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 int (*auth_decrypt)(enum crypto_dec_algo dec_algo, void *data_ptr, 245 size_t len, const void *key, unsigned int key_len, 246 unsigned int key_flags, const void *iv, 247 unsigned int iv_len, const void *tag, 248 unsigned int tag_len); 249 250These functions are registered in the CM using the macro: 251 252.. code:: c 253 254 REGISTER_CRYPTO_LIB(_name, 255 _init, 256 _verify_signature, 257 _calc_hash, 258 _verify_hash, 259 _auth_decrypt, 260 _convert_pk); 261 262``_name`` must be a string containing the name of the CL. This name is used for 263debugging purposes. 264 265Crypto module provides a function ``_calc_hash`` to calculate and 266return the hash of the given data using the provided hash algorithm. 267This function is mainly used in the ``MEASURED_BOOT`` and ``DRTM_SUPPORT`` 268features to calculate the hashes of various images/data. 269 270Optionally, a platform function can be provided to convert public key 271(_convert_pk). It is only used if the platform saves a hash of the ROTPK. 272Most platforms save the hash of the ROTPK, but some may save slightly different 273information - e.g the hash of the ROTPK plus some related information. 274Defining this function allows to transform the ROTPK used to verify 275the signature to the buffer (a platform specific public key) which 276hash is saved in OTP. 277 278.. code:: c 279 280 int (*convert_pk)(void *full_pk_ptr, unsigned int full_pk_len, 281 void **hashed_pk_ptr, unsigned int *hashed_pk_len); 282 283 284- ``full_pk_ptr``: Pointer to Distinguished Encoding Rules (DER) ROTPK. 285- ``full_pk_len``: DER ROTPK size. 286- ``hashed_pk_ptr``: to return a pointer to a buffer, which hash should be the one saved in OTP. 287- ``hashed_pk_len``: previous buffer size 288 289Image Parser Module (IPM) 290^^^^^^^^^^^^^^^^^^^^^^^^^ 291 292The IPM is responsible for: 293 294#. Checking the integrity of each image loaded by the IO framework. 295#. Extracting parameters used for authenticating an image based upon a 296 description provided by the platform in the CoT descriptor. 297 298Images may have different formats (for example, authentication images could be 299x509v3 certificates, signed ELF files or any other platform specific format). 300The IPM allows to register an Image Parser Library (IPL) for every image format 301used in the CoT. This library must implement the specific methods to parse the 302image. The IPM obtains the image format from the CoT and calls the right IPL to 303check the image integrity and extract the authentication parameters. 304 305See Section "Describing the image parsing methods" for more details about the 306mechanism the IPM provides to define and register IPLs. 307 308Authentication methods 309~~~~~~~~~~~~~~~~~~~~~~ 310 311The AM supports the following authentication methods: 312 313#. Hash 314#. Digital signature 315 316The platform may specify these methods in the CoT in case it decides to define 317a custom CoT instead of reusing a predefined one. 318 319If a data image uses multiple methods, then all the methods must be a part of 320the same CoT. The number and type of parameters are method specific. These 321parameters should be obtained from the parent image using the IPM. 322 323#. Hash 324 325 Parameters: 326 327 #. A pointer to data to hash 328 #. Length of the data 329 #. A pointer to the hash 330 #. Length of the hash 331 332 The hash will be represented by the DER encoding of the following ASN.1 333 type: 334 335 :: 336 337 DigestInfo ::= SEQUENCE { 338 digestAlgorithm DigestAlgorithmIdentifier, 339 digest Digest 340 } 341 342 This ASN.1 structure makes it possible to remove any assumption about the 343 type of hash algorithm used as this information accompanies the hash. This 344 should allow the Cryptography Library (CL) to support multiple hash 345 algorithm implementations. 346 347#. Digital Signature 348 349 Parameters: 350 351 #. A pointer to data to sign 352 #. Length of the data 353 #. Public Key Algorithm 354 #. Public Key value 355 #. Digital Signature Algorithm 356 #. Digital Signature value 357 358 The Public Key parameters will be represented by the DER encoding of the 359 following ASN.1 type: 360 361 :: 362 363 SubjectPublicKeyInfo ::= SEQUENCE { 364 algorithm AlgorithmIdentifier{PUBLIC-KEY,{PublicKeyAlgorithms}}, 365 subjectPublicKey BIT STRING } 366 367 The Digital Signature Algorithm will be represented by the DER encoding of 368 the following ASN.1 types. 