I installed Mosquitto broker in Raspberry in my local network. I generated certificates in Raspberry as follows:

sudo openssl genrsa -out ca.key 2048
sudo openssl req -new -x509 -days 3600 -key ca.key -out ca.crt
sudo openssl genrsa -out server.key 2048
sudo openssl req -new -out server.csr -key server.key
sudo openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 3600

I modified the Mosqutto configuration file (.conf) as follows:

allow_anonymous false
cafile /etc/mosquitto/certs/ca.crt
certfile /etc/mosquitto/certs/server.crt
keyfile /etc/mosquitto/certs/server.key
tls_version tlsv1.2

I restarted Mosquitto everything ok

I downloaded the ca.crt file from Raspberry to my PC with Filezilla from my PC I use MQTT.fx to connect I set the certificate. WORKS WELL

The problem: with ESP32 I can't connect when I set the certificate with espClient.setCACert(ca_cert); MQTT connecting ...

[E] [ssl_client.cpp: 33] handle_error (): X509 - Certificate verification failed, e.g. CRL, CA or signature check failed

Someone can help me, I can't understand if something is missing in the code or there is some error in generating the certificates

Please help many thanks Michael

this is my code:

#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <PubSubClient.h>
#include <ESPmDNS.h>

/* change it with your ssid-password */
const char* ssid = "XXXXXX";
const char* password = "XXXXXXX";
/* this is the MDNS name of PC where you installed MQTT Server */
const char* serverHostname = "home";

const char* ca_cert = \ 
"-----BEGIN CERTIFICATE-----\n" \
"A4IBDwAwggEKAoIBAQC2rPN7VQXtzLCl1LufJkzLK9xedrbW72K2vBZ34VNOl+tG\n" \
"ffNx2QQLQC0Jh5GzB79dZweqAjCRBbIEpx0IGvHWrHDUQMJrvg4BcmDr0eqNCup/\n" \
"tBItlEcCdvmWFfm3IBezOVBDFiQWmC4jvAC/G3DPlftHiPI1k3cDewKNJEJykxeD\n" \
"8H/4KTAz+wJXyGZiviZYv8OYOTRMa3nEhg1vaQTMgCwEVBavg5fsEQkZLArBNJQM\n" \
"Gmw0qDdJEMDGOGWPogxRAKrxvNnXr4mIebzPj9/UTrJnfzVOPEHZ9Uz5BU/psGCL\n" \
"cnMkBeNEa/JyLR14Hu8e+xa/pOCpJ9Dqhi/BTji5AgMBAAGjUzBRMB0GA1UdDgQW\n" \
"BBRIz+15SpOBqo0O+vOtgYvPj4LqWDAfBgNVHSMEGDAWgBRIz+15SpOBqo0O+vOt\n" \
"w7LPX+Bd80NwPuxgBmMU4U/pCw8AyzzSgckHmay0K8KuHXhSvZjdl6i0gjG/2xLa\n" \
"ghCtfFGIXPu1Lu226bhTWigmuFLYasG1MivTDz2LeUTFPQrtEO8KdsRUarPZW3Fj\n" \
"nOMyVDdzE4+HxvzvARDc1QbE9SX2ozQcNKQp4oRpunfT2+/OyzYgfMVA7MVKvehX\n" \
"VH49fRF1P1Ajizqr2eHDFImcMQTzepRT7IjfOiQ/8PoqrZGvGenMj5so1I9MtgyK\n" \
"gVUf1ZWPBgxmhVfla6vYBZiTGuZMHSDuF6/yFC1QnoXVq7K+bbIhAE8lsPNuOY7V\n" \
"dKidKIfBJiN378CC\n" \
"-----END CERTIFICATE-----\n";

/* create an instance of WiFiClientSecure */
WiFiClientSecure espClient;
PubSubClient client(espClient);

/*LED GPIO pin*/
const char led = 5;

/* topics */
#define COUNTER_TOPIC    "smarthome/room1/counter"
#define LED_TOPIC     "smarthome/room1/led" /* 1=on, 0=off */

long lastMsg = 0;
char msg[20];
int counter = 0;

void receivedCallback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message received: ");

