feature-remoteid/RemoteIDModule/transport.cpp

/*
  generic transport class for handling OpenDroneID messages
 */
#include <Arduino.h>
#include "transport.h"
#include "parameters.h"
#include "util.h"
#include "monocypher.h"

const char *Transport::parse_fail = "uninitialised";

uint32_t Transport::last_location_ms;
uint32_t Transport::last_basic_id_ms;
uint32_t Transport::last_self_id_ms;
uint32_t Transport::last_operator_id_ms;
uint32_t Transport::last_system_ms;

mavlink_open_drone_id_location_t Transport::location;
mavlink_open_drone_id_basic_id_t Transport::basic_id;
mavlink_open_drone_id_authentication_t Transport::authentication;
mavlink_open_drone_id_self_id_t Transport::self_id;
mavlink_open_drone_id_system_t Transport::system;
mavlink_open_drone_id_operator_id_t Transport::operator_id;

Transport::Transport()
{
}

/*
  check we are OK to arm
 */
uint8_t Transport::arm_status_check(const char *&reason)
{
    const uint32_t max_age_location_ms = 3000;
    const uint32_t max_age_other_ms = 22000;
    const uint32_t now_ms = millis();

    uint8_t status = MAV_ODID_PRE_ARM_FAIL_GENERIC;

    if (last_location_ms == 0 || now_ms - last_location_ms > max_age_location_ms) {
        reason = "missing location message";
    } else if (!g.have_basic_id_info() && (last_basic_id_ms == 0 || now_ms - last_basic_id_ms > max_age_other_ms)) {
        reason = "missing basic_id message";
    } else if (last_self_id_ms == 0  || now_ms - last_self_id_ms > max_age_other_ms) {
        reason = "missing self_id message";
    } else if (last_operator_id_ms == 0 || now_ms - last_operator_id_ms > max_age_other_ms) {
        reason = "missing operator_id message";
    } else if (last_system_ms == 0 || now_ms - last_system_ms > max_age_location_ms) {
        // we use location age limit for system as the operator location needs to come in as fast
        // as the vehicle location for FAA standard
        reason = "missing system message";
    } else if (location.latitude == 0 && location.longitude == 0) {
        reason = "Bad location";
    } else if (system.operator_latitude == 0 && system.operator_longitude == 0) {
        reason = "Bad operator location";
    } else if (reason == nullptr) {
        status = MAV_ODID_GOOD_TO_ARM;
    }

    return status;
}

/*
  make a session key
 */
void Transport::make_session_key(uint8_t key[8]) const
{
    struct {
        uint32_t time_us;
        uint8_t mac[8];
        uint32_t rand;
    } data {};
    static_assert(sizeof(data) % 4 == 0, "data must be multiple of 4 bytes");

    esp_efuse_mac_get_default(data.mac);
    data.time_us = micros();
    data.rand = random(0xFFFFFFFF);
    const uint64_t c64 = crc_crc64((const uint32_t *)&data, sizeof(data)/sizeof(uint32_t));
    memcpy(key, (uint8_t *)&c64, 8);
}

/*
  check signature in a command against public keys
 */
bool Transport::check_signature(uint8_t sig_length, uint8_t data_len, uint32_t sequence, uint32_t operation,
                                const uint8_t *data)
{
    if (g.no_public_keys()) {
        // allow through if no keys are setup
        return true;
    }
    if (sig_length != 64) {
        // monocypher signatures are 64 bytes
        return false;
    }
    
    /*
      loop over all public keys, if one matches then we are OK
     */
    for (uint8_t i=0; i<MAX_PUBLIC_KEYS; i++) {
        uint8_t key[32];
        if (!g.get_public_key(i, key)) {
            continue;
        }
        crypto_check_ctx ctx {};
        crypto_check_ctx_abstract *actx = (crypto_check_ctx_abstract*)&ctx;
        crypto_check_init(actx, &data[data_len], key);

        crypto_check_update(actx, (const uint8_t*)&sequence, sizeof(sequence));
        crypto_check_update(actx, (const uint8_t*)&operation, sizeof(operation));
        crypto_check_update(actx, data, data_len);
        if (operation != SECURE_COMMAND_GET_SESSION_KEY &&
            operation != SECURE_COMMAND_GET_REMOTEID_SESSION_KEY) {
            crypto_check_update(actx, session_key, sizeof(session_key));
        }
        if (crypto_check_final(actx) == 0) {
            // good signature
            return true;
        }
    }
    return false;
}