####################################### Trusted Firmware-M Profile Small Design ####################################### :Author: David Hu :Organization: Arm Limited :Contact: david.hu@arm.com ************ Introduction ************ The capabilities and resources may dramatically vary on different IoT devices. Some IoT devices may have very limited memory resource. The program on those devices should keep small memory footprint and basic functionalities. On the other hand, some devices may consist of more memory and extended storage, to support stronger software capabilities. Diverse IoT use cases also require different levels of security and requirements on device resource. For example, use cases require different cipher capabilities. Selecting cipher suites can be sensitive to memory footprint on devices with constrained resource. Trusted Firmware-M (TF-M) defines 3 general profiles, Profile Small, Profile Medium and Profile Large, to provide different levels of security to fit diverse device capabilities and use cases. Each profile specifies a predefined list of features, targeting typical use cases with specific hardware constraints. Profiles can serve as reference designs, based on which developers can continue further development and configurations, according to use case. As one of the TF-M Profiles, TF-M Profile Small (Profile S) consists of lightweight TF-M framework and basic Secure Services to keep smallest memory footprint, supporting fundamental security features on devices with ultra constrained resource. This profile enables connecting with Edge Gateways and IoT Cloud Services supporting secure connection based solely on symmetric cryptography. This document summarizes and discusses the features specified in TF-M Profile Small. ************** Overall design ************** TF-M Profile Small defines the following features: - Lightweight framework - Library model or Secure Function (SFN) model [2]_ - Level 1 isolation - Buffer sharing allowed - Single secure context - Crypto - Symmetric cipher only - Cipher suite for symmetric-key algorithms based protocols, such as cipher suites defined in TLS pre-shared key (TLS-PSK) [1]_. - Advanced Encryption Standard (AES) as symmetric crypto algorithm - SHA256 as Hash function - HMAC as Message Authentication Code algorithm - Internal Trusted Storage (ITS) - No encryption - No rollback protection - Decrease internal transient buffer size - Initial Attestation - Based on symmetric key algorithms - Lightweight boot - Single image boot - Anti-rollback protection is enabled Protected Storage, audit logging and other Secure Services provided by TF-M are disabled by default. ************** Design details ************** More details of TF-M Profile Small design are discussed in following sections. Lightweight framework ===================== TF-M framework model -------------------- Library model is selected by default in Profile Small implementation. Library model implements secure function calls, via which clients directly call secure services. It provides a more simple implementation of TF-M framework and may reduce memory footprint, compared with Inter-Process Communication (IPC) model [3]_. As Library model is TF-M specific implementation, please check some of its dedicated implementation details as described in `Appendix`_, before adopting Library model on your platforms. You can select SFN model instead of Library model in Profile Small. SFN model is defined in FF-M 1.1 extensions [2]_. It is a more simple implementation of TF-M framework and may also reduce memory footprint, compared with Inter-Process Communication (IPC) model [3]_. Level 1 isolation ----------------- PSA Security Model [4]_ defines 3 levels of isolation. - Level 1 isolation isolates Secure Processing Environment (SPE) from Non-secure Processing Environment (NSPE). - PSA Root of Trust (PSA RoT) and Application Root of Trust (ARoT) are isolated from each other in level 2 isolation. - Individual secure partitions are isolated from each other even within a particular security domain (PSA RoT, ARoT), in level 3 isolation. Profile Small dedicated use cases with simple service model may not require level 2 or level 3 isolation. Devices which Profile Small aims at may be unable to implement stricter isolation, limited by hardware capabilities. Level 1 isolation reduces requirements enforced by hardware isolation and cost of software for management. .. note :: **Security note** If a device or a use case enforces level 2 or level 3 isolation, it is suggested to apply other configurations, other than TF-M Profile Small. Crypto service ============== TF-M Profile Small only requires symmetric crypto since symmetric algorithms require shorter keys and less computational burden, compared with asymmetric