  Serial.print("payload: ");
  for (int i = 0; i < length; i++) {
  /* we got '1' -> on */
  if ((char)payload[0] == '1') {
    digitalWrite(led, HIGH); 
  } else {
    /* we got '0' -> on */
    digitalWrite(led, LOW);


void mqttconnect() {
  /* Loop until reconnected */
  while (!client.connected()) {
    Serial.print("MQTT connecting ...");
    /* client ID */
    String clientId = "ESP32Client";
    /* connect now */
    if (client.connect(clientId.c_str(),"XXXXX","XXXXXX")) {
      /* subscribe topic */
    } else {
      Serial.print("failed, status code =");
      Serial.println("try again in 5 seconds");
      /* Wait 5 seconds before retrying */

void setup() {
  // We start by connecting to a WiFi network
  Serial.print("Connecting to ");

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
  /* set led as output to control led on-off */
  pinMode(led, OUTPUT);

  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  /*setup MDNS for ESP32 */
  if (!MDNS.begin("esp32")) {
      Serial.println("Error setting up MDNS responder!");
      while(1) {
  /* get the IP address of server by MDNS name */
  Serial.println("mDNS responder started");
  IPAddress serverIp = MDNS.queryHost(serverHostname);
  Serial.print("IP address of server: ");
  /* set SSL/TLS certificate */
  /* configure the MQTT server with IPaddress and port */
  client.setServer(serverIp, 26391);
  /* this receivedCallback function will be invoked 
  when client received subscribed topic */

void loop() {
  /* if client was disconnected then try to reconnect again */
  if (!client.connected()) {
  /* this function will listen for incomming 
  subscribed topic-process-invoke receivedCallback */
  /* we increase counter every 3 secs
  we count until 3 secs reached to avoid blocking program if using delay()*/
  long now = millis();
  if (now - lastMsg > 3000) {
    lastMsg = now;
    if (counter < 100) {
      snprintf (msg, 20, "%d", counter);
      /* publish the message */
      client.publish(COUNTER_TOPIC, msg);
    }else {
      counter = 0;  
  • See github.com/espressif/arduino-esp32/issues/2875 and github.com/TwilioDevEd/sync-for-iot-examples/blob/master/…. Does it work when adding R in front of the string constant for the ca_cert? Sep 8, 2019 at 18:50
  • Hi Maximilian, I tried as recommended but I get the same error even with the R - MQTT connecting ...[E][ssl_client.cpp:33] handle_error(): X509 - Certificate verification failed, e.g. CRL, CA or signature check failed [E][ssl_client.cpp:35] handle_error(): MbedTLS message code: -9984 [E][WiFiClientSecure.cpp:132] connect(): start_ssl_client: -9984 - do you have any suggestions for me?
    – Michele
    Sep 9, 2019 at 19:15
  • Do you have a full mbedtls log for the whole session? You might need to set debug level to the most verbose in the Arduin IDE then. Sep 9, 2019 at 20:52
  • I found the solution, the problem was related to the generation of certificates. The OU must be different for each file generated. SUBJECT_CA="/C=SE/ST=Stockholm/L=Stockholm/O=himinds/OU=CA/CN=" SUBJECT_SERVER="/C=SE/ST=Stockholm/L=Stockholm/O=himinds/OU=Server/CN=" SUBJECT_CLIENT="/C=SE/ST=Stockholm/L=Stockholm/O=himinds/OU=Client/CN="
    – Michele
    Sep 15, 2019 at 18:22
  • Good morning, first I would like to say that I am Brazilian, so I'm using the translator, if something goes wrong please do not care. Come on, I have the same problem, I believe it is problem in the certificate rsrs, could you tell me please, how did you solve your problem? thanks. Sep 18, 2019 at 11:35

1 Answer 1


@sempaiscuba and Bruno I solved by regenerating the certificates in this way you have to do this comfortably script in Raspiberry



function generate_CA () {
   echo "$SUBJECT_CA"
   openssl req -x509 -nodes -sha256 -newkey rsa:2048 -subj "$SUBJECT_CA"  -days 365 -keyout ca.key -out ca.crt

function generate_server () {
   openssl req -nodes -sha256 -new -subj "$SUBJECT_SERVER" -keyout server.key -out server.csr
   openssl x509 -req -sha256 -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 365

function generate_client () {
   openssl req -new -nodes -sha256 -subj "$SUBJECT_CLIENT" -out client.csr -keyout client.key 
   openssl x509 -req -sha256 -in client.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out client.crt -days 365

function copy_keys_to_broker () {
   sudo cp ca.crt /etc/mosquitto/certs/
   sudo cp server.crt /etc/mosquitto/certs/
   sudo cp server.key /etc/mosquitto/certs/


as you see OU changes for each file. Remember to restart Mosquitto

sudo systemctl stop mosquitto.service
sudo systemctl start mosquitto.service

let me know how it goes bye Michele

Not the answer you're looking for? Browse other questions tagged or ask your own question.