369 370 :: 371 372 AlgorithmIdentifier {ALGORITHM:IOSet } ::= SEQUENCE { 373 algorithm ALGORITHM.&id({IOSet}), 374 parameters ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL 375 } 376 377 The digital signature will be represented by: 378 379 :: 380 381 signature ::= BIT STRING 382 383The authentication framework will use the image descriptor to extract all the 384information related to authentication. 385 386Specifying a Chain of Trust 387--------------------------- 388 389A CoT can be described as a set of image descriptors linked together in a 390particular order. The order dictates the sequence in which they must be 391verified. Each image has a set of properties which allow the AM to verify it. 392These properties are described below. 393 394The PP is responsible for defining a single or multiple CoTs for a data image. 395Unless otherwise specified, the data structures described in the following 396sections are populated by the PP statically. 397 398Describing the image parsing methods 399~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 400 401The parsing method refers to the format of a particular image. For example, an 402authentication image that represents a certificate could be in the X.509v3 403format. A data image that represents a boot loader stage could be in raw binary 404or ELF format. The IPM supports three parsing methods. An image has to use one 405of the three methods described below. An IPL is responsible for interpreting a 406single parsing method. There has to be one IPL for every method used by the 407platform. 408 409#. Raw format: This format is effectively a nop as an image using this method 410 is treated as being in raw binary format e.g. boot loader images used by 411 TF-A. This method should only be used by data images. 412 413#. X509V3 method: This method uses industry standards like X.509 to represent 414 PKI certificates (authentication images). It is expected that open source 415 libraries will be available which can be used to parse an image represented 416 by this method. Such libraries can be used to write the corresponding IPL 417 e.g. the X.509 parsing library code in mbed TLS. 418 419#. Platform defined method: This method caters for platform specific 420 proprietary standards to represent authentication or data images. For 421 example, The signature of a data image could be appended to the data image 422 raw binary. A header could be prepended to the combined blob to specify the 423 extents of each component. The platform will have to implement the 424 corresponding IPL to interpret such a format. 425 426The following enum can be used to define these three methods. 427 428.. code:: c 429 430 typedef enum img_type_enum { 431 IMG_RAW, /* Binary image */ 432 IMG_PLAT, /* Platform specific format */ 433 IMG_CERT, /* X509v3 certificate */ 434 IMG_MAX_TYPES, 435 } img_type_t; 436 437An IPL must provide functions with the following prototypes: 438 439.. code:: c 440 441 void init(void); 442 int check_integrity(void *img, unsigned int img_len); 443 int get_auth_param(const auth_param_type_desc_t *type_desc, 444 void *img, unsigned int img_len, 445 void **param, unsigned int *param_len); 446 447An IPL for each type must be registered using the following macro: 448 449.. code:: c 450 451 REGISTER_IMG_PARSER_LIB(_type, _name, _init, _check_int, _get_param) 452 453- ``_type``: one of the types described above. 454- ``_name``: a string containing the IPL name for debugging purposes. 455- ``_init``: initialization function pointer. 456- ``_check_int``: check image integrity function pointer. 457- ``_get_param``: extract authentication parameter function pointer. 458 459The ``init()`` function will be used to initialize the IPL. 460 461The ``check_integrity()`` function is passed a pointer to the memory where the 462image has been loaded by the IO framework and the image length. It should ensure 463that the image is in the format corresponding to the parsing method and has not 464been tampered with. For example, RFC-2459 describes a validation sequence for an 465X.509 certificate. 466 467The ``get_auth_param()`` function is passed a parameter descriptor containing 468information about the parameter (``type_desc`` and ``cookie``) to identify and 469extract the data corresponding to that parameter from an image. This data will 470be used to verify either the current or the next image in the CoT sequence. 