crypto. By default, TF-M Profile Small requires the same capabilities as defined in TLS-PSK, to support symmetric key algorithms based protocols. .. note :: **Implementation note** Please note that TF-M Profile Small doesn't require that TLS-PSK is mandatory in applications. Instead, Profile Small only requires the same capabilities as defined in TLS-PSK, such as one symmetric cipher algorithm and one hash function. TF-M Profile Small selects TLS-PSK cipher suite TLS_PSK_WITH_AES_128_CCM [6]_ as reference, which requires: - AES-128-CCM (AES CCM mode with 128-bit key) as symmetric crypto algorithm - SHA256 as Hash function - HMAC as Message Authentication Code algorithm TLS_PSK_WITH_AES_128_CCM is selected since it requires small key length and less hardware capabilities, while keeping enough level of security. .. note :: **Implementation note** Developers can replace default algorithms with others or implement more algorithms. Proper symmetric key algorithms and cipher suites should be selected according to device capabilities, the use case and the requirement of peers in connection. Refer to `Crypto service configuration`_ for implementation details of configuring algorithms and cipher suites. .. note :: **Security note** It is recommended not to use MD5 or SHA-1 for message digests as they are subject to collision attacks [7]_ [8]_. Secure Storage ============== TF-M Profile Small assumes that extremely constrained devices only contain basic on-chip storage, without external or removable storage. As a result, TF-M Profile Small includes ITS service and disables Protected Storage service. Encryption and rollback protection ---------------------------------- Neither encryption nor rollback protection is enabled in current ITS implementation. It is expected that ITS relies solely on the physical inaccessibility property of on-chip storage, together with PSA isolation, without requiring additional cryptographic protection. Internal transient buffer ------------------------- ITS implements a internal transient buffer [9]_ to hold the data read from/written to storage, especially for flash, to solve the alignment and security issues. The internal transient buffer is aligned to the flash device’s program unit. Copying data to it from the caller can align all write requests to the flash device’s program unit. The internal transient buffer can help protect Flash access from some attacks, such as TOCTOU attack. Although removing this internal buffer can save some memory consumption, typically 512 bytes, it may bring alignment or security issues. Therefore, to achieve a better trade-off between memory footprint and security, TF-M Profile Small optimizes the internal buffer size to 32 bytes by default. As discussed in `Crypto service`_, TF-M Profile Small requires AES-128 and SHA-256, which use 128-bit key and 256-bit key respectively. Besides, either long public/private keys or PKI-based certificates should be very rare as asymmetric crypto is not supported in Profile Small. Therefore, a 32-byte internal buffer should cover the assets in TF-M Profile Small use cases. The buffer size can be adjusted according to use case and device Flash attributes. Refer to `Internal Trusted Storage configurations`_ for more details. Initial Attestation =================== Profile Small requires an Initial Attestation secure service based on symmetric key algorithms. Refer to PSA Attestation API document [10]_ for details of Initial Attestation based on symmetric key algorithms. It can heavily increase memory footprint to support Initial Attestation based on asymmetric key algorithms, due to asymmetric ciphers and related PKI modules. .. note :: **Implementation note** As pointed out by PSA Attestation API document [10]_, the use cases of Initial Attestation based on symmetric key algorithms can be limited due to the associated infrastructure costs for key management and operational complexities. It may also restrict the ability to interoperate with scenarios that involve third parties. If asymmetric key algorithms based Initial Attestation is required in use scenarios, it is recommended to select other TF-M Profiles which support asymmetric key algorithms. .. note :: **Implementation note** It is recommended to utilize the same MAC algorithm supported in Crypto service to complete the signing in ``COSE_Mac0``, to minimize memory footprint. Lightweight boot ================ If MCUBoot provided by TF-M is enabled, single image boot [11]_ is selected by default in Profile Small. In case of single image boot, secure and non-secure images are handled as a single blob and signed together during image generation. However, secure and non-secure images must be updated together in single image boot. It may decrease the flexibility of image