Crypto Service Integration Guide

Introduction

The TF-M Crypto service allows Non Secure world applications and Secure services to use cryptographic functionalities such as symmetric ciphering, message signing and verification, asymmetric encryption and decryption, cryptographic hashes, message authentication codes (MACs), key derivation and agreement, authenticated encryption with associated data (AEAD). It exposes the PSA Crypto APIs [1].

The secure service resides in the Crypto partition as a single entry point and is made of different components:

• An interface layer that exposes the PSA Crypto API to either NS or S entities and is implemented in interface/src/tfm_crypto_api.c. The interface is based on the Uniform Secure Service Signature and communicates with the Secure Partition Manager available in TF-M

• An init module secure_fw/partitions/crypto/crypto_init.c that implements functionalities requested by TF-M during the initialisation phase, and an API dispatcher that at runtime receives the requests from the interface and dispatches them to the component that processes that particular API request

• A set of components that process cryptographic API requests, each component dispatching to a subset of functionalities, i.e. AEAD, Asymmetric, Ciphering, Hashing, Key derivation, Key management, MACs, and Random Number Generation

• An alloc module secure_fw/partitions/crypto/crypto_alloc.c that manages the partition secure memory, storing multipart application contexts, input / outputs of the APIs being requested, inaccessible from NS or other secure partitions

• A library abstraction module secure_fw/partitions/crypto/crypto_library.c which is used to abstract the details of the cryptographic library used by the service backend to provide the actual implementation of the crypto functionalities. The backend library must expose the PSA Crypto APIs, and must provide support to encode key ownership into key identifiers. This is not standardized by the PSA Crypto APIs so it must be provided as an extension to the APIs. The backend library also needs to provide the subsystem for key storage and retrieval, and, in case, the interface to the cryptographic accelerator of the underlying platform, using the PSA cryptoprocessor driver interface specification [2]. For this reason, it must provide a mechanism to access platform builtin keys, and permanent key slots using the TF-M Internal Trusted Storage (ITS) service, if available.

Code structure

The PSA interfaces for the Crypto service are located in interface/include. The only header to be included by applications that want to use functions from the PSA API is psa/crypto.h. The TF-M Crypto service source files are located in secure_fw/partitions/crypto.

PSA interfaces

The TF-M Crypto service exposes the PSA interfaces detailed in the header psa/crypto.h. This header, in turn, includes several other headers which are not meant to be included directly by user applications. For a detailed description of the PSA API interface, please refer to the comments in the psa/crypto.h header itself.

Service source files

A brief description of what is implemented by each source file is as below:

• crypto_cipher.c : Dispatcher for symmetric crypto operations

• crypto_hash.c : Dispatcher for hash operations

• crypto_mac.c : Dispatcher for MAC operations

• crypto_aead.c : dispatcher for AEAD operations

• crypto_key_derivation.c : Dispatcher for key derivation and key agreement operations

• crypto_key_management.c : Dispatcher for key management operations towards the key slot management system provided by the backend library

• crypto_rng.c : Dispatcher for the random number generation requests

• crypto_asymmetric.c : Dispatcher for message signature/verification and encryption/decryption using asymmetric crypto

• crypto_init.c : Init module for the service. The modules stores also the internal buffer used to allocate temporarily the IOVECs needed, which is not required in case of SFN model. The size of this buffer is controlled by the CRYPTO_IOVEC_BUFFER_SIZE config define

• crypto_library.c : Library abstractions to interface the dispatchers towards the underlying library providing backend crypto functions. Currently this only supports the Mbed TLS library. In particular, the mbed TLS library requires to provide a static buffer to be used as heap for its internal allocation. The size of this buffer is controlled by the CRYPTO_ENGINE_BUF_SIZE config define

• crypto_alloc.c : Takes care of storing multipart operation contexts in a secure memory not visible outside of the crypto service. The CRYPTO_CONC_OPER_NUM config define determines how many concurrent contexts are supported at once. In a multipart operation, the client view of the contexts is much simpler (i.e. just an handle), and the Alloc module keeps track of the association between handles and contexts

• tfm_crypto_api.c : This module is contained in interface/src and implements the PSA Crypto API client interface exposed to both S/NS clients. This module allows a configuration option CONFIG_TFM_CRYPTO_API_RENAME to be set to 1 in case the NS environment or integrators want to rename the API symbols exported by the TF-M Crypto service. The renaming adds a default prefix, tfm_crypto__ to all functions. The prefix can be changed editing the interface file. This config option is for the NS environment or integration setup only, hence it is not accessible through the TF-M config

• tfm_mbedcrypto_alt.c : This module is specific to the Mbed TLS [3] library integration and provides some alternative implementation of Mbed TLS APIs that can be used when a optimised profile is chosen. Through the _ALT mechanism it is possible to replace at link time default implementations available in Mbed TLS with the ones available in this file

  Note

  The _ALT mechanism will be deprecated in future releases of the Mbed TLS library

Considerations on service configuration

Crypto backend library configuration

The TF-M Crypto service relies on a cryptographic library to provide the functionalities specific by the PSA Crypto API spec and the PSA cryptoprocessor driver interface spec. At the moment, the only supported library is mbed TLS [3].

The configuration of the backend library is supplied using the TFM_MBEDCRYPTO_CONFIG_PATH and TFM_MBEDCRYPTO_PSA_CRYPTO_CONFIG_PATH config option that point to configuration headers following the legacy Mbed TLS configuration scheme or the new PSA based configuration scheme.

Platforms can specify an extra config file by setting the TFM_MBEDCRYPTO_PLATFORM_EXTRA_CONFIG_PATH variable (which is a wrapper around the MBEDTLS_USER_CONFIG_FILE option). This is preferred for platform configuration over TFM_MBEDCRYPTO_CONFIG_PATH and TFM_MBEDCRYPTO_PSA_CRYPTO_CONFIG_PATH as it does not interfere with config changes due to TFM Profile.

Note

The default entropy source configured for Mbed TLS is MBEDTLS_ENTROPY_NV_SEED with a unique seed. For production devices, an alternative hardware entropy source can be specified using the config option MBEDTLS_ENTROPY_HARDWARE_ALT

Note

Starting from Mbed TLS 3.3.0, the Python package jsonschema must be available when building as it is required by the autogen framework for the driver integrations into the PSA Crypto core and driver wrapper modules

Crypto service build time options

• CRYPTO_STACK_SIZE : Defines the stack size of the Crypto Secure Partition. This value might depend on several parameters such as the build type, the compiler being used, the cryptographic functionalities that are enabled at build time

• CRYPTO_<COMPONENT>_MODULE_ENABLED : A series of defines, one per each <COMPONENT> that processes cryptographic operations, that are used to disable modules at build time. Each define corresponds to a component as described in the components list.

Crypto service builtin keys integration

A detailed description of how the service interacts with builtin keys is available in the tfm_builtin_key_loader design document.

Note

The crypto service integration with builtin keys relies on implementation details of Mbed TLS that are not standardized in the spec and might change between releases due to ongoing work [4]

References

[1]

PSA Crypto APIs: https://armmbed.github.io/mbed-crypto/html/

[2]

PSA cryptoprocessor driver interface: https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-interface.md

[3] (1,2)

Mbed TLS library: https://www.trustedfirmware.org/projects/mbed-tls/

[4]

Interface for platform keys: https://github.com/ARM-software/psa-crypto-api/issues/550

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