471 472Each image in the CoT will specify the parsing method it uses. This information 473will be used by the IPM to find the right parser descriptor for the image. 474 475Describing the authentication method(s) 476~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 477 478As part of the CoT, each image has to specify one or more authentication methods 479which will be used to verify it. As described in the Section "Authentication 480methods", there are three methods supported by the AM. 481 482.. code:: c 483 484 typedef enum { 485 AUTH_METHOD_NONE, 486 AUTH_METHOD_HASH, 487 AUTH_METHOD_SIG, 488 AUTH_METHOD_NUM 489 } auth_method_type_t; 490 491The AM defines the type of each parameter used by an authentication method. It 492uses this information to: 493 494#. Specify to the ``get_auth_param()`` function exported by the IPM, which 495 parameter should be extracted from an image. 496 497#. Correctly marshall the parameters while calling the verification function 498 exported by the CM and PP. 499 500#. Extract authentication parameters from a parent image in order to verify a 501 child image e.g. to verify the certificate image, the public key has to be 502 obtained from the parent image. 503 504.. code:: c 505 506 typedef enum { 507 AUTH_PARAM_NONE, 508 AUTH_PARAM_RAW_DATA, /* Raw image data */ 509 AUTH_PARAM_SIG, /* The image signature */ 510 AUTH_PARAM_SIG_ALG, /* The image signature algorithm */ 511 AUTH_PARAM_HASH, /* A hash (including the algorithm) */ 512 AUTH_PARAM_PUB_KEY, /* A public key */ 513 } auth_param_type_t; 514 515The AM defines the following structure to identify an authentication parameter 516required to verify an image. 517 518.. code:: c 519 520 typedef struct auth_param_type_desc_s { 521 auth_param_type_t type; 522 void *cookie; 523 } auth_param_type_desc_t; 524 525``cookie`` is used by the platform to specify additional information to the IPM 526which enables it to uniquely identify the parameter that should be extracted 527from an image. For example, the hash of a BL3x image in its corresponding 528content certificate is stored in an X509v3 custom extension field. An extension 529field can only be identified using an OID. In this case, the ``cookie`` could 530contain the pointer to the OID defined by the platform for the hash extension 531field while the ``type`` field could be set to ``AUTH_PARAM_HASH``. A value of 0 for 532the ``cookie`` field means that it is not used. 533 534For each method, the AM defines a structure with the parameters required to 535verify the image. 536 537.. code:: c 538 539 /* 540 * Parameters for authentication by hash matching 541 */ 542 typedef struct auth_method_param_hash_s { 543 auth_param_type_desc_t *data; /* Data to hash */ 544 auth_param_type_desc_t *hash; /* Hash to match with */ 545 } auth_method_param_hash_t; 546 547 /* 548 * Parameters for authentication by signature 549 */ 550 typedef struct auth_method_param_sig_s { 551 auth_param_type_desc_t *pk; /* Public key */ 552 auth_param_type_desc_t *sig; /* Signature to check */ 553 auth_param_type_desc_t *alg; /* Signature algorithm */ 554 auth_param_type_desc_t *tbs; /* Data signed */ 555 } auth_method_param_sig_t; 556 557The AM defines the following structure to describe an authentication method for 558verifying an image 559 560.. code:: c 561 562 /* 563 * Authentication method descriptor 564 */ 565 typedef struct auth_method_desc_s { 566 auth_method_type_t type; 567 union { 568 auth_method_param_hash_t hash; 569 auth_method_param_sig_t sig; 570 } param; 571 } auth_method_desc_t; 572 573Using the method type specified in the ``type`` field, the AM finds out what field 574needs to access within the ``param`` union. 575 576Storing Authentication parameters 577~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 578 579A parameter described by ``auth_param_type_desc_t`` to verify an image could be 580obtained from either the image itself or its parent image. The memory allocated 581for loading the parent image will be reused for loading the child image. Hence 582parameters which are obtained from the parent for verifying a child image need 583to have memory allocated for them separately where they can be stored. This 584memory must be statically allocated by the platform port. 585 586The AM defines the following structure to store the data corresponding to an 587authentication parameter. 