####################################### Trusted Firmware-M Profile Small Design ####################################### :Authors: 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 - 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 ===================== Library model ------------- Profile Small selects Library model in TF-M. 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 [2]_. .. 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. Level 1 isolation ----------------- So far, TF-M Library model only supports level 1 isolation [2]_, which isolates Secure Processing Environment (SPE) from Non-secure Processing Environment (NSPE). Neither level 2 nor level 3 isolation [2]_ is implemented in TF-M Library model. 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. 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. [3]_ 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. 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 [4]_ 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 [5]_ [6]_. 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 [7]_ 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 [8]_ 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 [8]_, 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 [9]_ 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 top-level CMake configuration files under folder ``configs`` 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 TF-M Profile Small and disable the unnecessary ones, during TF-M build. An alternative option is to set only a global flag in top-level Profile Small CMake file and configure the remaining configurations in dedicated CMake files of each module/secure service. However, since configuration flags are distributed in multiple CMake files, it will become difficult for a platform/use case to overwrite default configurations. Therefore it is more reasonable to explicitly set all critical configurations in a top-level CMake file. 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_PROFILE_CONFIG_EXT`` in build command line. The top-level config file will include the device configuration extension file to load platform/use case specific configurations. The overall build flow of Profile Small is shown as the flowchart below. .. uml:: @startuml title Overall build flow start :Profile Small CMake file; note left Top-level CMake config file under ""configs"". Set configurations to default values. endnote if (Platform config\nextension specified?) then (Yes) :Include platform specific\nconfig extension file; note left Platform specific configuration extension file is provided via ""TFM_PROFILE_CONFIG_EXT"" in build command line. endnote :Overwrite default configurations; else (No) endif :CommonConfig.cmake; note left Normal building sequence endnote stop @enduml The control flags set in the top-level Profile Small config file are listed below. The details will be covered in each module in `Implementation details`_. .. list-table:: Config flags in Profile S top-level CMake config file :widths: 20 15 30 :header-rows: 1 * - Configs - Default value - Descriptions * - ``CORE_IPC`` - ``False`` - Library model is selected * - ``TFM_LVL`` - ``1`` - Level 1 isolation * - ``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 * - ``CRYPTO_ASYMMETRIC_MODULE_DISABLED`` - ``ON`` - Disable asymmetric cipher in Crypto service * - ``CRYPTO_AEAD_MODULE_DISABLED`` - ``OFF`` - Enable AEAD in Crypto service * - ``MBEDTLS_CONFIG_FILE`` - ``tfm_profile_s_mbedcrypto_config`` - Default mbed-crypto config file for Profile Small under ``platform/ext/common`` * - ``TFM_PARTITION_AUDIT_LOG`` - ``OFF`` - Disable Audit Logging Logging service * - ``TFM_PARTITION_SECURE_STORAGE`` - ``OFF`` - Disable Protected Storage service * - ``TFM_PARTITION_INITIAL_ATTESTATION`` - ``ON`` - Enable Initial Attestation service * - ``SYMMETRIC_INITIAL_ATTESTATION`` - ``ON`` - Select Initial Attestation based on symmetric key algorithms * - ``TFM_PARTITION_PLATFORM`` - ``OFF`` - Disable Platform service Test cases settings in top-level Profile Small config files are listed below. The ``Default config`` stands for configuration without tests and the ``Regression config`` stands for configuration with regression tests. .. list-table:: Test config flags in Profile S top-level CMake config file :widths: 20 20 15 15 :header-rows: 1 * - Test cases - Configs - Default config - Regression config * - Regression test - ``REGRESSION`` - ``OFF`` - ``ON`` * - Core test - ``CORE_TEST`` - ``OFF`` - ``ON`` * - PSA API test - ``PSA_API_TEST`` - ``OFF`` - ``OFF`` Implementation details ====================== This section discusses the details of Profile Small implementation. .. 