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 (*calc_hash)(enum crypto_md_algo alg, void *data_ptr, 240 unsigned int data_len, 241 unsigned char output[CRYPTO_MD_MAX_SIZE]) 242 int (*verify_hash)(void *data_ptr, unsigned int data_len, 243 void *digest_info_ptr, unsigned int digest_info_len); 244 245These functions are registered in the CM using the macro: 246 247.. code:: c 248 249 REGISTER_CRYPTO_LIB(_name, 250 _init, 251 _verify_signature, 252 _calc_hash, 253 _verify_hash); 254 255``_name`` must be a string containing the name of the CL. This name is used for 256debugging purposes. 257 258Crypto module provides a function ``_calc_hash`` to calculate and 259return the hash of the given data using the provided hash algorithm. 260This function is mainly used in the ``MEASURED_BOOT`` and ``DRTM_SUPPORT`` 261features to calculate the hashes of various images/data. 262 263Image Parser Module (IPM) 264^^^^^^^^^^^^^^^^^^^^^^^^^ 265 266The IPM is responsible for: 267 268#. Checking the integrity of each image loaded by the IO framework. 269#. Extracting parameters used for authenticating an image based upon a 270 description provided by the platform in the CoT descriptor. 271 272Images may have different formats (for example, authentication images could be 273x509v3 certificates, signed ELF files or any other platform specific format). 274The IPM allows to register an Image Parser Library (IPL) for every image format 275used in the CoT. This library must implement the specific methods to parse the 276image. The IPM obtains the image format from the CoT and calls the right IPL to 277check the image integrity and extract the authentication parameters. 278 279See Section "Describing the image parsing methods" for more details about the 280mechanism the IPM provides to define and register IPLs. 281 282Authentication methods 283~~~~~~~~~~~~~~~~~~~~~~ 284 285The AM supports the following authentication methods: 286 287#. Hash 288#. Digital signature 289 290The platform may specify these methods in the CoT in case it decides to define 291a custom CoT instead of reusing a predefined one. 292 293If a data image uses multiple methods, then all the methods must be a part of 294the same CoT. The number and type of parameters are method specific. These 295parameters should be obtained from the parent image using the IPM. 296 297#. Hash 298 299 Parameters: 300 301 #. A pointer to data to hash 302 #. Length of the data 303 #. A pointer to the hash 304 #. Length of the hash 305 306 The hash will be represented by the DER encoding of the following ASN.1 307 type: 308 309 :: 310 311 DigestInfo ::= SEQUENCE { 312 digestAlgorithm DigestAlgorithmIdentifier, 313 digest Digest 314 } 315 316 This ASN.1 structure makes it possible to remove any assumption about the 317 type of hash algorithm used as this information accompanies the hash. This 318 should allow the Cryptography Library (CL) to support multiple hash 319 algorithm implementations. 320 321#. Digital Signature 322 323 Parameters: 324 325 #. A pointer to data to sign 326 #. Length of the data 327 #. Public Key Algorithm 328 #. Public Key value 329 #. Digital Signature Algorithm 330 #. Digital Signature value 331 332 The Public Key parameters will be represented by the DER encoding of the 333 following ASN.1 type: 334 335 :: 336 337 SubjectPublicKeyInfo ::= SEQUENCE { 338 algorithm AlgorithmIdentifier{PUBLIC-KEY,{PublicKeyAlgorithms}}, 339 subjectPublicKey BIT STRING } 340 341 The Digital Signature Algorithm will be represented by the DER encoding of 342 the following ASN.1 types. 343 344 :: 345 346 AlgorithmIdentifier {ALGORITHM:IOSet } ::= SEQUENCE { 347 algorithm ALGORITHM.&id({IOSet}), 348 parameters ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL 349 } 350 351 The digital signature will be represented by: 352 353 :: 354 355 signature ::= BIT STRING 356 357The authentication framework will use the image descriptor to extract all the 358information related to authentication. 359 360Specifying a Chain of Trust 361--------------------------- 362 363A CoT can be described as a set of image descriptors linked together in a 364particular order. The order dictates the sequence in which they must be 365verified. Each image has a set of properties which allow the AM to verify it. 366These properties are described below. 