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Show how to extract curve information from an ecp_keypair

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It's not pretty.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2023-06-20 23:38:39 +02:00
parent 603f0fca6e
commit 909cf5a3ec
1 changed files with 17 additions and 3 deletions
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20
docs/psa-transition.md
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@ -800,16 +800,30 @@ psa_import_key(&attributes, buf + sizeof(buf) - length, length, &key_id);
mbedtls_pk_free(&pk);
```
For an ECC private key (a future version of Mbed TLS [will provide a function to calculate the curve family](https://github.com/Mbed-TLS/mbedtls/issues/7764)):
For an ECC private key (a future version of Mbed TLS [will provide a more direct way to find the curve family](https://github.com/Mbed-TLS/mbedtls/issues/7764)):
```
unsigned char buf[PSA_BITS_TO_BYTES(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
size_t length = PSA_BITS_TO_BYTES(mbedtls_pk_bitlen(&pk));
mbedtls_ecp_keypair *ec = mbedtls_pk_ec(&pk);
psa_ecc_curve_t curve;
{
mbedtls_ecp_group grp;
mbedtls_ecp_group_init(&grp);
mbedtls_ecp_point Q;
mbedtls_ecp_point_init(&Q);
mbedtls_mpi d;
mbedtls_mpi_init(&d);
mbedtls_ecp_export(ec, &grp, &d, &Q);
size_t bits;
curve = mbedtls_ecc_group_to_psa(grp.id, &bits);
mbedtls_ecp_group_free(&grp);
mbedtls_ecp_point_free(&Q);
mbedtls_mpi_free(&d);
}
mbedtls_ecp_write_key(ec, buf, length);
psa_ecc_curve_t curve = ...; // need to determine the curve family manually
psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
psa_set_key_attributes(&attributes, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
psa_set_key_type(&attributes, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_... | ...);
psa_set_key_algorithm(&attributes, PSA_ALGORITHM_...);
psa_key_id_t key_id = 0;