docs/driver-only-builds.md

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)
---------------------------------

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.