This document explains how to create builds of Mbed TLS where some cryptographic mechanisms are provided only by PSA drivers (that is, no built-in implementation of those algorithms), from a user's perspective. This is useful to save code size for people who are using either a hardware accelerator, or an alternative software implementation that's more aggressively optimized for code size than the default one in Mbed TLS. General considerations ---------------------- This document assumes that you already have a working driver. Otherwise, please see the [PSA driver example and guide](psa-driver-example-and-guide.md) for information on writing a driver. In order to have some mechanism provided only by a driver, you'll want the following compile-time configuration options enabled: - `MBEDTLS_PSA_CRYPTO_C` (enabled by default) - this enables PSA Crypto. - `MBEDTLS_USE_PSA_CRYPTO` (disabled by default) - this makes PK, X.509 and TLS use PSA Crypto. You need to enable this if you're using PK, X.509 or TLS and want them to have access to the algorithms provided by your driver. (See [the dedicated document](use-psa-crypto.md) for details.) - `MBEDTLS_PSA_CRYPTO_CONFIG` (disabled by default) - this enables configuration of cryptographic algorithms using `PSA_WANT` macros in `include/psa/crypto_config.h`. See [Conditional inclusion of cryptographic mechanism through the PSA API in Mbed TLS](proposed/psa-conditional-inclusion-c.md) for details. In addition, for each mechanism you want provided only by your driver: - Define the corresponding `PSA_WANT` macro in `psa/crypto_config.h` - this means the algorithm will be available in the PSA Crypto API. - Define the corresponding `MBEDTLS_PSA_ACCEL` in your build. This could be defined in `psa/crypto_config.h` or your compiler's command line. This informs the PSA code that an accelerator is available for this mechanism. - Undefine / comment out the corresponding `MBEDTLS_xxx_C` macro in `mbedtls/mbedtls_config.h`. This ensures the built-in implementation is not included in the build. For example, if you want SHA-256 to be provided only by a driver, you'll want `PSA_WANT_ALG_SHA_256` and `MBEDTLS_PSA_ACCEL_SHA_256` defined, and `MBEDTLS_SHA256_C` undefined. In addition to these compile-time considerations, at runtime you'll need to make sure you call `psa_crypto_init()` before any function that uses the driver-only mechanisms. Note that this is already a requirement for any use of the PSA Crypto API, as well as for use of the PK, X.509 and TLS modules when `MBEDTLS_USE_PSA_CRYPTO` is enabled, so in most cases your application will already be doing this. Mechanisms covered ------------------ For now, only the following (families of) mechanisms are supported: - hashes: SHA-3, SHA-2, SHA-1, MD5, etc. - elliptic-curve cryptography (ECC): ECDH, ECDSA, EC J-PAKE, ECC key types. - finite-field Diffie-Hellman: FFDH algorithm, DH key types. Supported means that when those are provided only by drivers, everything (including PK, X.509 and TLS if `MBEDTLS_USE_PSA_CRYPTO` is enabled) should work in the same way as if the mechanisms where built-in, except as documented in the "Limitations" sub-sections of the sections dedicated to each family below. In the near future (end of 2023), we are planning to also add support for ciphers (AES) and AEADs (GCM, CCM, ChachaPoly). Currently (mid-2023) we don't have plans to extend this to RSA. If you're interested in driver-only support for RSA, please let us know. Hashes ------ TODO Elliptic-curve cryptography (ECC) --------------------------------- Note: things are still evolving. This section describes the situation right after #7452 has been merged. It will be updated again in #7757 when bignum is done. It is possible to have most ECC operations provided only by a driver: - the ECDH, ECDSA and EC J-PAKE algorithms; - key import, export, and random generation. More precisely: - you can enable `PSA_WANT_ALG_ECDH` without `MBEDTLS_ECDH_C` provided `MBEDTLS_PSA_ACCEL_ALG_ECDH` is enabled; - you can enable `PSA_WANT_ALG_ECDSA` without `MBEDTLS_ECDSA_C` provided `MBEDTLS_PSA_ACCEL_ALG_ECDSA` is enabled; - you can enable `PSA_WANT_ALG_JPAKE` without `MBEDTLS_ECJPAKE_C` provided `MBEDTLS_PSA_ACCEL_ALG_JPAKE` is enabled. In addition, if none of `MBEDTLS_ECDH_C`, `MBEDTLS_ECDSA_C`, `MBEDTLS_ECJPAKE_C` are enabled, you can enable: - `PSA_WANT_KEY_TYPE_ECC_PUBLIC_KEY`; - `PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_BASIC`; - `PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_IMPORT`; - `PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_EXPORT`; - `PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_GENERATE`; without `MBEDTLS_ECP_C` provided the corresponding `MBEDTLS_PSA_ACCEL_KEY_TYPE_xxx` are enabled. [Coming soon] If `MBEDTLS_ECP_C` is disabled and `ecp.c` is fully removed (see "Limitations regarding fully removing `ecp.c`" below), and you're not using RSA or FFDH, then you can also disable `MBEDTLS_BIGNUM_C` for further code size saving. ### Limitations regarding fully removing `ecp.c` A limited subset of `ecp.c` will still be automatically re-enabled if any of the following is enabled: - `MBEDTLS_PK_PARSE_EC_COMPRESSED` - support for parsing ECC keys where the public part is in compressed format; - `MBEDTLS_PK_PARSE_EC_EXTENDED` - support for parsing ECC keys where the curve is identified not by name, but by explicit parameters; - `PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_DERIVE` - support for deterministic derivation of an ECC keypair with `psa_key_derivation_output_key()`. Note: when any of the above options is enabled, a subset of `ecp.c` will automatically be included in the build in order to support it. Therefore you can still disable `MBEDTLS_ECP_C` in `mbedtls_config.h` and this will result in some code size savings, but not as much as when none of the above features are enabled. We do have plans to support each of these with `ecp.c` fully removed in the future, however there is no established timeline. If you're interested, please let us know, so we can take it into consideration in our planning. ### Limitations regarding restartable / interruptible ECC operations At the moment, there is not driver support for interruptible operations (see `psa_sign_hash_start()` + `psa_sign_hash_complete()` etc.) so as a consequence these are not supported in builds without `MBEDTLS_ECDSA_C`. Similarly, there is no PSA support for interruptible ECDH operations so these are not supported without `ECDH_C`. See also limitations regarding restartable operations with `MBEDTLS_USE_PSA_CRYPTO` in [its documentation](use-psa-crypto.md). Again, we have plans to support this in the future but not with an established timeline, please let us know if you're interested. ### Limitations regarding the selection of curves TODO: apparently we don't really support having some curves built-in and others driver-only... investigate and describe the situation. See also #7899. Finite-field Diffie-Hellman --------------------------- TODO