechanism the IPM provides to define and register IPLs. 269 270Authentication methods 271~~~~~~~~~~~~~~~~~~~~~~ 272 273The AM supports the following authentication methods: 274 275#. Hash 276#. Digital signature 277 278The platform may specify these methods in the CoT in case it decides to define 279a custom CoT instead of reusing a predefined one. 280 281If a data image uses multiple methods, then all the methods must be a part of 282the same CoT. The number and type of parameters are method specific. These 283parameters should be obtained from the parent image using the IPM. 284 285#. Hash 286 287 Parameters: 288 289 #. A pointer to data to hash 290 #. Length of the data 291 #. A pointer to the hash 292 #. Length of the hash 293 294 The hash will be represented by the DER encoding of the following ASN.1 295 type: 296 297 :: 298 299 DigestInfo ::= SEQUENCE { 300 digestAlgorithm DigestAlgorithmIdentifier, 301 digest Digest 302 } 303 304 This ASN.1 structure makes it possible to remove any assumption about the 305 type of hash algorithm used as this information accompanies the hash. This 306 should allow the Cryptography Library (CL) to support multiple hash 307 algorithm implementations. 308 309#. Digital Signature 310 311 Parameters: 312 313 #. A pointer to data to sign 314 #. Length of the data 315 #. Public Key Algorithm 316 #. Public Key value 317 #. Digital Signature Algorithm 318 #. Digital Signature value 319 320 The Public Key parameters will be represented by the DER encoding of the 321 following ASN.1 type: 322 323 :: 324 325 SubjectPublicKeyInfo ::= SEQUENCE { 326 algorithm AlgorithmIdentifier{PUBLIC-KEY,{PublicKeyAlgorithms}}, 327 subjectPublicKey BIT STRING } 328 329 The Digital Signature Algorithm will be represented by the DER encoding of 330 the following ASN.1 types. 331 332 :: 333 334 AlgorithmIdentifier {ALGORITHM:IOSet } ::= SEQUENCE { 335 algorithm ALGORITHM.&id({IOSet}), 336 parameters ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL 337 } 338 339 The digital signature will be represented by: 340 341 :: 342 343 signature ::= BIT STRING 344 345The authentication framework will use the image descriptor to extract all the 346information related to authentication. 347 348Specifying a Chain of Trust 349--------------------------- 350 351A CoT can be described as a set of image descriptors linked together in a 352particular order. The order dictates the sequence in which they must be 353verified. Each image has a set of properties which allow the AM to verify it. 354These properties are described below. 355 356The PP is responsible for defining a single or multiple CoTs for a data image. 357Unless otherwise specified, the data structures described in the following 358sections are populated by the PP statically. 359 360Describing the image parsing methods 361~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 362 363The parsing method refers to the format of a particular image. For example, an 364authentication image that represents a certificate could be in the X.509v3 365format. A data image that represents a boot loader stage could be in raw binary 366or ELF format. The IPM supports three parsing methods. An image has to use one 367of the three methods described below. An IPL is responsible for interpreting a 368single parsing method. There has to be one IPL for every method used by the 369platform. 370 371#. Raw format: This format is effectively a nop as an image using this method 372 is treated as being in raw binary format e.g. boot loader images used by 373 TF-A. This method should only be used by data images. 374 375#. X509V3 method: This method uses industry standards like X.509 to represent 376 PKI certificates (authentication images). It is expected that open source 377 libraries will be available which can be used to parse an image represented 378 by this method. Such libraries can be used to write the corresponding IPL 379 e.g. the X.509 parsing library code in mbed TLS. 380 381#. Platform defined method: This method caters for platform specific 382 proprietary standards to represent authentication or data images. For 383 example, The signature of a data image could be appended to the data image 384 raw binary. A header could be prepended to the combined blob to specify the 385 extents of each component. The platform will have to implement the 386 corresponding IPL to interpret such a format. 