Docs: TF-M Profile Small design document
Add a document to discuss TF-M Profile Small design and
implementation.
Change-Id: If2114cafc1c6604e2cb6da22424d8299a8e29534
Signed-off-by: David Hu <david.hu@arm.com>
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+#######################################
+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_RAM_FS``
+ - ``OFF``
+ - Stop using RAM to simulate Flash
+ * - ``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
+ * - ``CRYPTO_KEY_DERIVATION_MODULE_DISABLED``
+ - ``ON``
+ - Disable key derivation 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
+ - Disable key derivation
+
+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) <https://tools.ietf.org/html/rfc4279>`_
+
+.. [2] `DEN0063 Arm Platform Security Architecture Firmware Framework 1.0 <https://developer.arm.com/-/media/Files/pdf/DeviceSecurityArchitecture/Architect/DEN0063-PSA_Firmware_Framework-1.0.0-2.pdf?revision=2d1429fa-4b5b-461a-a60e-4ef3d8f7f4b4>`_
+
+.. [3] `PSA analyze stage <https://developer.arm.com/architectures/security-architectures/platform-security-architecture#analyze>`_
+
+.. [4] `AES-CCM Cipher Suites for Transport Layer Security (TLS) <https://tools.ietf.org/html/rfc6655>`_
+
+.. [5] `Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms <https://tools.ietf.org/html/rfc6151>`_
+
+.. [6] `Transitioning the Use of Cryptographic Algorithms and Key Lengths <https://www.nist.gov/publications/transitioning-use-cryptographic-algorithms-and-key-lengths>`_
+
+.. [7] :doc:`ITS integration guide </docs/user_guides/services/tfm_its_integration_guide>`
+
+.. [8] `PSA Attestation API 1.0 (ARM IHI 0085) <https://developer.arm.com/-/media/Files/pdf/PlatformSecurityArchitecture/Implement/IHI0085-PSA_Attestation_API-1.0.2.pdf?revision=eef78753-c77e-4b24-bcf0-65596213b4c1&la=en&hash=E5E0353D612077AFDCE3F2F3708A50C77A74B2A3>`_
+
+.. [9] :doc:`Secure boot </docs/user_guides/tfm_secure_boot>`
+
+.. [10] :doc:`Crypto design </docs/design_documents/tfm_crypto_design>`
+
+.. [11] :doc:`TF-M build instruction </docs/user_guides/tfm_build_instruction>`
+
+--------------
+
+*Copyright (c) 2020, Arm Limited. All rights reserved.*