367 368The PP is responsible for defining a single or multiple CoTs for a data image. 369Unless otherwise specified, the data structures described in the following 370sections are populated by the PP statically. 371 372Describing the image parsing methods 373~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 374 375The parsing method refers to the format of a particular image. For example, an 376authentication image that represents a certificate could be in the X.509v3 377format. A data image that represents a boot loader stage could be in raw binary 378or ELF format. The IPM supports three parsing methods. An image has to use one 379of the three methods described below. An IPL is responsible for interpreting a 380single parsing method. There has to be one IPL for every method used by the 381platform. 382 383#. Raw format: This format is effectively a nop as an image using this method 384 is treated as being in raw binary format e.g. boot loader images used by 385 TF-A. This method should only be used by data images. 386 387#. X509V3 method: This method uses industry standards like X.509 to represent 388 PKI certificates (authentication images). It is expected that open source 389 libraries will be available which can be used to parse an image represented 390 by this method. Such libraries can be used to write the corresponding IPL 391 e.g. the X.509 parsing library code in mbed TLS. 392 393#. Platform defined method: This method caters for platform specific 394 proprietary standards to represent authentication or data images. For 395 example, The signature of a data image could be appended to the data image 396 raw binary. A header could be prepended to the combined blob to specify the 397 extents of each component. The platform will have to implement the 398 corresponding IPL to interpret such a format. 399 400The following enum can be used to define these three methods. 401 402.. code:: c 403 404 typedef enum img_type_enum { 405 IMG_RAW, /* Binary image */ 406 IMG_PLAT, /* Platform specific format */ 407 IMG_CERT, /* X509v3 certificate */ 408 IMG_MAX_TYPES, 409 } img_type_t; 410 411An IPL must provide functions with the following prototypes: 412 413.. code:: c 414 415 void init(void); 416 int check_integrity(void *img, unsigned int img_len); 417 int get_auth_param(const auth_param_type_desc_t *type_desc, 418 void *img, unsigned int img_len, 419 void **param, unsigned int *param_len); 420 421An IPL for each type must be registered using the following macro: 422 423.. code:: c 424 425 REGISTER_IMG_PARSER_LIB(_type, _name, _init, _check_int, _get_param) 426 427- ``_type``: one of the types described above. 428- ``_name``: a string containing the IPL name for debugging purposes. 429- ``_init``: initialization function pointer. 430- ``_check_int``: check image integrity function pointer. 431- ``_get_param``: extract authentication parameter function pointer. 432 433The ``init()`` function will be used to initialize the IPL. 434 435The ``check_integrity()`` function is passed a pointer to the memory where the 436image has been loaded by the IO framework and the image length. It should ensure 437that the image is in the format corresponding to the parsing method and has not 438been tampered with. For example, RFC-2459 describes a validation sequence for an 439X.509 certificate. 440 441The ``get_auth_param()`` function is passed a parameter descriptor containing 442information about the parameter (``type_desc`` and ``cookie``) to identify and 443extract the data corresponding to that parameter from an image. This data will 444be used to verify either the current or the next image in the CoT sequence. 445 446Each image in the CoT will specify the parsing method it uses. This information 447will be used by the IPM to find the right parser descriptor for the image. 448 449Describing the authentication method(s) 450~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 451 452As part of the CoT, each image has to specify one or more authentication methods 453which will be used to verify it. As described in the Section "Authentication 454methods", there are three methods supported by the AM. 455 456.. code:: c 457 458 typedef enum { 459 AUTH_METHOD_NONE, 460 AUTH_METHOD_HASH, 461 AUTH_METHOD_SIG, 462 AUTH_METHOD_NUM 463 } auth_method_type_t; 464 465The AM defines the type of each parameter used by an authentication method. It 466uses this information to: 467 468#. Specify to the ``get_auth_param()`` function exported by the IPM, which 469 parameter should be extracted from an image. 