387 388The following enum can be used to define these three methods. 389 390.. code:: c 391 392 typedef enum img_type_enum { 393 IMG_RAW, /* Binary image */ 394 IMG_PLAT, /* Platform specific format */ 395 IMG_CERT, /* X509v3 certificate */ 396 IMG_MAX_TYPES, 397 } img_type_t; 398 399An IPL must provide functions with the following prototypes: 400 401.. code:: c 402 403 void init(void); 404 int check_integrity(void *img, unsigned int img_len); 405 int get_auth_param(const auth_param_type_desc_t *type_desc, 406 void *img, unsigned int img_len, 407 void **param, unsigned int *param_len); 408 409An IPL for each type must be registered using the following macro: 410 411.. code:: c 412 413 REGISTER_IMG_PARSER_LIB(_type, _name, _init, _check_int, _get_param) 414 415- ``_type``: one of the types described above. 416- ``_name``: a string containing the IPL name for debugging purposes. 417- ``_init``: initialization function pointer. 418- ``_check_int``: check image integrity function pointer. 419- ``_get_param``: extract authentication parameter function pointer. 420 421The ``init()`` function will be used to initialize the IPL. 422 423The ``check_integrity()`` function is passed a pointer to the memory where the 424image has been loaded by the IO framework and the image length. It should ensure 425that the image is in the format corresponding to the parsing method and has not 426been tampered with. For example, RFC-2459 describes a validation sequence for an 427X.509 certificate. 428 429The ``get_auth_param()`` function is passed a parameter descriptor containing 430information about the parameter (``type_desc`` and ``cookie``) to identify and 431extract the data corresponding to that parameter from an image. This data will 432be used to verify either the current or the next image in the CoT sequence. 433 434Each image in the CoT will specify the parsing method it uses. This information 435will be used by the IPM to find the right parser descriptor for the image. 436 437Describing the authentication method(s) 438~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 439 440As part of the CoT, each image has to specify one or more authentication methods 441which will be used to verify it. As described in the Section "Authentication 442methods", there are three methods supported by the AM. 443 444.. code:: c 445 446 typedef enum { 447 AUTH_METHOD_NONE, 448 AUTH_METHOD_HASH, 449 AUTH_METHOD_SIG, 450 AUTH_METHOD_NUM 451 } auth_method_type_t; 452 453The AM defines the type of each parameter used by an authentication method. It 454uses this information to: 455 456#. Specify to the ``get_auth_param()`` function exported by the IPM, which 457 parameter should be extracted from an image. 458 459#. Correctly marshall the parameters while calling the verification function 460 exported by the CM and PP. 461 462#. Extract authentication parameters from a parent image in order to verify a 463 child image e.g. to verify the certificate image, the public key has to be 464 obtained from the parent image. 465 466.. code:: c 467 468 typedef enum { 469 AUTH_PARAM_NONE, 470 AUTH_PARAM_RAW_DATA, /* Raw image data */ 471 AUTH_PARAM_SIG, /* The image signature */ 472 AUTH_PARAM_SIG_ALG, /* The image signature algorithm */ 473 AUTH_PARAM_HASH, /* A hash (including the algorithm) */ 474 AUTH_PARAM_PUB_KEY, /* A public key */ 475 } auth_param_type_t; 476 477The AM defines the following structure to identify an authentication parameter 478required to verify an image. 