Crypto Service design ===================== :Author: Antonio de Angelis :Organization: Arm Limited :Contact: Antonio de Angelis .. contents:: Table of Contents Abstract -------- This document describes the design of the TF-M Cryptographic Secure Service (in short, TF-M Crypto service). Introduction ------------ The TF-M Crypto service provides an implementation of the PSA Crypto API in a PSA RoT secure partition in TF-M. It is based on Mbed Crypto, which is a reference implementation of the PSA Crypto API. For more details on the PSA Crypto API or the Mbed Crypto implementation, please refer directly to the ``mbed-crypto`` GitHub repository [1]_ . The service can be used by other services running in the SPE, or by applications running in the NSPE, to provide cryptographic functionalities. Components ---------- The TF-M Crypto service is implemented by a number of different software components, which are listed below: .. table:: Components table :widths: auto +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | **Component name** | **Description** | **Location** | +=============================+===============================================================+======================================================================+ | Client API interface | This module exports the client API of PSA Crypto to the users.| ``./interface/src/tfm_crypto_api.c`` | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | Mbed Crypto | The Mbed Crypto library is used in the service as a | Needed as dependency at the same level of the TF-M folder, | | | cryptographic library exposing the PSA Crypto API interface. | i.e. ``../mbed-crypto`` | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | Init module | This module handles the initialisation of the service objects | ``./secure_fw/partitions/crypto/crypto_init.c`` | | | during TF-M boot and provides the infrastructure to service | | | | requests when TF-M is built for IPC model. | | | | The dispatching mechanism of IPC requests is based on a look | | | | up table of function pointers. | | | | This design allows for better scalability and support of a | | | | higher number of Secure functions with minimal overhead and | | | | duplication of code. | | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | Alloc module | This module handles the allocation of contexts for multipart | ``./secure_fw/partitions/crypto/crypto_alloc.c`` | | | operations in the Secure world. | | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | Service modules | These modules (AEAD, Asymmetric, Cipher, Key Deriv, Hash, Key,| ``./secure_fw/partitions/crypto/crypto_aead.c`` | | | MAC) represent a thin layer which is in charge of servicing | ``./secure_fw/partitions/crypto/crypto_asymmetric.c`` | | | the calls from the SPE/NSPE client API interfaces. | ``./secure_fw/partitions/crypto/crypto_cipher.c`` | | | They provide parameter sanitation and context retrieval for | ``./secure_fw/partitions/crypto/crypto_key_derivation.c`` | | | multipart operations, and dispatching to the corresponding | ``./secure_fw/partitions/crypto/crypto_hash.c`` | | | library function exposed by Mbed Crypto for the desired | ``./secure_fw/partitions/crypto/crypto_key.c`` | | | functionality. | ``./secure_fw/partitions/crypto/crypto_mac.c`` | | | | ''./secure_fw/partitions/crypto/crypto_key_management.c'' | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | Manifest | The manifest file is a description of the service components. | ``./secure_fw/partitions/crypto/manifest.yaml`` | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | CMake files and headers | The CMake files are used by the TF-M CMake build system to | ``./secure_fw/partitions/crypto/CMakeLists.inc`` | | | build the service as part of the Secure FW build. The service | ``./secure_fw/partitions/crypto/CMakeLists.txt`` | | | is built as a static library (``tfm_crypto.a``). | ``./interface/include/tfm_crypto_defs.h`` | | | The build system allows to build as well the Mbed Crypto | ``./secure_fw/partitions/crypto/tfm_crypto_api.h`` | | | library as part of the Secure FW build process and archive it | ``./secure_fw/partitions/crypto/tfm_crypto_signal.h`` | | | with the static library of the Crypto service. | ``./secure_fw/partitions/crypto/spe_crypto.h`` | | | The headers are used to export the public prototypes of the | | | | functions in the Service modules ``tfm_crypto_api.h``, and | | | | to provide the necessary defines (i.e. ``TFM_CRYPTO_SIG``). | | | | In particular ``TFM_CRYPTO_SIG`` identifies the signal on | | | | which the service handler waits for requests when the service | | | | is built for IPC model. | | | | The header available in the interface, ``tfm_crypto_defs.h`` | | | | , contains types and defines for building the NSPE interface | | | | as part of a Non-Secure application. | | | | Finally, the ``crypto_spe.h`` header is used during the | | | | build of the Mbed Crypto library, when the Mbed Crypto config | | | | option ``MBEDTLS_PSA_CRYPTO_SPM`` is defined, to add a | | | | custom prefix to the PSA API symbols so that duplication of | | | | symbol names is avoided. | | | | The prefix used for the PSA API symbols of the Mbed Crypto | | | | library is chosen to be ``mbedcrypto__``. | | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ | Documentation | The integration guide contains the description of the TF-M | ``./user_guides/services/tfm_crypto_integration_guide.rst`` | | | Crypto service modules and interfaces. | | +-----------------------------+---------------------------------------------------------------+----------------------------------------------------------------------+ The interaction between the different components is described by the following block diagram: .. figure:: /design_docs/media/tfm_crypto_design.png Block diagram of the different components of the TF-M Crypto service. A dotted line is used to indicate the interaction with a library. Note: in IPC model, the interaction between components is slightly different, as the Service modules are not called directly through the TF-M Secure Partition Manager but through the IPC handler which resides in the Init module. Service API description ----------------------- Most of the APIs exported by the TF-M Crypto service (i.e. from the Service modules) have a direct correspondence with the PSA Crypto API. The Alloc and Init modules instead export some APIs which are specific to the TF-M Crypto service, and are available only to the Service modules or the