470 471#. Correctly marshall the parameters while calling the verification function 472 exported by the CM and PP. 473 474#. Extract authentication parameters from a parent image in order to verify a 475 child image e.g. to verify the certificate image, the public key has to be 476 obtained from the parent image. 477 478.. code:: c 479 480 typedef enum { 481 AUTH_PARAM_NONE, 482 AUTH_PARAM_RAW_DATA, /* Raw image data */ 483 AUTH_PARAM_SIG, /* The image signature */ 484 AUTH_PARAM_SIG_ALG, /* The image signature algorithm */ 485 AUTH_PARAM_HASH, /* A hash (including the algorithm) */ 486 AUTH_PARAM_PUB_KEY, /* A public key */ 487 } auth_param_type_t; 488 489The AM defines the following structure to identify an authentication parameter 490required to verify an image. 491 492.. code:: c 493 494 typedef struct auth_param_type_desc_s { 495 auth_param_type_t type; 496 void *cookie; 497 } auth_param_type_desc_t; 498 499``cookie`` is used by the platform to specify additional information to the IPM 500which enables it to uniquely identify the parameter that should be extracted 501from an image. For example, the hash of a BL3x image in its corresponding 502content certificate is stored in an X509v3 custom extension field. An extension 503field can only be identified using an OID. In this case, the ``cookie`` could 504contain the pointer to the OID defined by the platform for the hash extension 505field while the ``type`` field could be set to ``AUTH_PARAM_HASH``. A value of 0 for 506the ``cookie`` field means that it is not used. 507 508For each method, the AM defines a structure with the parameters required to 509verify the image. 510 511.. code:: c 512 513 /* 514 * Parameters for authentication by hash matching 515 */ 516 typedef struct auth_method_param_hash_s { 517 auth_param_type_desc_t *data; /* Data to hash */ 518 auth_param_type_desc_t *hash; /* Hash to match with */ 519 } auth_method_param_hash_t; 520 521 /* 522 * Parameters for authentication by signature 523 */ 524 typedef struct auth_method_param_sig_s { 525 auth_param_type_desc_t *pk; /* Public key */ 526 auth_param_type_desc_t *sig; /* Signature to check */ 527 auth_param_type_desc_t *alg; /* Signature algorithm */ 528 auth_param_type_desc_t *tbs; /* Data signed */ 529 } auth_method_param_sig_t; 530 531The AM defines the following structure to describe an authentication method for 532verifying an image 533 534.. code:: c 535 536 /* 537 * Authentication method descriptor 538 */ 539 typedef struct auth_method_desc_s { 540 auth_method_type_t type; 541 union { 542 auth_method_param_hash_t hash; 543 auth_method_param_sig_t sig; 544 } param; 545 } auth_method_desc_t; 546 547Using the method type specified in the ``type`` field, the AM finds out what field 548needs to access within the ``param`` union. 549 550Storing Authentication parameters 551~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 552 553A parameter described by ``auth_param_type_desc_t`` to verify an image could be 554obtained from either the image itself or its parent image. The memory allocated 555for loading the parent image will be reused for loading the child image. Hence 556parameters which are obtained from the parent for verifying a child image need 557to have memory allocated for them separately where they can be stored. This 558memory must be statically allocated by the platform port. 559 560The AM defines the following structure to store the data corresponding to an 561authentication parameter. 562 563.. code:: c 564 565 typedef struct auth_param_data_desc_s { 566 void *auth_param_ptr; 567 unsigned int auth_param_len; 568 } auth_param_data_desc_t; 569 570The ``auth_param_ptr`` field is initialized by the platform. The ``auth_param_len`` 571field is used to specify the length of the data in the memory. 572 573For parameters that can be obtained from the child image itself, the IPM is 574responsible for populating the ``auth_param_ptr`` and ``auth_param_len`` fields 575while executing the ``img_get_auth_param()`` function. 