479 480.. code:: c 481 482 typedef struct auth_param_type_desc_s { 483 auth_param_type_t type; 484 void *cookie; 485 } auth_param_type_desc_t; 486 487``cookie`` is used by the platform to specify additional information to the IPM 488which enables it to uniquely identify the parameter that should be extracted 489from an image. For example, the hash of a BL3x image in its corresponding 490content certificate is stored in an X509v3 custom extension field. An extension 491field can only be identified using an OID. In this case, the ``cookie`` could 492contain the pointer to the OID defined by the platform for the hash extension 493field while the ``type`` field could be set to ``AUTH_PARAM_HASH``. A value of 0 for 494the ``cookie`` field means that it is not used. 495 496For each method, the AM defines a structure with the parameters required to 497verify the image. 498 499.. code:: c 500 501 /* 502 * Parameters for authentication by hash matching 503 */ 504 typedef struct auth_method_param_hash_s { 505 auth_param_type_desc_t *data; /* Data to hash */ 506 auth_param_type_desc_t *hash; /* Hash to match with */ 507 } auth_method_param_hash_t; 508 509 /* 510 * Parameters for authentication by signature 511 */ 512 typedef struct auth_method_param_sig_s { 513 auth_param_type_desc_t *pk; /* Public key */ 514 auth_param_type_desc_t *sig; /* Signature to check */ 515 auth_param_type_desc_t *alg; /* Signature algorithm */ 516 auth_param_type_desc_t *tbs; /* Data signed */ 517 } auth_method_param_sig_t; 518 519The AM defines the following structure to describe an authentication method for 520verifying an image 521 522.. code:: c 523 524 /* 525 * Authentication method descriptor 526 */ 527 typedef struct auth_method_desc_s { 528 auth_method_type_t type; 529 union { 530 auth_method_param_hash_t hash; 531 auth_method_param_sig_t sig; 532 } param; 533 } auth_method_desc_t; 534 535Using the method type specified in the ``type`` field, the AM finds out what field 536needs to access within the ``param`` union. 537 538Storing Authentication parameters 539~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 540 541A parameter described by ``auth_param_type_desc_t`` to verify an image could be 542obtained from either the image itself or its parent image. The memory allocated 543for loading the parent image will be reused for loading the child image. Hence 544parameters which are obtained from the parent for verifying a child image need 545to have memory allocated for them separately where they can be stored. This 546memory must be statically allocated by the platform port. 547 548The AM defines the following structure to store the data corresponding to an 549authentication parameter. 550 551.. code:: c 552 553 typedef struct auth_param_data_desc_s { 554 void *auth_param_ptr; 555 unsigned int auth_param_len; 556 } auth_param_data_desc_t; 557 558The ``auth_param_ptr`` field is initialized by the platform. The ``auth_param_len`` 559field is used to specify the length of the data in the memory. 560 561For parameters that can be obtained from the child image itself, the IPM is 562responsible for populating the ``auth_param_ptr`` and ``auth_param_len`` fields 563while executing the ``img_get_auth_param()`` function. 564 565The AM defines the following structure to enable an image to describe the 566parameters that should be extracted from it and used to verify the next image 567(child) in a CoT. 568 569.. code:: c 570 571 typedef struct auth_param_desc_s { 572 auth_param_type_desc_t type_desc; 573 auth_param_data_desc_t data; 574 } auth_param_desc_t; 575 576Describing an image in a CoT 577~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 578 579An image in a CoT is a consolidation of the following aspects of a CoT described 580above. 581 582#. A unique identifier specified by the platform which allows the IO framework 583 to locate the image in a FIP and load it in the memory reserved for the data 584 image in the CoT. 585 586#. A parsing method which is used by the AM to find the appropriate IPM. 587 588#. Authentication methods and their parameters as described in the previous 589 section. These are used to verify the current image. 