588 589.. code:: c 590 591 typedef struct auth_param_data_desc_s { 592 void *auth_param_ptr; 593 unsigned int auth_param_len; 594 } auth_param_data_desc_t; 595 596The ``auth_param_ptr`` field is initialized by the platform. The ``auth_param_len`` 597field is used to specify the length of the data in the memory. 598 599For parameters that can be obtained from the child image itself, the IPM is 600responsible for populating the ``auth_param_ptr`` and ``auth_param_len`` fields 601while executing the ``img_get_auth_param()`` function. 602 603The AM defines the following structure to enable an image to describe the 604parameters that should be extracted from it and used to verify the next image 605(child) in a CoT. 606 607.. code:: c 608 609 typedef struct auth_param_desc_s { 610 auth_param_type_desc_t type_desc; 611 auth_param_data_desc_t data; 612 } auth_param_desc_t; 613 614Describing an image in a CoT 615~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 616 617An image in a CoT is a consolidation of the following aspects of a CoT described 618above. 619 620#. A unique identifier specified by the platform which allows the IO framework 621 to locate the image in a FIP and load it in the memory reserved for the data 622 image in the CoT. 623 624#. A parsing method which is used by the AM to find the appropriate IPM. 625 626#. Authentication methods and their parameters as described in the previous 627 section. These are used to verify the current image. 628 629#. Parameters which are used to verify the next image in the current CoT. These 630 parameters are specified only by authentication images and can be extracted 631 from the current image once it has been verified. 632 633The following data structure describes an image in a CoT. 634 635.. code:: c 636 637 typedef struct auth_img_desc_s { 638 unsigned int img_id; 639 const struct auth_img_desc_s *parent; 640 img_type_t img_type; 641 const auth_method_desc_t *const img_auth_methods; 642 const auth_param_desc_t *const authenticated_data; 643 } auth_img_desc_t; 644 645A CoT is defined as an array of pointers to ``auth_image_desc_t`` structures 646linked together by the ``parent`` field. Those nodes with no parent must be 647authenticated using the ROTPK stored in the platform. 648 649Implementation example 650---------------------- 651 652This section is a detailed guide explaining a trusted boot implementation using 653the authentication framework. This example corresponds to the Applicative 654Functional Mode (AFM) as specified in the TBBR-Client document. It is 655recommended to read this guide along with the source code. 656 657The TBBR CoT 658~~~~~~~~~~~~ 659 660CoT specific to BL1 and BL2 can be found in ``drivers/auth/tbbr/tbbr_cot_bl1.c`` 661and ``drivers/auth/tbbr/tbbr_cot_bl2.c`` respectively. The common CoT used across 662BL1 and BL2 can be found in ``drivers/auth/tbbr/tbbr_cot_common.c``. 663This CoT consists of an array of pointers to image descriptors and it is 664registered in the framework using the macro ``REGISTER_COT(cot_desc)``, where 665``cot_desc`` must be the name of the array (passing a pointer or any other 666type of indirection will cause the registration process to fail). 667 668The number of images participating in the boot process depends on the CoT. 669There is, however, a minimum set of images that are mandatory in TF-A and thus 670all CoTs must present: 671 672- ``BL2`` 673- ``SCP_BL2`` (platform specific) 674- ``BL31`` 675- ``BL32`` (optional) 676- ``BL33`` 677 678The TBBR specifies the additional certificates that must accompany these images 679for a proper authentication. Details about the TBBR CoT may be found in the 680:ref:`Trusted Board Boot` document. 681 682Following the :ref:`Porting Guide`, a platform must provide unique 683identifiers for all the images and certificates that will be loaded during the 684boot process. If a platform is using the TBBR as a reference for trusted boot, 685these identifiers can be obtained from ``include/common/tbbr/tbbr_img_def.h``. 686Arm platforms include this file in ``include/plat/arm/common/arm_def.h``. Other 687platforms may also include this file or provide their own identifiers. 688 689**Important**: the authentication module uses these identifiers to index the 690CoT array, so the descriptors location in the array must match the identifiers. 691 692Each image descriptor must specify: 693 694- ``img_id``: the corresponding image unique identifier defined by the platform. 