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. Top-level Profile Small CMake config file ----------------------------------------- There are two top-level Profile Small CMake config files under folder ``configs``. - ``ConfigDefaultProfileS.cmake`` completes Profile Small default configurations without test cases. - ``ConfigRegressionProfileS.cmake`` enables regression and core test cases for the features defined Profile Small, besides default configurations. The details of configuration control flags set in top-level configuration file are listed in following sections. Device configuration extension ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ To overwrite default configurations and add platform specific configurations, a platform can set the path to its own configuration extension file in parameter ``TFM_PROFILE_CONFIG_EXT`` in command line. A platform can also add its device specific configurations into its specific CMake file under ``platform/ext/`` folder. TF-M framework setting ---------------------- The top-level Profile Small CMake config file selects Library model and level 1 isolation. 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 [10]_ 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_profile_s_mbedcrypto_config.h`` under ``platform/ext/common``. TF-M Profile Small specifies ``tfm_profile_s_mbedcrypto_config.h`` as the default Mbed Crypto config in ``MBEDTLS_CONFIG_FILE`` in top-level CMake config file, instead of the common one ``tfm_mbedcrypto_config.h`` [10]_. 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 replace Profile Small default Mbed Crypto config file with its specific one to overwrite the default configurations. Alternatively, a device can overwrite the configurations by appending a config file via ``MBEDTLS_USER_CONFIG_FILE``. Internal Trusted Storage configurations --------------------------------------- ITS service is enabled in top-level Profile Small CMake config file. The internal transient buffer size ``ITS_BUF_SIZE`` [7]_ 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 [7]_. 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 [8]_. 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. 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 or in its specific CMake file under ``platform/ext/`` folder. If MCUBoot provided by TF-M is enabled, single image boot is selected in TF-M Profile Small top-level CMake config file. The following table lists the configurations specified in Profile Small top-level config file for MCUBoot provided by TF-M. .. list-table:: MCUBoot config flags in Profile S top-level CMake config file :widths: 30 15 30 :header-rows: 1 * - Configs - Default value - Descriptions * - ``BL2`` - ``True`` - MCUBoot is enabled * - ``MCUBOOT_IMAGE_NUMBER`` - ``1`` - Single image boot If a device implements its own boot loader, the configurations are implementation defined. **************** Platform support **************** To enable Profile Small on a platform, the platform specific CMake file should be added into the platform support list in top-level Profile Small CMake config file. Building Profile Small ====================== To build Profile Small, argument ``PROJ_CONFIG`` in build command line should be set to ``ConfigRegressionProfileS.cmake`` or ``ConfigRegressionProfileS.cmake``. Take AN521 as an example. The following commands build Profile Small without test cases on **AN521** with build type **MinSizeRel**, built by **Armclang**. .. code-block:: bash cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigDefaultProfileS.cmake` \ -DTARGET_PLATFORM=AN521 \ -DCMAKE_BUILD_TYPE=MinSizeRel \ -DCOMPILER=ARMCLANG ../ cmake --build ./ -- install The following commands build Profile Small with regression test cases on **AN521** with build type **MinSizeRel**, built by **Armclang**. .. code-block:: bash cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigRegressionProfileS.cmake` \ -DTARGET_PLATFORM=AN521 \ -DCMAKE_BUILD_TYPE=MinSizeRel \ -DCOMPILER=ARMCLANG ../ cmake --build ./ -- install More details of building instructions and parameters can be found TF-M build instruction guide [11]_. The following commands include platform specific configuration extension file via ``TFM_PROFILE_CONFIG_EXT`` in command line. ``TFM_PROFILE_CONFIG_EXT`` can be an absolute path or a relative one to TF-M code root directory. .. code-block:: bash cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigDefaultProfileS.cmake` \ -DTARGET_PLATFORM=AN521 \ -DCMAKE_BUILD_TYPE=MinSizeRel \ -DCOMPILER=ARMCLANG \ -DTFM_PROFILE_CONFIG_EXT=path/to/config_ext_file ../ cmake --build ./ -- install ********* Reference ********* .. [1] `Pre-Shared Key Ciphersuites for Transport Layer Security (TLS) `_ .. [2] `DEN0063 Arm Platform Security Architecture Firmware Framework 1.0 `_ .. [3] `PSA analyze stage `_ .. [4] `AES-CCM Cipher Suites for Transport Layer Security (TLS) `_ .. [5] `Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms `_ .. [6] `Transitioning the Use of Cryptographic Algorithms and Key Lengths `_ .. [7] :doc:`ITS integration guide ` .. [8] `PSA Attestation API 1.0 (ARM IHI 0085) `_ .. [9] :doc:`Secure boot ` .. [10] :doc:`Crypto design ` .. [11] :doc:`TF-M build instruction ` -------------- *Copyright (c) 2020, Arm Limited. All rights reserved.*