590 591#. Parameters which are used to verify the next image in the current CoT. These 592 parameters are specified only by authentication images and can be extracted 593 from the current image once it has been verified. 594 595The following data structure describes an image in a CoT. 596 597.. code:: c 598 599 typedef struct auth_img_desc_s { 600 unsigned int img_id; 601 const struct auth_img_desc_s *parent; 602 img_type_t img_type; 603 const auth_method_desc_t *const img_auth_methods; 604 const auth_param_desc_t *const authenticated_data; 605 } auth_img_desc_t; 606 607A CoT is defined as an array of pointers to ``auth_image_desc_t`` structures 608linked together by the ``parent`` field. Those nodes with no parent must be 609authenticated using the ROTPK stored in the platform. 610 611Implementation example 612---------------------- 613 614This section is a detailed guide explaining a trusted boot implementation using 615the authentication framework. This example corresponds to the Applicative 616Functional Mode (AFM) as specified in the TBBR-Client document. It is 617recommended to read this guide along with the source code. 618 619The TBBR CoT 620~~~~~~~~~~~~ 621 622CoT specific to BL1 and BL2 can be found in ``drivers/auth/tbbr/tbbr_cot_bl1.c`` 623and ``drivers/auth/tbbr/tbbr_cot_bl2.c`` respectively. The common CoT used across 624BL1 and BL2 can be found in ``drivers/auth/tbbr/tbbr_cot_common.c``. 625This CoT consists of an array of pointers to image descriptors and it is 626registered in the framework using the macro ``REGISTER_COT(cot_desc)``, where 627``cot_desc`` must be the name of the array (passing a pointer or any other 628type of indirection will cause the registration process to fail). 629 630The number of images participating in the boot process depends on the CoT. 631There is, however, a minimum set of images that are mandatory in TF-A and thus 632all CoTs must present: 633 634- ``BL2`` 635- ``SCP_BL2`` (platform specific) 636- ``BL31`` 637- ``BL32`` (optional) 638- ``BL33`` 639 640The TBBR specifies the additional certificates that must accompany these images 641for a proper authentication. Details about the TBBR CoT may be found in the 642:ref:`Trusted Board Boot` document. 643 644Following the :ref:`Porting Guide`, a platform must provide unique 645identifiers for all the images and certificates that will be loaded during the 646boot process. If a platform is using the TBBR as a reference for trusted boot, 647these identifiers can be obtained from ``include/common/tbbr/tbbr_img_def.h``. 648Arm platforms include this file in ``include/plat/arm/common/arm_def.h``. Other 649platforms may also include this file or provide their own identifiers. 650 651**Important**: the authentication module uses these identifiers to index the 652CoT array, so the descriptors location in the array must match the identifiers. 653 654Each image descriptor must specify: 655 656- ``img_id``: the corresponding image unique identifier defined by the platform. 657- ``img_type``: the image parser module uses the image type to call the proper 658 parsing library to check the image integrity and extract the required 659 authentication parameters. Three types of images are currently supported: 660 661 - ``IMG_RAW``: image is a raw binary. No parsing functions are available, 662 other than reading the whole image. 663 - ``IMG_PLAT``: image format is platform specific. The platform may use this 664 type for custom images not directly supported by the authentication 665 framework. 666 - ``IMG_CERT``: image is an x509v3 certificate. 667 668- ``parent``: pointer to the parent image descriptor. The parent will contain 669 the information required to authenticate the current image. If the parent 670 is NULL, the authentication parameters will be obtained from the platform 671 (i.e. the BL2 and Trusted Key certificates are signed with the ROT private 672 key, whose public part is stored in the platform). 