695- ``img_type``: the image parser module uses the image type to call the proper 696 parsing library to check the image integrity and extract the required 697 authentication parameters. Three types of images are currently supported: 698 699 - ``IMG_RAW``: image is a raw binary. No parsing functions are available, 700 other than reading the whole image. 701 - ``IMG_PLAT``: image format is platform specific. The platform may use this 702 type for custom images not directly supported by the authentication 703 framework. 704 - ``IMG_CERT``: image is an x509v3 certificate. 705 706- ``parent``: pointer to the parent image descriptor. The parent will contain 707 the information required to authenticate the current image. If the parent 708 is NULL, the authentication parameters will be obtained from the platform 709 (i.e. the BL2 and Trusted Key certificates are signed with the ROT private 710 key, whose public part is stored in the platform). 711- ``img_auth_methods``: this points to an array which defines the 712 authentication methods that must be checked to consider an image 713 authenticated. Each method consists of a type and a list of parameter 714 descriptors. A parameter descriptor consists of a type and a cookie which 715 will point to specific information required to extract that parameter from 716 the image (i.e. if the parameter is stored in an x509v3 extension, the 717 cookie will point to the extension OID). Depending on the method type, a 718 different number of parameters must be specified. This pointer should not be 719 NULL. 720 Supported methods are: 721 722 - ``AUTH_METHOD_HASH``: the hash of the image must match the hash extracted 723 from the parent image. The following parameter descriptors must be 724 specified: 725 726 - ``data``: data to be hashed (obtained from current image) 727 - ``hash``: reference hash (obtained from parent image) 728 729 - ``AUTH_METHOD_SIG``: the image (usually a certificate) must be signed with 730 the private key whose public part is extracted from the parent image (or 731 the platform if the parent is NULL). The following parameter descriptors 732 must be specified: 733 734 - ``pk``: the public key (obtained from parent image) 735 - ``sig``: the digital signature (obtained from current image) 736 - ``alg``: the signature algorithm used (obtained from current image) 737 - ``data``: the data to be signed (obtained from current image) 738 739- ``authenticated_data``: this array pointer indicates what authentication 740 parameters must be extracted from an image once it has been authenticated. 741 Each parameter consists of a parameter descriptor and the buffer 742 address/size to store the parameter. The CoT is responsible for allocating 743 the required memory to store the parameters. This pointer may be NULL. 744 745In the ``tbbr_cot*.c`` file, a set of buffers are allocated to store the parameters 746extracted from the certificates. In the case of the TBBR CoT, these parameters 747are hashes and public keys. In DER format, an RSA-4096 public key requires 550 748bytes, and a hash requires 51 bytes. Depending on the CoT and the authentication 749process, some of the buffers may be reused at different stages during the boot. 750 751Next in that file, the parameter descriptors are defined. These descriptors will 752be used to extract the parameter data from the corresponding image. 753 754Example: the BL31 Chain of Trust 755^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 756 757Four image descriptors form the BL31 Chain of Trust: 758 759.. code:: c 760 761 static const auth_img_desc_t trusted_key_cert = { 762 .img_id = TRUSTED_KEY_CERT_ID, 763 .img_type = IMG_CERT, 764 .parent = NULL, 765 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 766 [0] = { 767 .type = AUTH_METHOD_SIG, 768 .param.sig = { 769 .pk = &subject_pk, 770 .sig = &sig, 771 .alg = &sig_alg, 772 .data = &raw_data 773 } 774 }, 775 [1] = { 776 .type = AUTH_METHOD_NV_CTR, 777 .param.nv_ctr = { 778 .cert_nv_ctr = &trusted_nv_ctr, 779 .plat_nv_ctr = &trusted_nv_ctr 780 } 781 } 782 }, 783 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 784 [0] = { 785 .type_desc = &trusted_world_pk, 786 .data = { 787 .ptr = (void *)trusted_world_pk_buf, 788 .len = (unsigned int)PK_DER_LEN 789 } 790 }, 791 [1] = { 792 .type_desc = &non_trusted_world_pk, 793 .data = { 794 .ptr = (void *)non_trusted_world_pk_buf, 795 .len = (unsigned int)PK_DER_LEN 796 } 797 } 798 } 799 }; 800 static const auth_img_desc_t soc_fw_key_cert = { 801 .img_id = SOC_FW_KEY_CERT_ID, 802 .img_type = IMG_CERT, 803 .parent = &trusted_key_cert, 