576 577The AM defines the following structure to enable an image to describe the 578parameters that should be extracted from it and used to verify the next image 579(child) in a CoT. 580 581.. code:: c 582 583 typedef struct auth_param_desc_s { 584 auth_param_type_desc_t type_desc; 585 auth_param_data_desc_t data; 586 } auth_param_desc_t; 587 588Describing an image in a CoT 589~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 590 591An image in a CoT is a consolidation of the following aspects of a CoT described 592above. 593 594#. A unique identifier specified by the platform which allows the IO framework 595 to locate the image in a FIP and load it in the memory reserved for the data 596 image in the CoT. 597 598#. A parsing method which is used by the AM to find the appropriate IPM. 599 600#. Authentication methods and their parameters as described in the previous 601 section. These are used to verify the current image. 602 603#. Parameters which are used to verify the next image in the current CoT. These 604 parameters are specified only by authentication images and can be extracted 605 from the current image once it has been verified. 606 607The following data structure describes an image in a CoT. 608 609.. code:: c 610 611 typedef struct auth_img_desc_s { 612 unsigned int img_id; 613 const struct auth_img_desc_s *parent; 614 img_type_t img_type; 615 const auth_method_desc_t *const img_auth_methods; 616 const auth_param_desc_t *const authenticated_data; 617 } auth_img_desc_t; 618 619A CoT is defined as an array of pointers to ``auth_image_desc_t`` structures 620linked together by the ``parent`` field. Those nodes with no parent must be 621authenticated using the ROTPK stored in the platform. 622 623Implementation example 624---------------------- 625 626This section is a detailed guide explaining a trusted boot implementation using 627the authentication framework. This example corresponds to the Applicative 628Functional Mode (AFM) as specified in the TBBR-Client document. It is 629recommended to read this guide along with the source code. 630 631The TBBR CoT 632~~~~~~~~~~~~ 633 634CoT specific to BL1 and BL2 can be found in ``drivers/auth/tbbr/tbbr_cot_bl1.c`` 635and ``drivers/auth/tbbr/tbbr_cot_bl2.c`` respectively. The common CoT used across 636BL1 and BL2 can be found in ``drivers/auth/tbbr/tbbr_cot_common.c``. 637This CoT consists of an array of pointers to image descriptors and it is 638registered in the framework using the macro ``REGISTER_COT(cot_desc)``, where 639``cot_desc`` must be the name of the array (passing a pointer or any other 640type of indirection will cause the registration process to fail). 641 642The number of images participating in the boot process depends on the CoT. 643There is, however, a minimum set of images that are mandatory in TF-A and thus 644all CoTs must present: 645 646- ``BL2`` 647- ``SCP_BL2`` (platform specific) 648- ``BL31`` 649- ``BL32`` (optional) 650- ``BL33`` 651 652The TBBR specifies the additional certificates that must accompany these images 653for a proper authentication. Details about the TBBR CoT may be found in the 654:ref:`Trusted Board Boot` document. 655 656Following the :ref:`Porting Guide`, a platform must provide unique 657identifiers for all the images and certificates that will be loaded during the 658boot process. If a platform is using the TBBR as a reference for trusted boot, 659these identifiers can be obtained from ``include/common/tbbr/tbbr_img_def.h``. 660Arm platforms include this file in ``include/plat/arm/common/arm_def.h``. Other 661platforms may also include this file or provide their own identifiers. 662 663**Important**: the authentication module uses these identifiers to index the 664CoT array, so the descriptors location in the array must match the identifiers. 665 666Each image descriptor must specify: 667 668- ``img_id``: the corresponding image unique identifier defined by the platform. 669- ``img_type``: the image parser module uses the image type to call the proper 670 parsing library to check the image integrity and extract the required 671 authentication parameters. Three types of images are currently supported: 672 673 - ``IMG_RAW``: image is a raw binary. No parsing functions are available, 674 other than reading the whole image. 675 - ``IMG_PLAT``: image format is platform specific. The platform may use this 676 type for custom images not directly supported by the authentication 677 framework. 