673- ``img_auth_methods``: this points to an array which defines the 674 authentication methods that must be checked to consider an image 675 authenticated. Each method consists of a type and a list of parameter 676 descriptors. A parameter descriptor consists of a type and a cookie which 677 will point to specific information required to extract that parameter from 678 the image (i.e. if the parameter is stored in an x509v3 extension, the 679 cookie will point to the extension OID). Depending on the method type, a 680 different number of parameters must be specified. This pointer should not be 681 NULL. 682 Supported methods are: 683 684 - ``AUTH_METHOD_HASH``: the hash of the image must match the hash extracted 685 from the parent image. The following parameter descriptors must be 686 specified: 687 688 - ``data``: data to be hashed (obtained from current image) 689 - ``hash``: reference hash (obtained from parent image) 690 691 - ``AUTH_METHOD_SIG``: the image (usually a certificate) must be signed with 692 the private key whose public part is extracted from the parent image (or 693 the platform if the parent is NULL). The following parameter descriptors 694 must be specified: 695 696 - ``pk``: the public key (obtained from parent image) 697 - ``sig``: the digital signature (obtained from current image) 698 - ``alg``: the signature algorithm used (obtained from current image) 699 - ``data``: the data to be signed (obtained from current image) 700 701- ``authenticated_data``: this array pointer indicates what authentication 702 parameters must be extracted from an image once it has been authenticated. 703 Each parameter consists of a parameter descriptor and the buffer 704 address/size to store the parameter. The CoT is responsible for allocating 705 the required memory to store the parameters. This pointer may be NULL. 706 707In the ``tbbr_cot*.c`` file, a set of buffers are allocated to store the parameters 708extracted from the certificates. In the case of the TBBR CoT, these parameters 709are hashes and public keys. In DER format, an RSA-4096 public key requires 550 710bytes, and a hash requires 51 bytes. Depending on the CoT and the authentication 711process, some of the buffers may be reused at different stages during the boot. 712 713Next in that file, the parameter descriptors are defined. These descriptors will 714be used to extract the parameter data from the corresponding image. 715 716Example: the BL31 Chain of Trust 717^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 718 719Four image descriptors form the BL31 Chain of Trust: 720 721.. code:: c 722 723 static const auth_img_desc_t trusted_key_cert = { 724 .img_id = TRUSTED_KEY_CERT_ID, 725 .img_type = IMG_CERT, 726 .parent = NULL, 727 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 728 [0] = { 729 .type = AUTH_METHOD_SIG, 730 .param.sig = { 731 .pk = &subject_pk, 732 .sig = &sig, 733 .alg = &sig_alg, 734 .data = &raw_data 735 } 736 }, 737 [1] = { 738 .type = AUTH_METHOD_NV_CTR, 739 .param.nv_ctr = { 740 .cert_nv_ctr = &trusted_nv_ctr, 741 .plat_nv_ctr = &trusted_nv_ctr 742 } 743 } 744 }, 745 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 746 [0] = { 747 .type_desc = &trusted_world_pk, 748 .data = { 749 .ptr = (void *)trusted_world_pk_buf, 750 .len = (unsigned int)PK_DER_LEN 751 } 752 }, 753 [1] = { 754 .type_desc = &non_trusted_world_pk, 755 .data = { 756 .ptr = (void *)non_trusted_world_pk_buf, 757 .len = (unsigned int)PK_DER_LEN 758 } 759 } 760 } 761 }; 762 static const auth_img_desc_t soc_fw_key_cert = { 763 .img_id = SOC_FW_KEY_CERT_ID, 764 .img_type = IMG_CERT, 765 .parent = &trusted_key_cert, 766 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 767 [0] = { 768 .type = AUTH_METHOD_SIG, 769 .param.sig = { 770 .pk = &trusted_world_pk, 771 .sig = &sig, 772 .alg = &sig_alg, 773 .data = &raw_data 774 } 775 }, 776 [1] = { 777 .type = AUTH_METHOD_NV_CTR, 778 .param.nv_ctr = { 779 .cert_nv_ctr = &trusted_nv_ctr, 780 .plat_nv_ctr = &trusted_nv_ctr 781 } 782 } 783 }, 784 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 785 [0] = { 786 .type_desc = &soc_fw_content_pk, 787 .data = { 788 .ptr = (void *)content_pk_buf, 789 .len = (unsigned int)PK_DER_LEN 790 } 791 } 792 } 793 }; 794 static const