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Sketch some optimizations relevant to MD light

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2022-11-25 23:05:14 +01:00
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docs/architecture/psa-migration/md-cipher-dispatch.md
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@ -406,3 +406,40 @@ Optionally, code that currently tests on `MBEDTLS_USE_PSA_CRYPTO` just to determ
#### Remove `legacy_or_psa.h`
It's no longer used.
### MD light optimizations
This section is not necessary to implement MD light, but will cut down its code size.
#### Split names out of MD light
Remove hash names from `mbedtls_md_info_t`. Use a simple switch-case or a separate list to implement `mbedtls_md_info_from_string` and `mbedtls_md_get_name`.
#### Remove metadata from the info structure
In `mbedtls_md_get_size` and in modules that want a hash's block size, instead of looking up hash metadata in the info structure, call the PSA macros.
#### Optimize type conversions
To allow optimizing conversions between `mbedtls_md_type_t` and `psa_algorithm_t`, renumber the `mbedtls_md_type_t` enum so that the values are the 8 lower bits of the PSA encoding.
With this optimization,
```
static inline psa_algorithm_t psa_alg_of_md_info(
const mbedtls_md_info_t *md_info )
{
if( md_info == NULL )
return( PSA_ALG_NONE );
return( PSA_ALG_CATEGORY_HASH | md_info->type );
}
```
Work in progress on this conversion is at https://github.com/gilles-peskine-arm/mbedtls/tree/hash-unify-ids-wip-1
#### Get rid of the hash_info module
The hash_info module is redundant with MD light. Move `mbedtls_md_error_from_psa` to `md.c`, defined only when `MBEDTLS_MD_SOME_PSA` is defined. The rest is no longer used.
#### Unify HMAC with PSA
PSA has its own HMAC implementation. In builds with both `MBEDTLS_MD_C` and `PSA_WANT_ALG_HMAC` not fully provided by drivers, we should have a single implementation. Replace the one in `md.h` by calls to the PSA driver interface. This will also give mixed-domain modules access to HMAC accelerated directly by a PSA driver (eliminating the need to a HMAC interface in software if all supported hashes have an accelerator that includes HMAC support).