update and cost longer update process. Since the image sizes should usually be small with limited functionalities in Profile Small dedicated use case, the cost may still be reasonable. BL2 implementation can be device specific. Devices may implement diverse boot processes with different features and configurations. However, anti-rollback protection is required as a mandatory feature of boot loader. Boot loader should be able to prevent unauthorized rollback, to protect devices from being downgraded to earlier versions with known vulnerabilities. ************** Implementation ************** Overview ======== The basic idea is to add dedicated profile CMake configuration files under folder ``config/profile`` for TF-M Profile Small default configuration. The top-level Profile Small config file collects all the necessary configuration flags and set them to default values, to explicitly enable the features required in Profile Small and disable the unnecessary ones, during TF-M build. A platform/use case can provide a configuration extension file to overwrite Profile Small default setting and append other configurations. This configuration extension file can be added via parameter ``TFM_EXTRA_CONFIG_PATH`` in build command line. The behavior of the Profile Small build flow (particularly the order of configuration loading and overriding) can be found at :ref:`tfm_cmake_configuration` The details of configurations will be covered in each module in `Implementation details`_. Implementation details ====================== This section discusses the details of Profile Small implementation. Top-level configuration files ----------------------------- The firmware framework configurations in ``config/profile/profile_small`` are shown below. .. table:: TFM options in Profile Small top-level CMake config file :widths: auto :align: center +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | Configs | Default value | Descriptions | +============================================+=====================================================================================================+=====================================+ | ``TFM_ISOLATION_LEVEL`` | ``1`` | Select level 2 isolation | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_LIB_MODEL`` | ``ON`` | Select Library model | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_PARTITION_INTERNAL_TRUSTED_STORAGE`` | ``ON`` | Enable ITS SP | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``ITS_BUF_SIZE`` | ``32`` | ITS internal transient buffer size | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_PARTITION_CRYPTO`` | ``ON`` | Enable Crypto service | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_MBEDCRYPTO_CONFIG_PATH`` | ``${CMAKE_SOURCE_DIR}/lib/ext/mbedcrypto/mbedcrypto_config/tfm_mbedcrypto_config_profile_small.h`` | Mbed Crypto config file path | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``CRYPTO_ASYM_SIGN_MODULE_DISABLED`` | ``ON`` | Disable asymmetric signature | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``CRYPTO_ASYM_ENCRYPT_MODULE_DISABLED`` | ``ON`` | Disable asymmetric encryption | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_PARTITION_INITIAL_ATTESTATION`` | ``ON`` | Enable Initial Attestation service | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``SYMMETRIC_INITIAL_ATTESTATION`` | ``ON`` | Enable symmetric attestation | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_PARTITION_PROTECTED_STORAGE`` | ``OFF`` | Enable PS service | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_PARTITION_PLATFORM`` | ``OFF`` | Enable TF-M Platform SP | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_PARTITION_AUDIT_LOG`` | ``OFF`` | Disable TF-M audit logging service | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ .. note :: **Implementation note** The following sections focus on the feature selection via configuration setting. Dedicated optimization on memory footprint is not covered in this document. Device configuration extension ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ To change default configurations and add platform specific configurations, a platform can add a platform configuration file at ``platform/ext/config.cmake`` TF-M framework setting ---------------------- The top-level Profile Small CMake config file selects Library model and level 1 isolation. Users can set ``-DCONFIG_TFM_SPM_BACKEND=SFN`` in build command to select SFN model instead. Crypto service configuration ---------------------------- Crypto Secure Partition ^^^^^^^^^^^^^^^^^^^^^^^ TF-M Profile Small enables Crypto Secure Partition (SP) in its top-level CMake config file. Crypto SP modules not supported in TF-M Profile Small are disabled. The disabled modules are shown below. - Disable asymmetric cipher Other modules and