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" \
"MIIDmDCCAoCgAwIBAgIJAKV4GE+4y/G4MA0GCSqGSIb3DQEBCwUAMGExCzAJBgNV\n" \
"BAYTAlRWMRAwDgYDVQQIDAdUcmV2aXNvMRUwEwYDVQQHDAxDYXN0ZWxmcmFuY28x\n" \
"DDAKBgNVBAoMA1BTUzEMMAoGA1UECwwDUFNTMQ0wCwYDVQQDDARob21lMB4XDTE5\n" \
"MDkwODE3NDIzOFoXDTI5MDcxNzE3NDIzOFowYTELMAkGA1UEBhMCVFYxEDAOBgNV\n" \
"BAgMB1RyZXZpc28xFTATBgNVBAcMDENhc3RlbGZyYW5jbzEMMAoGA1UECgwDUFNT\n" \
"MQwwCgYDVQQLDANQU1MxDTALBgNVBAMMBGhvbWUwggEiMA0GCSqGSIb3DQEBAQUA\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" \
"gYvPj4LqWDAPBgNVHRMBAf8EBTADAQH/MA0GCSqGSIb3DQEBCwUAA4IBAQC08Crs\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.println(topic);

  Serial.print("payload: ");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
  /* 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")) {
      Serial.println("connected");
      /* subscribe topic */
      client.subscribe(LED_TOPIC);
    } else {
      Serial.print("failed, status code =");
      Serial.print(client.state());
      Serial.println("try again in 5 seconds");
      /* Wait 5 seconds before retrying */
      delay(5000);
    }
  }
}

void setup() {
  Serial.begin(9600);
  // We start by connecting to a WiFi network
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  /* set led as output to control led on-off */
  pinMode(led, OUTPUT);

  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
  /*setup MDNS for ESP32 */
  if (!MDNS.begin("esp32")) {
      Serial.println("Error setting up MDNS responder!");
      while(1) {
          delay(1000);
      }
  }
  /* 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: ");
  Serial.println(serverIp.toString());
  /* set SSL/TLS certificate */
  espClient.setCACert(ca_cert);
  /* configure the MQTT server with IPaddress and port */
  client.setServer(serverIp, 26391);
  /* this receivedCallback function will be invoked 
  when client received subscribed topic */
  client.setCallback(receivedCallback);
  
}
void loop() {
  /* if client was disconnected then try to reconnect again */
  if (!client.connected()) {
    mqttconnect();
  }
  /* this function will listen for incomming 
  subscribed topic-process-invoke receivedCallback */
  client.loop();
  /* 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) {
      counter++;
      snprintf (msg, 20, "%d", counter);
      /* publish the message */
      client.publish(COUNTER_TOPIC, msg);
    }else {
      counter = 0;  
    }
  }
}