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 is 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 ------ It is possible to have all hash operations provided only by a driver. More precisely: - you can enable `PSA_WANT_ALG_SHA_256` without `MBEDTLS_SHA256_C`, provided you have `MBEDTLS_PSA_ACCEL_ALG_SHA_256` enabled; - and similarly for all supported hash algorithms: `MD5`, `RIPEMD160`, `SHA_1`, `SHA_224`, `SHA_256`, `SHA_384`, `SHA_512`, `SHA3_224`, `SHA3_256`, `SHA3_384`, `SHA3_512`. In such a build, all crypto operations (via the PSA Crypto API, or non-PSA APIs), as well as X.509 and TLS, will work as usual, except that direct calls to low-level hash APIs (`mbedtls_sha256()` etc.) are not possible for the modules that are disabled. You need to call `psa_crypto_init()` before any crypto operation that uses a hash algorithm that is provided only by a driver, as mentioned in [General considerations](#general-considerations) above. If you want to check at compile-time whether a certain hash algorithm is available in the present build of Mbed TLS, regardless of whether it's provided by a driver or built-in, you should use the following macros: - for code that uses only the PSA Crypto API: `PSA_WANT_ALG_xxx` from `psa/crypto.h`; - for code that uses non-PSA crypto APIs: `MBEDTLS_MD_CAN_xxx` from `mbedtls/md.h`. Elliptic-curve cryptography (ECC) --------------------------------- 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. [Coming soon] As noted in the "Limitations regarding the selection of curves" section below, there is an upcoming requirement for all the required curves to also be accelerated in the PSA driver in order to exclude the builtin algs support. ### 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 There is ongoing work which is trying to establish the links and constraints between the list of supported curves and supported algorithms both in the builtin and PSA sides. In particular: - #8014 ensures that the curves supported on the PSA side (`PSA_WANT_ECC_xxx`) are always a superset of the builtin ones (`MBEDTLS_ECP_DP_xxx`) - #8016 forces builtin alg support as soon as there is at least one builtin curve. In other words, in order to exclue all builtin algs, all the required curves should be supported and accelerated by the PSA driver. Finite-field Diffie-Hellman --------------------------- Support is pretty similar to the "Elliptic-curve cryptography (ECC)" section above. Key management and usage can be enabled by means of the usual `PSA_WANT` + `MBEDTLS_PSA_ACCEL` pairs: - `[PSA_WANT|MBEDTLS_PSA_ACCEL]_KEY_TYPE_DH_PUBLIC_KEY`; - `[PSA_WANT|MBEDTLS_PSA_ACCEL]_KEY_TYPE_DH_KEY_PAIR_BASIC`; - `[PSA_WANT|MBEDTLS_PSA_ACCEL]_KEY_TYPE_DH_KEY_PAIR_IMPORT`; - `[PSA_WANT|MBEDTLS_PSA_ACCEL]_KEY_TYPE_DH_KEY_PAIR_EXPORT`; - `[PSA_WANT|MBEDTLS_PSA_ACCEL]_KEY_TYPE_DH_KEY_PAIR_GENERATE`; The same holds for the associated algorithm: `[PSA_WANT|MBEDTLS_PSA_ACCEL]_ALG_FFDH` allow builds accelerating FFDH and removing builtin support (i.e. `MBEDTLS_DHM_C`). ### Limitations Support for deterministic derivation of a DH keypair (i.e. `PSA_WANT_KEY_TYPE_RSA_KEY_PAIR_DERIVE`) is not supported.