678 - ``IMG_CERT``: image is an x509v3 certificate. 679 680- ``parent``: pointer to the parent image descriptor. The parent will contain 681 the information required to authenticate the current image. If the parent 682 is NULL, the authentication parameters will be obtained from the platform 683 (i.e. the BL2 and Trusted Key certificates are signed with the ROT private 684 key, whose public part is stored in the platform). 685- ``img_auth_methods``: this points to an array which defines the 686 authentication methods that must be checked to consider an image 687 authenticated. Each method consists of a type and a list of parameter 688 descriptors. A parameter descriptor consists of a type and a cookie which 689 will point to specific information required to extract that parameter from 690 the image (i.e. if the parameter is stored in an x509v3 extension, the 691 cookie will point to the extension OID). Depending on the method type, a 692 different number of parameters must be specified. This pointer should not be 693 NULL. 694 Supported methods are: 695 696 - ``AUTH_METHOD_HASH``: the hash of the image must match the hash extracted 697 from the parent image. The following parameter descriptors must be 698 specified: 699 700 - ``data``: data to be hashed (obtained from current image) 701 - ``hash``: reference hash (obtained from parent image) 702 703 - ``AUTH_METHOD_SIG``: the image (usually a certificate) must be signed with 704 the private key whose public part is extracted from the parent image (or 705 the platform if the parent is NULL). The following parameter descriptors 706 must be specified: 707 708 - ``pk``: the public key (obtained from parent image) 709 - ``sig``: the digital signature (obtained from current image) 710 - ``alg``: the signature algorithm used (obtained from current image) 711 - ``data``: the data to be signed (obtained from current image) 712 713- ``authenticated_data``: this array pointer indicates what authentication 714 parameters must be extracted from an image once it has been authenticated. 715 Each parameter consists of a parameter descriptor and the buffer 716 address/size to store the parameter. The CoT is responsible for allocating 717 the required memory to store the parameters. This pointer may be NULL. 718 719In the ``tbbr_cot*.c`` file, a set of buffers are allocated to store the parameters 720extracted from the certificates. In the case of the TBBR CoT, these parameters 721are hashes and public keys. In DER format, an RSA-4096 public key requires 550 722bytes, and a hash requires 51 bytes. Depending on the CoT and the authentication 723process, some of the buffers may be reused at different stages during the boot. 724 725Next in that file, the parameter descriptors are defined. These descriptors will 726be used to extract the parameter data from the corresponding image. 727 728Example: the BL31 Chain of Trust 729^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 730 731Four image descriptors form the BL31 Chain of Trust: 732 733.. code:: c 734 735 static const auth_img_desc_t trusted_key_cert = { 736 .img_id = TRUSTED_KEY_CERT_ID, 737 .img_type = IMG_CERT, 738 .parent = NULL, 739 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 740 [0] = { 741 .type = AUTH_METHOD_SIG, 742 .param.sig = { 743 .pk = &subject_pk, 744 .sig = &sig, 745 .alg = &sig_alg, 746 .data = &raw_data 747 } 748 }, 749 [1] = { 750 .type = AUTH_METHOD_NV_CTR, 751 .param.nv_ctr = { 752 .cert_nv_ctr = &trusted_nv_ctr, 753 .plat_nv_ctr = &trusted_nv_ctr 754 } 755 } 756 }, 757 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 758 [0] = { 759 .type_desc = &trusted_world_pk, 760 .data = { 761 .ptr = (void *)trusted_world_pk_buf, 762 .len = (unsigned int)PK_DER_LEN 763 } 764 }, 765 [1] = { 766 .type_desc = &non_trusted_world_pk, 767 .data = { 768 .ptr = (void *)non_trusted_world_pk_buf, 769 .len = (unsigned int)PK_DER_LEN 770 } 771 } 772 } 773 }; 774 static const auth_img_desc_t soc_fw_key_cert = { 775 .img_id = SOC_FW_KEY_CERT_ID, 776 .img_type = IMG_CERT, 777 .parent = &trusted_key_cert, 778 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 779 [0] = { 780 .type = AUTH_METHOD_SIG, 781 .param.sig = { 782 .pk = &trusted_world_pk, 783 .sig = &sig, 784 .alg = &sig_alg, 785 .data = &raw_data 786 } 