804 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 805 [0] = { 806 .type = AUTH_METHOD_SIG, 807 .param.sig = { 808 .pk = &trusted_world_pk, 809 .sig = &sig, 810 .alg = &sig_alg, 811 .data = &raw_data 812 } 813 }, 814 [1] = { 815 .type = AUTH_METHOD_NV_CTR, 816 .param.nv_ctr = { 817 .cert_nv_ctr = &trusted_nv_ctr, 818 .plat_nv_ctr = &trusted_nv_ctr 819 } 820 } 821 }, 822 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 823 [0] = { 824 .type_desc = &soc_fw_content_pk, 825 .data = { 826 .ptr = (void *)content_pk_buf, 827 .len = (unsigned int)PK_DER_LEN 828 } 829 } 830 } 831 }; 832 static const auth_img_desc_t soc_fw_content_cert = { 833 .img_id = SOC_FW_CONTENT_CERT_ID, 834 .img_type = IMG_CERT, 835 .parent = &soc_fw_key_cert, 836 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 837 [0] = { 838 .type = AUTH_METHOD_SIG, 839 .param.sig = { 840 .pk = &soc_fw_content_pk, 841 .sig = &sig, 842 .alg = &sig_alg, 843 .data = &raw_data 844 } 845 }, 846 [1] = { 847 .type = AUTH_METHOD_NV_CTR, 848 .param.nv_ctr = { 849 .cert_nv_ctr = &trusted_nv_ctr, 850 .plat_nv_ctr = &trusted_nv_ctr 851 } 852 } 853 }, 854 .authenticated_data = (const auth_param_desc_t[COT_MAX_VERIFIED_PARAMS]) { 855 [0] = { 856 .type_desc = &soc_fw_hash, 857 .data = { 858 .ptr = (void *)soc_fw_hash_buf, 859 .len = (unsigned int)HASH_DER_LEN 860 } 861 }, 862 [1] = { 863 .type_desc = &soc_fw_config_hash, 864 .data = { 865 .ptr = (void *)soc_fw_config_hash_buf, 866 .len = (unsigned int)HASH_DER_LEN 867 } 868 } 869 } 870 }; 871 static const auth_img_desc_t bl31_image = { 872 .img_id = BL31_IMAGE_ID, 873 .img_type = IMG_RAW, 874 .parent = &soc_fw_content_cert, 875 .img_auth_methods = (const auth_method_desc_t[AUTH_METHOD_NUM]) { 876 [0] = { 877 .type = AUTH_METHOD_HASH, 878 .param.hash = { 879 .data = &raw_data, 880 .hash = &soc_fw_hash 881 } 882 } 883 } 884 }; 885 886The **Trusted Key certificate** is signed with the ROT private key and contains 887the Trusted World public key and the Non-Trusted World public key as x509v3 888extensions. This must be specified in the image descriptor using the 889``img_auth_methods`` and ``authenticated_data`` arrays, respectively. 890 891The Trusted Key certificate is authenticated by checking its digital signature 892using the ROTPK. Four parameters are required to check a signature: the public 893key, the algorithm, the signature and the data that has been signed. Therefore, 894four parameter descriptors must be specified with the authentication method: 895 896- ``subject_pk``: parameter descriptor of type ``AUTH_PARAM_PUB_KEY``. This type 897 is used to extract a public key from the parent image. If the cookie is an 898 OID, the key is extracted from the corresponding x509v3 extension. If the 899 cookie is NULL, the subject public key is retrieved. In this case, because 900 the parent image is NULL, the public key is obtained from the platform 901 (this key will be the ROTPK). 902- ``sig``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to extract 903 the signature from the certificate. 904- ``sig_alg``: parameter descriptor of type ``AUTH_PARAM_SIG``. It is used to 905 extract the signature algorithm from the certificate. 906- ``raw_data``: parameter descriptor of type ``AUTH_PARAM_RAW_DATA``. It is used 907 to extract the data to be signed from the certificate. 908 909Once the signature has been checked and the certificate authenticated, the 910Trusted World public key needs to be extracted from the certificate. A new entry 911is created in the ``authenticated_data`` array for that purpose. In that entry, 912the corresponding parameter descriptor must be specified along with the buffer 913address to store the parameter value. In this case, the ``trusted_world_pk`` 914descriptor is used to extract the public key from an x509v3 extension with OID 915``TRUSTED_WORLD_PK_OID``. The BL31 key certificate will use this descriptor as 916parameter in the signature authentication method. The key is stored in the 917``trusted_world_pk_buf`` buffer. 918 919The **BL31 Key certificate** is authenticated by checking its digital signature 920using the Trusted World public key obtained previously from the Trusted Key 921certificate. In the image descriptor, we specify a single authentication method 922by signature whose public key is the ``trusted_world_pk``. Once this certificate 923has been authenticated, we have to extract the BL31 public key, stored in the 924extension specified by ``soc_fw_content_pk``. This key will be copied to the 925``content_pk_buf`` buffer. 