configurations [12]_ are kept as default values. Additional configuration flags with more fine granularity can be added to control building of specific crypto algorithms and corresponding test cases. Mbed Crypto configurations ^^^^^^^^^^^^^^^^^^^^^^^^^^ TF-M Profile Small adds a dedicated Mbed Crypto config file ``tfm_mbedcrypto_config_profile_small.h`` at ``/lib/ext/mbedcrypto/mbedcrypto_config`` file, instead of the common one ``tfm_mbedcrypto_config_default.h`` [12]_. Major Mbed Crypto configurations are set as listed below: - Enable SHA256 - Enable generic message digest wrappers - Enable AES - Enable CCM mode for symmetric ciphers - Disable other modes for symmetric ciphers - Disable asymmetric ciphers - Disable HMAC-based key derivation function (HKDF) Other configurations can be selected to optimize the memory footprint of Crypto module. A device/use case can append an extra config header to the Profile Small default Mbed Crypto config file. This can be done by setting the ``TFM_MBEDCRYPTO_PLATFORM_EXTRA_CONFIG_PATH`` cmake variable in the platform config file ``platform/ext/config.cmake``. This cmake variable is a wrapper around the ``MBEDTLS_USER_CONFIG_FILE`` options, but is preferred as it keeps all configuration in cmake. Internal Trusted Storage configurations --------------------------------------- ITS service is enabled in top-level Profile Small CMake config file. The internal transient buffer size ``ITS_BUF_SIZE`` [9]_ is set to 32 bytes by default. A platform/use case can overwrite the buffer size in its specific configuration extension according to its actual requirement of assets and Flash attributes. Profile Small CMake config file won't touch the configurations of device specific Flash hardware attributes [9]_. Initial Attestation secure service ---------------------------------- TF-M Profile Small provides a reference implementation of symmetric key algorithms based Initial Attestation, using HMAC SHA-256 as MAC algorithm in ``COSE_Mac0`` structure. The implementation follows PSA Attestation API document [10]_. Profile Small top-level config file enables Initial Attestation secure service and selects symmetric key algorithms based Initial Attestation by default. - Set ``TFM_PARTITION_INITIAL_ATTESTATION`` to ``ON`` - Set ``SYMMETRIC_INITIAL_ATTESTATION`` to ``ON`` Symmetric and asymmetric key algorithms based Initial Attestation can share the same generations of token claims, except Instance ID claim. Profile Small may implement the procedure or rely on a 3rd-party tool to construct and sign ``COSE_Mac0`` structure. Details of symmetric key algorithms based Initial Attestation design will be covered in a dedicated document. Disabled secure services ------------------------ Audit logging, Protected Storage, and Platform Service are disabled by default in Profile Small top-level CMake config file. Test configuration ------------------ Some cryptography tests are disabled due to the reduced Mbed Crypto config. Some of them are shown in the table below. .. table:: TFM options in Profile Small top-level CMake config file :widths: auto :align: center +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | Configs | Default value | Descriptions | +============================================+=====================================================================================================+=====================================+ | ``TFM_CRYPTO_TEST_ALG_CBC`` | ``OFF`` | Test CBC cryptography mode | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_ALG_CCM`` | ``ON`` | Test CCM cryptography mode | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_ALG_CFB`` | ``OFF`` | Test CFB cryptography mode | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_ALG_CTR`` | ``OFF`` | Test CTR cryptography mode | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_ALG_GCM`` | ``OFF`` | Test GCM cryptography mode | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_ALG_SHA_512`` | ``OFF`` | Test SHA-512 cryptography algorithm | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_HKDF`` | ``OFF`` | Test HKDF key derivation algorithm | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``TFM_CRYPTO_TEST_ECDH`` | ``OFF`` | Test ECDH key agreement algorithm | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ BL2 setting ----------- Profile Small enables MCUBoot provided by TF-M by default. A platform can overwrite this configuration by disabling MCUBoot in its configuration extension file ``platform/ext/config.cmake``. If MCUBoot provided by TF-M is enabled, single image boot is selected in TF-M Profile Small top-level CMake config file. If a device implements its own boot loader, the configurations are implementation defined. .. table:: BL2 options in Profile Small top-level CMake config file :widths: auto :align: center +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | Configs | Default value | Descriptions | +============================================+=====================================================================================================+=====================================+ | ``BL2`` | ``ON`` | Enable MCUBoot bootloader | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ | ``MCUBOOT_IMAGE_NUMBER`` | ``1`` | Combine S and NS images | +--------------------------------------------+-----------------------------------------------------------------------------------------------------+-------------------------------------+ **************** Platform support **************** Building Profile Small ====================== To build Profile Small, argument ``TFM_PROFILE`` in build command line should be set to ``profile_small``. Take AN521 as an example. The following commands build Profile Small without test cases on **AN521** with build type **MinSizeRel**, built by **Armclang**. Library model is selected by default. .. code-block:: bash cd mkdir build && cd build cmake -DTFM_PLATFORM=arm/mps2/an521 \ -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \ -DTFM_PROFILE=profile_small \ -DCMAKE_BUILD_TYPE=MinSizeRel \ ../ cmake --build ./ -- install The following commands build Profile Small with regression test cases on **AN521** with build type **MinSizeRel**, built by **Armclang**. Library model is selected by default. .. code-block:: bash cd mkdir build && cd build cmake -DTFM_PLATFORM=arm/mps2/an521 \ -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \ -DTFM_PROFILE=profile_small \ -DCMAKE_BUILD_TYPE=MinSizeRel \ -DTEST_NS=ON \ ../ cmake --build ./ -- install .. Note:: - For devices with more constrained memory and flash requirements, it is possible to build with either only TEST_S enabled or only TEST_NS enabled. This will decrease the size of the test images. Note that both test suites must still be run to ensure correct operation. The following commands build Profile Small with SFN model on **AN521** with build type **MinSizeRel**, built by **GNU Arm compiler**. .. code-block:: bash cd mkdir build && cd build cmake -DTFM_PLATFORM=arm/mps2/an521 \ -DTFM_PROFILE=profile_small \ -DCMAKE_BUILD_TYPE=MinSizeRel \ -DCONFIG_TFM_SPM_BACKEND=SFN \ ../ cmake --build ./ -- install More details of building instructions and parameters can be found TF-M build instruction guide [13]_. ******** Appendix ******** TF-M Library model implementation details ========================================= .. note :: **Implementation note** Please note that there is no public dedicated specification for Library model. The design, interfaces and implementation of Library model in TF-M may change. Buffer sharing allowed ---------------------- To simplify interface and reduce memory footprint, TF-M Library model directly handles client call input vectors from non-secure client buffers and later writes results back to those buffers, without keeping a copy in a transient buffer inside TF-M. .. note :: **Security note** There can be security vulnerabilities if non-secure client buffers are directly shared between NSPE and SPE, such as Time-of-check to time-of-use (TOCTOU) attack. Developers need to check if this can meet the Security Functional Requirements (SFR) of the integration of their devices. Some SFRs are listed in a set of example Threat Models and Security Analyses (TMSA) offered by PSA for common IoT use cases. [5]_ Single secure context --------------------- TF-M Library model only supports single secure context. It cannot support multiple contexts or the scheduling implemented in IPC model. It neither can support multiple outstanding PSA client calls. But correspondingly, it can save memory footprint and runtime complexity in context management and scheduling. .. note :: **Security note** Non-secure software should prevent triggering multiple outstanding PSA client calls concurrently. Otherwise, it may crash current running secure context. ********* Reference ********* .. [1] `Pre-Shared Key Ciphersuites for Transport Layer Security (TLS) `_ .. [2] `Arm Firmware Framework for M 1.1 Extensions `_ .. [3] `Arm Platform Security Architecture Firmware Framework 1.0 `_ .. [4] `Platform Security Model 1.1 `_ .. [5] `PSA analyze stage `_ .. [6] `AES-CCM Cipher Suites for Transport Layer Security (TLS) `_ .. [7] `Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms `_ .. [8] `Transitioning the Use of Cryptographic Algorithms and Key Lengths `_ .. [9] :doc:`ITS integration guide ` .. [10] `PSA Attestation API 1.0 (ARM IHI 0085) `_ .. [11] :doc:`Secure boot ` .. [12] :doc:`Crypto design ` .. [13] :doc:`TF-M build instruction ` -------------- *Copyright (c) 2020-2022, Arm Limited. All rights reserved.*