787 }, 788 [1] = { 789 .type = AUTH_METHOD_NV_CTR, 790 .param.nv_ctr = { 791 .cert_nv_ctr = &trusted_nv_ctr, 792 .plat_nv_ctr = &trusted_nv_ctr 793 } 794 } 795 }, 796 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 797 [0] = { 798 .type_desc = &soc_fw_content_pk, 799 .data = { 800 .ptr = (void *)content_pk_buf, 801 .len = (unsigned int)PK_DER_LEN 802 } 803 } 804 } 805 }; 806 static const auth_img_desc_t soc_fw_content_cert = { 807 .img_id = SOC_FW_CONTENT_CERT_ID, 808 .img_type = IMG_CERT, 809 .parent = &soc_fw_key_cert, 810 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 811 [0] = { 812 .type = AUTH_METHOD_SIG, 813 .param.sig = { 814 .pk = &soc_fw_content_pk, 815 .sig = &sig, 816 .alg = &sig_alg, 817 .data = &raw_data 818 } 819 }, 820 [1] = { 821 .type = AUTH_METHOD_NV_CTR, 822 .param.nv_ctr = { 823 .cert_nv_ctr = &trusted_nv_ctr, 824 .plat_nv_ctr = &trusted_nv_ctr 825 } 826 } 827 }, 828 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 829 [0] = { 830 .type_desc = &soc_fw_hash, 831 .data = { 832 .ptr = (void *)soc_fw_hash_buf, 833 .len = (unsigned int)HASH_DER_LEN 834 } 835 }, 836 [1] = { 837 .type_desc = &soc_fw_config_hash, 838 .data = { 839 .ptr = (void *)soc_fw_config_hash_buf, 840 .len = (unsigned int)HASH_DER_LEN 841 } 842 } 843 } 844 }; 845 static const auth_img_desc_t bl31_image = { 846 .img_id = BL31_IMAGE_ID, 847 .img_type = IMG_RAW, 848 .parent = &soc_fw_content_cert, 849 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 850 [0] = { 851 .type = AUTH_METHOD_HASH, 852 .param.hash = { 853 .data = &raw_data, 854 .hash = &soc_fw_hash 855 } 856 } 857 } 858 }; 859 860The **Trusted Key certificate** is signed with the ROT private key and contains 861the Trusted World public key and the Non-Trusted World public key as x509v3 862extensions. This must be specified in the image descriptor using the 863``img_auth_methods`` and ``authenticated_data`` arrays, respectively. 864 865The Trusted Key certificate is authenticated by checking its digital signature 866using the ROTPK. Four parameters are required to check a signature: the public 867key, the algorithm, the signature and the data that has been signed. Therefore, 868four parameter descriptors must be specified with the authentication method: 869 870- ``subject_pk``: parameter descriptor of type ``AUTH_PARAM_PUB_KEY``. This type 871 is used to extract a public key from the parent image. If the cookie is an 872 OID, the key is extracted from the corresponding x509v3 extension. If the 873 cookie is NULL, the subject public key is retrieved. In this case, because 874 the parent image is NULL, the public key is obtained from the platform 875 (this key will be the ROTPK). 876- ``sig``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to extract 877 the signature from the certificate. 878- ``sig_alg``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to 879 extract the signature algorithm from the certificate. 880- ``raw_data``: parameter descriptor of type ``AUTH_PARAM_RAW_DATA``. It is used 881 to extract the data to be signed from the certificate. 882 883Once the signature has been checked and the certificate authenticated, the 884Trusted World public key needs to be extracted from the certificate. A new entry 885is created in the ``authenticated_data`` array for that purpose. In that entry, 886the corresponding parameter descriptor must be specified along with the buffer 887address to store the parameter value. In this case, the ``trusted_world_pk`` 888descriptor is used to extract the public key from an x509v3 extension with OID 889``TRUSTED_WORLD_PK_OID``. The BL31 key certificate will use this descriptor as 890parameter in the signature authentication method. The key is stored in the 891``trusted_world_pk_buf`` buffer. 892 893The **BL31 Key certificate** is authenticated by checking its digital signature 894using the Trusted World public key obtained previously from the Trusted Key 895certificate. In the image descriptor, we specify a single authentication method 896by signature whose public key is the ``trusted_world_pk``. Once this certificate 897has been authenticated, we have to extract the BL31 public key, stored in the 898extension specified by ``soc_fw_content_pk``. This key will be copied to the 899``content_pk_buf`` buffer. 