926 927The **BL31 certificate** is authenticated by checking its digital signature 928using the BL31 public key obtained previously from the BL31 Key certificate. 929We specify the authentication method using ``soc_fw_content_pk`` as public key. 930After authentication, we need to extract the BL31 hash, stored in the extension 931specified by ``soc_fw_hash``. This hash will be copied to the 932``soc_fw_hash_buf`` buffer. 933 934The **BL31 image** is authenticated by calculating its hash and matching it 935with the hash obtained from the BL31 certificate. The image descriptor contains 936a single authentication method by hash. The parameters to the hash method are 937the reference hash, ``soc_fw_hash``, and the data to be hashed. In this case, 938it is the whole image, so we specify ``raw_data``. 939 940The image parser library 941~~~~~~~~~~~~~~~~~~~~~~~~ 942 943The image parser module relies on libraries to check the image integrity and 944extract the authentication parameters. The number and type of parser libraries 945depend on the images used in the CoT. Raw images do not need a library, so 946only an x509v3 library is required for the TBBR CoT. 947 948Arm platforms will use an x509v3 library based on mbed TLS. This library may be 949found in ``drivers/auth/mbedtls/mbedtls_x509_parser.c``. It exports three 950functions: 951 952.. code:: c 953 954 void init(void); 955 int check_integrity(void *img, unsigned int img_len); 956 int get_auth_param(const auth_param_type_desc_t *type_desc, 957 void *img, unsigned int img_len, 958 void **param, unsigned int *param_len); 959 960The library is registered in the framework using the macro 961``REGISTER_IMG_PARSER_LIB()``. Each time the image parser module needs to access 962an image of type ``IMG_CERT``, it will call the corresponding function exported 963in this file. 964 965The build system must be updated to include the corresponding library and 966mbed TLS sources. Arm platforms use the ``arm_common.mk`` file to pull the 967sources. 968 969The cryptographic library 970~~~~~~~~~~~~~~~~~~~~~~~~~ 971 972The cryptographic module relies on a library to perform the required operations, 973i.e. verify a hash or a digital signature. Arm platforms will use a library 974based on mbed TLS, which can be found in 975``drivers/auth/mbedtls/mbedtls_crypto.c``. This library is registered in the 976authentication framework using the macro ``REGISTER_CRYPTO_LIB()`` and exports 977below functions: 978 979.. code:: c 980 981 void init(void); 982 int verify_signature(void *data_ptr, unsigned int data_len, 983 void *sig_ptr, unsigned int sig_len, 984 void *sig_alg, unsigned int sig_alg_len, 985 void *pk_ptr, unsigned int pk_len); 986 int crypto_mod_calc_hash(enum crypto_md_algo alg, void *data_ptr, 987 unsigned int data_len, 988 unsigned char output[CRYPTO_MD_MAX_SIZE]) 989 int verify_hash(void *data_ptr, unsigned int data_len, 990 void *digest_info_ptr, unsigned int digest_info_len); 991 int auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr, 992 size_t len, const void *key, unsigned int key_len, 993 unsigned int key_flags, const void *iv, 994 unsigned int iv_len, const void *tag, 995 unsigned int tag_len) 996 997The mbedTLS library algorithm support is configured by both the 998``TF_MBEDTLS_KEY_ALG`` and ``TF_MBEDTLS_KEY_SIZE`` variables. 999 1000- ``TF_MBEDTLS_KEY_ALG`` can take in 3 values: `rsa`, `ecdsa` or `rsa+ecdsa`. 1001 This variable allows the Makefile to include the corresponding sources in 1002 the build for the various algorithms. Setting the variable to `rsa+ecdsa` 1003 enables support for both rsa and ecdsa algorithms in the mbedTLS library. 1004 1005- ``TF_MBEDTLS_KEY_SIZE`` sets the supported RSA key size for TFA. Valid values 1006 include 1024, 2048, 3072 and 4096. 1007 1008- ``TF_MBEDTLS_USE_AES_GCM`` enables the authenticated decryption support based 1009 on AES-GCM algorithm. Valid values are 0 and 1. 1010 1011.. note:: 1012 If code size is a concern, the build option ``MBEDTLS_SHA256_SMALLER`` can 1013 be defined in the platform Makefile. It will make mbed TLS use an 1014 implementation of SHA-256 with smaller memory footprint (~1.5 KB less) but 1015 slower (~30%). 1016 1017-------------- 1018 1019*Copyright (c) 2017-2023, Arm Limited and Contributors. All rights reserved.* 1020 1021.. _TBBR-Client specification: https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a 1022
