stse_ecc_establish_shared_secret STSE_ECC_KT_CURVE25519 support

[simeonfelis]

In stse_ecc_establish_share_secrete the key_type STSE_ECC_KT_CURVE25519 is explicitely unsupported. However in stsafea_ecc_establish_shared_secret this key_type is explicitely allowed.

I guess there is a typo in the stse_ecc api level, because the service level works with with CURVE25519. I could verify STSAFE120 ECDH with CURVE25519 with openssl on linux hosts. My modification:

stse_ReturnCode_t stse_ecc_establish_shared_secret(
    stse_Handler_t *pSTSE,
    PLAT_UI8 private_key_slot_number,
    stse_ecc_key_type_t key_type,
    PLAT_UI8 *pPublic_key,
    PLAT_UI8 *pShared_secret) {
    stse_ReturnCode_t ret;

    /* - Check stsafe handler initialization */
    if (pSTSE == NULL) {
        return (STSE_API_HANDLER_NOT_INITIALISED);
    }

#ifdef STSE_CONF_STSAFE_L_SUPPORT
    /* - Check device type */
    if (pSTSE->device_type == STSAFE_L010) {
        return (STSE_API_INCOMPATIBLE_DEVICE_TYPE);
    }
#endif /* STSE_CONF_STSAFE_L_SUPPORT */

    if (pPublic_key == NULL || pShared_secret == NULL
// typo? Maybe ED25519 because this is not supported on A120 slot 0xff for ephemerals
//#ifdef STSE_CONF_ECC_CURVE_25519
//        || key_type == STSE_ECC_KT_CURVE25519
//#endif /* STSE_CONF_ECC_CURVE_25519 */
    ) {
        return (STSE_SERVICE_INVALID_PARAMETER);
    }

    ret = stsafea_ecc_establish_shared_secret(pSTSE, private_key_slot_number, key_type, pPublic_key, pShared_secret);

    return ret;
}

[nils-cercariolo-st]

Indeed it seems like there's no restrictions for the service stsafea_ecc_establish_shared_secret, let me double check and come back to you

[TofMassilia13320]

In this case, this is the STSAFE-A SE itself which reject the command with a specific error code.
The service layer (which include stsafea_ecc_establish_shared_secret) check potential issue that could cause MCU hardfault; the SE itself will perform input parameters verification and firstly return a status code on the COM bus.

[nils-cercariolo-st]

Yes if the secure element (SE) returns an error on that, it should be handled by it, but here, the API blocks it before the SE can even execute the command. @TofMassilia13320 I think we should remove this check:
#ifdef STSE_CONF_ECC_CURVE_25519
|| key_type == STSE_ECC_KT_CURVE25519
#endif /* STSE_CONF_ECC_CURVE_25519 */
) {
return (STSE_API_INVALID_PARAMETER);
}