900 901The **BL31 certificate** is authenticated by checking its digital signature 902using the BL31 public key obtained previously from the BL31 Key certificate. 903We specify the authentication method using ``soc_fw_content_pk`` as public key. 904After authentication, we need to extract the BL31 hash, stored in the extension 905specified by ``soc_fw_hash``. This hash will be copied to the 906``soc_fw_hash_buf`` buffer. 907 908The **BL31 image** is authenticated by calculating its hash and matching it 909with the hash obtained from the BL31 certificate. The image descriptor contains 910a single authentication method by hash. The parameters to the hash method are 911the reference hash, ``soc_fw_hash``, and the data to be hashed. In this case, 912it is the whole image, so we specify ``raw_data``. 913 914The image parser library 915~~~~~~~~~~~~~~~~~~~~~~~~ 916 917The image parser module relies on libraries to check the image integrity and 918extract the authentication parameters. The number and type of parser libraries 919depend on the images used in the CoT. Raw images do not need a library, so 920only an x509v3 library is required for the TBBR CoT. 921 922Arm platforms will use an x509v3 library based on mbed TLS. This library may be 923found in ``drivers/auth/mbedtls/mbedtls_x509_parser.c``. It exports three 924functions: 925 926.. code:: c 927 928 void init(void); 929 int check_integrity(void *img, unsigned int img_len); 930 int get_auth_param(const auth_param_type_desc_t *type_desc, 931 void *img, unsigned int img_len, 932 void **param, unsigned int *param_len); 933 934The library is registered in the framework using the macro 935``REGISTER_IMG_PARSER_LIB()``. Each time the image parser module needs to access 936an image of type ``IMG_CERT``, it will call the corresponding function exported 937in this file. 938 939The build system must be updated to include the corresponding library and 940mbed TLS sources. Arm platforms use the ``arm_common.mk`` file to pull the 941sources. 942 943The cryptographic library 944~~~~~~~~~~~~~~~~~~~~~~~~~ 945 946The cryptographic module relies on a library to perform the required operations, 947i.e. verify a hash or a digital signature. Arm platforms will use a library 948based on mbed TLS, which can be found in 949``drivers/auth/mbedtls/mbedtls_crypto.c``. This library is registered in the 950authentication framework using the macro ``REGISTER_CRYPTO_LIB()`` and exports 951below functions: 952 953.. code:: c 954 955 void init(void); 956 int verify_signature(void *data_ptr, unsigned int data_len, 957 void *sig_ptr, unsigned int sig_len, 958 void *sig_alg, unsigned int sig_alg_len, 959 void *pk_ptr, unsigned int pk_len); 960 int crypto_mod_calc_hash(enum crypto_md_algo alg, void *data_ptr, 961 unsigned int data_len, 962 unsigned char output[CRYPTO_MD_MAX_SIZE]) 963 int verify_hash(void *data_ptr, unsigned int data_len, 964 void *digest_info_ptr, unsigned int digest_info_len); 965 int auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr, 966 size_t len, const void *key, unsigned int key_len, 967 unsigned int key_flags, const void *iv, 968 unsigned int iv_len, const void *tag, 969 unsigned int tag_len) 970 971The mbedTLS library algorithm support is configured by both the 972``TF_MBEDTLS_KEY_ALG`` and ``TF_MBEDTLS_KEY_SIZE`` variables. 973 974- ``TF_MBEDTLS_KEY_ALG`` can take in 3 values: `rsa`, `ecdsa` or `rsa+ecdsa`. 975 This variable allows the Makefile to include the corresponding sources in 976 the build for the various algorithms. Setting the variable to `rsa+ecdsa` 977 enables support for both rsa and ecdsa algorithms in the mbedTLS library. 978 979- ``TF_MBEDTLS_KEY_SIZE`` sets the supported RSA key size for TFA. Valid values 980 include 1024, 2048, 3072 and 4096. 981 982- ``TF_MBEDTLS_USE_AES_GCM`` enables the authenticated decryption support based 983 on AES-GCM algorithm. Valid values are 0 and 1. 984 985.. note:: 986 If code size is a concern, the build option ``MBEDTLS_SHA256_SMALLER`` can 987 be defined in the platform Makefile. It will make mbed TLS use an 988 implementation of SHA-256 with smaller memory footprint (~1.5 KB less) but 989 slower (~30%). 990 991-------------- 992 993*Copyright (c) 2017-2023, Arm Limited and Contributors. All rights reserved.* 994 995.. _TBBR-Client specification: https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a 996