An Example of Using TLS with the Paho MQTT Embedded C++ Client

Understandably, I’ve been asked a few times whether the Paho embedded client library will work with TLS. It will, but the only platform where I’ve written the code to do it so far is on mbed mbed. The reason why it hasn’t been widely publicised is that it uses the CyaSSL TLS library, which is licensed under the GPL. This means binaries linked with CyaSSL also have to be GPL, rather than the Apache license usually used on mbed, or the Eclipse licenses used for Paho.

But I can show you the code I did write. Now the client library is structured to be portable to any network library without changing the core code. It seems I need to explain this better, and further, certainly on the main Paho website as well as this blog. (Soon, I promise.) The following module is one I wrote to create a network class for CyaSSL which can be used by the embedded client class, MQTTClient. The two core methods which are needed are read and write; these are called whenever the library needs data, or has data to send. This is a basic connect — no client certificates are involved, but should serve as a model for what can be done. Here is the complete module — skip afterwards to see how to use it.

#if !defined(MQTTSSL_H)
#define MQTTSSL_H

#include "MQTT_mbed.h"
#include "mbed.h"
#include "EthernetInterface.h"


static TCPSocketConnection mysock; 

static int SocketReceive(CYASSL* ssl, char *buf, int len, void *ctx)
    int rc = mysock.receive(buf, len);
    if (rc == -1)
        rc = -2;  // -2 is WANT_READ
    return rc;
static int SocketSend(CYASSL* ssl, char *buf, int len, void *ctx)
    int rc = mysock.send(buf, len);
    return rc;

    MQTTSSL() : eth()
        ssl = 0;
        ctx = 0;
        eth.init();                          // Use DHCP
        method = CyaTLSv1_2_client_method();
    int connect(char* hostname, int port, int timeout=1000)
        int rc = -1;
        /* Initialize CyaSSL Context */
        if ( (ctx = CyaSSL_CTX_new(method)) == NULL)
            WARN("unable to get ctx");
            goto exit;
        CyaSSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, 0);
        CyaSSL_SetIORecv(ctx, SocketReceive); 
        CyaSSL_SetIOSend(ctx, SocketSend);
        mysock.set_blocking(false, timeout);    
        if ( (rc = mysock.connect(hostname, port)) != 0)
            goto exit;
        if ( (ssl = CyaSSL_new(ctx)) == NULL)
            ERROR("unable to get SSL object");
            rc = -1; goto exit;
        CyaSSL_set_using_nonblock(ssl, 1);
        if ( (rc = CyaSSL_connect(ssl)) != SSL_SUCCESS)
            rc = CyaSSL_get_error(ssl, 0);
            WARN("err = %d, %s\n", rc, CyaSSL_ERR_error_string(rc, "\n"));
            WARN("SSL Connection Error\n");
            rc = -1;
            LOG("SSL Connected\n") ;
            rc = 0;
        return rc;

    int read(unsigned char* buffer, int len, int timeout)
        int rc = 0;
        mysock.set_blocking(false, timeout);  
        rc = CyaSSL_read(ssl, buffer, len);
        DEBUG("called CyaSSL_read len %d rc %d\n", len, rc);
        return rc;
    int write(unsigned char* buffer, int len, int timeout)
        int rc = 0;
        mysock.set_blocking(false, timeout);  
        rc = CyaSSL_write(ssl, buffer, len);
        DEBUG("called CyaSSL_write len %d rc %d\n", len, rc);
        return rc;
    int disconnect()
        int rc = mysock.close();
        CyaSSL_CTX_free(ctx) ;
        return rc;
    EthernetInterface& getEth()
        return eth;

    EthernetInterface eth;

    CYASSL_METHOD*  method;
    CYASSL_CTX*     ctx;
    CYASSL*         ssl;


To use it, you need to create an instance of MQTTSSL, and pass the class as a template parameter when you create an MQTT client instance.

MQTTSSL ipstack;
MQTT::Client client(ipstack);

Now follows the sort of function you might use to connect. You have to make the network connection first, before calling the MQTT connect.

int connect(MQTT::Client* client, MQTTSSL* ipstack)
    const char* iot_ibm = "";
    // Network connect first
    char hostname[strlen(org) + strlen(iot_ibm) + 1];
    sprintf(hostname, "%s%s", org, iot_ibm);
    int rc = ipstack->connect(hostname, IBM_IOT_PORT);
    if (rc != 0)
        return rc;
    // Construct clientId - d:org:type:id
    char clientId[strlen(org) + strlen(type) + strlen(id) + 5];
    sprintf(clientId, "d:%s:%s:%s", org, type, id);
    // Now MQTT Connect
    MQTTPacket_connectData data = MQTTPacket_connectData_initializer;
    data.clientID.cstring = clientId;    
    if ( (rc = client->connect(&data)) == 0) 
    return rc;

If this is successful, you can now make other MQTT calls, like subscribe and publish.

if ( (rc = client.subscribe("iot-2/cmd/+/fmt/json", MQTT::QOS1, messageArrived)) != 0)
           WARN("rc from MQTT subscribe is %d\n", rc); 

MQTT::Message message;
char* pubTopic = "iot-2/evt/status/fmt/json";
char buf[250];
     "{\"d\":{\"myName\":\"IoT mbed\",\"accelX\":%0.4f,\"accelY\":%0.4f,\"accelZ\":%0.4f,\"temp\":%0.4f,\"joystick\":\"%s\",\"potentiometer1\":%0.4f,\"potentiometer2\":%0.4f}}",
            MMA.x(), MMA.y(), MMA.z(), sensor.temp(), joystickPos,,;
message.qos = MQTT::QOS0;
message.retained = false;
message.dup = false;
message.payload = (void*)buf;
message.payloadlen = strlen(buf);
client.publish(pubTopic, &message);

I will write this up more fully soon. With any luck, this has also given you an idea of what needs to be done to port the client to a different network API.

8 Replies to “An Example of Using TLS with the Paho MQTT Embedded C++ Client”

      1. Hi Ian,

        I am long searching for a way to send email using ssl/ tls on an arduino due. Have you done anything similar to that?

        1. Hi.

          there are a number of TLS libraries that can be used in embedded environments. mbed-TLS is the latest one. CyaSSL is another example. You should be able to use one of these libraries to connect to a POP or IMAP server.


  1. Hello Ian,

    Knowing that mbed-TLS (formerly polarSSL) should soon change to the Apache license, I am looking at this library as a solution to support TLS connections in paho.

    Though the sample code you provided should get me going, I was wondering if there is any chance you would have seen an implementation of paho MQTT using mbed TLS?

    1. Hello Francois,

      I don’t know of an implementation yet. Once the license has changed to Apache, I expect to have an implementation soon after. Of course, we are always open to contributions as well.

  2. Hi dear Ian,

    Is there any sample code that implemented RSA or AES or any other cryptography or hash algorithm inside AT91SAM7S MCUs using mbed TLS library?

    1. Hi Abraham,

      I don’t know of any specific code that does that. If that processor runs the mbed operating system, then the example I have shown should work.

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