SPM. For a detailed description of the prototypes please refer to the ``tfm_crypto_api.h`` header. .. table:: Init and Alloc modules APIs :widths: auto +--------------------------------+--------------+-----------------+------------------------------------------------------+ | **Function** | **Module** | **Caller** | **Scope** | +================================+==============+=================+======================================================+ | tfm_crypto_init() | Init | SPM | Called during TF-M boot for initialisation. In IPC | | | | | model, it calls the IPC service request handler. | +--------------------------------+--------------+-----------------+------------------------------------------------------+ | tfm_crypto_init_alloc() | Alloc | Init | Called by tfm_crypto_init(), it initialises the | | | | | concurrent operation contexts storage area. | +--------------------------------+--------------+-----------------+------------------------------------------------------+ | tfm_crypto_operation_alloc() | Alloc | Service modules | It allocates a new operation context for a multipart | | | | | operation. It returns an handle to the allocated | | | | | context in secure memory. | +--------------------------------+--------------+-----------------+------------------------------------------------------+ | tfm_crypto_operation_lookup() | Alloc | Service modules | It retrieves a previously allocated operation context| | | | | of a multipart operation, based on the handle given | | | | | as input. | +--------------------------------+--------------+-----------------+------------------------------------------------------+ | tfm_crypto_operation_release() | Alloc | Service modules | It releases a previously allocated operation context | | | | | of a multipart operation, based on the handle given | | | | | as input. | +--------------------------------+--------------+-----------------+------------------------------------------------------+ Configuration parameters ------------------------ The TF-M Crypto service exposes some configuration parameters to tailor the service configuration in terms of supported functionalities and hence FLASH/RAM size to meet the requirements of different platforms and use cases. These parameters can be provided via CMake parameters during the CMake configuration step and as a configuration header to allow the configuration of the Mbed Crypto library. .. table:: Configuration parameters table :widths: auto +------------------------------------+---------------------------+----------------------------------------------------------------+-----------------------------------------+----------------------------------------------------------------------------+ | **Parameter** | **Type** | **Description** | **Scope** | **Default** | +====================================+===========================+================================================================+=========================================+============================================================================+ | ``CRYPTO_ENGINE_BUF_SIZE`` | CMake build | Buffer used by Mbed Crypto for its own allocations at runtime. | To be configured based on the desired | 8096 (bytes) | | | configuration parameter | This is a buffer allocated in static memory. | use case and application requirements. | | +------------------------------------+---------------------------+----------------------------------------------------------------+-----------------------------------------+----------------------------------------------------------------------------+ | ``CRYPTO_CONC_OPER_NUM`` | CMake build | This parameter defines the maximum number of possible | To be configured based on the desire | 8 | | | configuration parameter | concurrent operation contexts (cipher, MAC, hash and key deriv)| use case and platform requirements. | | | | | for multi-part operations, that can be allocated simultaneously| | | | | | at any time. | | | +------------------------------------+---------------------------+----------------------------------------------------------------+-----------------------------------------+----------------------------------------------------------------------------+ | ``CRYPTO_IOVEC_BUFFER_SIZE`` | CMake build | This parameter applies only to IPC model builds. In IPC model, | To be configured based on the desired | 5120 (bytes) | | | configuration parameter | during a Service call, input and outputs are allocated | use case and application requirements. | | | | | temporarily in an internal scratch buffer whose size is | | | | | | determined by this parameter. | | | +------------------------------------+---------------------------+----------------------------------------------------------------+-----------------------------------------+----------------------------------------------------------------------------+ | ``MBEDTLS_CONFIG_FILE`` | Configuration header | The Mbed Crypto library can be configured to support different | To be configured based on the | ``./lib/ext/mbedcrypto/mbedcrypto_config/tfm_mbedcrypto_config_default.h`` | | | | algorithms through the usage of a a configuration header file | application and platform requirements. | | | | | at build time. This allows for tailoring FLASH/RAM requirements| | | | | | for different platforms and use cases. | | | +------------------------------------+---------------------------+----------------------------------------------------------------+-----------------------------------------+----------------------------------------------------------------------------+ | ``MBEDTLS_PSA_CRYPTO_CONFIG_FILE`` | Configuration header | This header file specifies which cryptographic mechanisms are | To be configured based on the | ``./lib/ext/mbedcrypto/mbedcrypto_config/crypto_config_default.h`` | | | | available through the PSA API when #MBEDTLS_PSA_CRYPTO_CONFIG | application and platform requirements. | | | | | is enabled, and is not used when #MBEDTLS_PSA_CRYPTO_CONFIG is | | | | | | disabled. | | | +------------------------------------+---------------------------+----------------------------------------------------------------+-----------------------------------------+----------------------------------------------------------------------------+ References ---------- .. [1] ``mbed-crypto`` repository which holds the PSA Crypto API specification and the Mbed Crypto reference implementation: \ https://github.com/Mbed-TLS -------------- *Copyright (c) 2019-2022, Arm Limited. All rights reserved.*