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Add documentation on driver-only ECC

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Just one part left for later.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2023-07-07 16:43:56 +02:00
parent 6d5f4946e6
commit 7a82e27a10
1 changed files with 69 additions and 1 deletions

70
docs/driver-only-builds.md

@ -75,4 +75,72 @@ TODO
Elliptic-curve cryptography (ECC)
---------------------------------
TODO
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` is 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 one of the above options is enabled, you can still `MBEDTLS_ECP_C`
in `mbedtls_config.h`, and it will still result in some code size savings, but
not as much as when none of these are enabled, as a subset of `ecp.c` will
still be included in the build in order to support these.
We do have plans to support each of these with `ecp.c` fully removed in the
future, however 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, the 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 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.