auth_img_desc_t soc_fw_content_cert = { 795 .img_id = SOC_FW_CONTENT_CERT_ID, 796 .img_type = IMG_CERT, 797 .parent = &soc_fw_key_cert, 798 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 799 [0] = { 800 .type = AUTH_METHOD_SIG, 801 .param.sig = { 802 .pk = &soc_fw_content_pk, 803 .sig = &sig, 804 .alg = &sig_alg, 805 .data = &raw_data 806 } 807 }, 808 [1] = { 809 .type = AUTH_METHOD_NV_CTR, 810 .param.nv_ctr = { 811 .cert_nv_ctr = &trusted_nv_ctr, 812 .plat_nv_ctr = &trusted_nv_ctr 813 } 814 } 815 }, 816 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 817 [0] = { 818 .type_desc = &soc_fw_hash, 819 .data = { 820 .ptr = (void *)soc_fw_hash_buf, 821 .len = (unsigned int)HASH_DER_LEN 822 } 823 }, 824 [1] = { 825 .type_desc = &soc_fw_config_hash, 826 .data = { 827 .ptr = (void *)soc_fw_config_hash_buf, 828 .len = (unsigned int)HASH_DER_LEN 829 } 830 } 831 } 832 }; 833 static const auth_img_desc_t bl31_image = { 834 .img_id = BL31_IMAGE_ID, 835 .img_type = IMG_RAW, 836 .parent = &soc_fw_content_cert, 837 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 838 [0] = { 839 .type = AUTH_METHOD_HASH, 840 .param.hash = { 841 .data = &raw_data, 842 .hash = &soc_fw_hash 843 } 844 } 845 } 846 }; 847 848The **Trusted Key certificate** is signed with the ROT private key and contains 849the Trusted World public key and the Non-Trusted World public key as x509v3 850extensions. This must be specified in the image descriptor using the 851``img_auth_methods`` and ``authenticated_data`` arrays, respectively. 852 853The Trusted Key certificate is authenticated by checking its digital signature 854using the ROTPK. Four parameters are required to check a signature: the public 855key, the algorithm, the signature and the data that has been signed. Therefore, 856four parameter descriptors must be specified with the authentication method: 857 858- ``subject_pk``: parameter descriptor of type ``AUTH_PARAM_PUB_KEY``. This type 859 is used to extract a public key from the parent image. If the cookie is an 860 OID, the key is extracted from the corresponding x509v3 extension. If the 861 cookie is NULL, the subject public key is retrieved. In this case, because 862 the parent image is NULL, the public key is obtained from the platform 863 (this key will be the ROTPK). 864- ``sig``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to extract 865 the signature from the certificate. 866- ``sig_alg``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to 867 extract the signature algorithm from the certificate. 868- ``raw_data``: parameter descriptor of type ``AUTH_PARAM_RAW_DATA``. It is used 869 to extract the data to be signed from the certificate. 870 871Once the signature has been checked and the certificate authenticated, the 872Trusted World public key needs to be extracted from the certificate. A new entry 873is created in the ``authenticated_data`` array for that purpose. In that entry, 874the corresponding parameter descriptor must be specified along with the buffer 875address to store the parameter value. In this case, the ``trusted_world_pk`` 876descriptor is used to extract the public key from an x509v3 extension with OID 877``TRUSTED_WORLD_PK_OID``. The BL31 key certificate will use this descriptor as 878parameter in the signature authentication method. The key is stored in the 879``trusted_world_pk_buf`` buffer. 880 881The **BL31 Key certificate** is authenticated by checking its digital signature 882using the Trusted World public key obtained previously from the Trusted Key 883certificate. In the image descriptor, we specify a single authentication method 884by signature whose public key is the ``trusted_world_pk``. Once this certificate 885has been authenticated, we have to extract the BL31 public key, stored in the 886extension specified by ``soc_fw_content_pk``. This key will be copied to the 887``content_pk_buf`` buffer. 888 889The **BL31 certificate** is authenticated by checking its digital signature 890using the BL31 public key obtained previously from the BL31 Key certificate. 891We specify the authentication method using ``soc_fw_content_pk`` as public key. 892After authentication, we need to extract the BL31 hash, stored in the extension 893specified by ``soc_fw_hash``. This hash will be copied to the 894``soc_fw_hash_buf`` buffer. 895 896The **BL31 image** is authenticated by calculating its hash and matching it 897with the hash obtained from the BL31 certificate. The image descriptor contains 898a single authentication method by hash. The parameters to the hash method are 899the reference hash, ``soc_fw_hash``, and the data to be hashed. In this case, 900it is the whole image, so we specify ``raw_data``. 901 902The image parser library 903~~~~~~~~~~~~~~~~~~~~~~~~ 904 905The image parser module relies on libraries to check the image integrity and 906extract the authentication parameters. The number and type of parser libraries 907depend on the images used in the CoT. Raw images do not need a library, so 908only an x509v3 library is required for the TBBR CoT. 909 910Arm platforms will use an x509v3 library based on mbed TLS. This library may be 911found in ``drivers/auth/mbedtls/mbedtls_x509_parser.c``. It exports three 912functions: 913 914.. code:: c 915 916 void init(void); 917 int check_integrity(void *img, unsigned int img_len); 918 int get_auth_param(const auth_param_type_desc_t *type_desc, 919 void *img, unsigned int img_len, 920 void **param, unsigned int *param_len); 921 922The library is registered in the framework using the macro 923``REGISTER_IMG_PARSER_LIB()``. Each time the image parser module needs to access 924an image of type ``IMG_CERT``, it will call the corresponding function exported 925in this file. 926 927The build system must be updated to include the corresponding library and 928mbed TLS sources. Arm platforms use the ``arm_common.mk`` file to pull the 929sources. 930 931The cryptographic library 932~~~~~~~~~~~~~~~~~~~~~~~~~ 933 934The cryptographic module relies on a library to perform the required operations, 935i.e. verify a hash or a digital signature. Arm platforms will use a library 936based on mbed TLS, which can be found in 937``drivers/auth/mbedtls/mbedtls_crypto.c``. This library is registered in the 938authentication framework using the macro ``REGISTER_CRYPTO_LIB()`` and exports 939four functions: 940 941.. code:: c 942 943 void init(void); 944 int verify_signature(void *data_ptr, unsigned int data_len, 945 void *sig_ptr, unsigned int sig_len, 946 void *sig_alg, unsigned int sig_alg_len, 947 void *pk_ptr, unsigned int pk_len); 948 int verify_hash(void *data_ptr, unsigned int data_len, 949 void *digest_info_ptr, unsigned int digest_info_len); 950 int auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr, 951 size_t len, const void *key, unsigned int key_len, 952 unsigned int key_flags, const void *iv, 953 unsigned int iv_len, const void *tag, 954 unsigned int tag_len) 955 956The mbedTLS library algorithm support is configured by both the 957``TF_MBEDTLS_KEY_ALG`` and ``TF_MBEDTLS_KEY_SIZE`` variables. 958 959- ``TF_MBEDTLS_KEY_ALG`` can take in 3 values: `rsa`, `ecdsa` or `rsa+ecdsa`. 960 This variable allows the Makefile to include the corresponding sources in 961 the build for the various algorithms. Setting the variable to `rsa+ecdsa` 962 enables support for both rsa and ecdsa algorithms in the mbedTLS library. 963 964- ``TF_MBEDTLS_KEY_SIZE`` sets the supported RSA key size for TFA. Valid values 965 include 1024, 2048, 3072 and 4096. 966 967- ``TF_MBEDTLS_USE_AES_GCM`` enables the authenticated decryption support based 968 on AES-GCM algorithm. Valid values are 0 and 1. 969 970.. note:: 971 If code size is a concern, the build option ``MBEDTLS_SHA256_SMALLER`` can 972 be defined in the platform Makefile. It will make mbed TLS use an 973 implementation of SHA-256 with smaller memory footprint (~1.5 KB less) but 974 slower (~30%). 975 976-------------- 977 978*Copyright (c) 2017-2020, Arm Limited and Contributors. All rights reserved.* 979 980.. _TBBR-Client specification: https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a 981