SSL: Secure Sockets Layer

Sming supports multiple SSL implementations, currently with adapters for:

If you want to use SSL then take a look at the Basic SSL sample for creating SSL clients, and HttpServer Config Network for SSL servers.


For server applications you’ll require an X509 certificate and private key. This is typically generated using openssl, then the resulting binary files added to your project. The Basic AWS demonstrates this approach.


Clearly private key information must be guarded so take care not to publish this in a public repository!

Sming also has a convenience script you can use:

make generate-cert

This generates binary certificate and key information in out/ssl, and creates include/ssl/cert.h and include/ssl/private_key.h. See MQTT Client Hello for example code.

Enabling SSL in HttpServer

The TcpServer::listen() method, and the child HttpServer class, accepts a second optional useSsl parameter. If you look at the startWebServer function in samples/HttpServer_WebSockets/app/application.cpp, it can be changed to something like:

void startWebServer()
    // TODO: Make sure to set a server certificate and key
    server.listen(443, true);

And what is left is the actual setting of the server certificate:

void startWebServer()
    // Assign the certificate
    server.setSslInitHandler([](Ssl::Session& session) {
         session.keyCert.assign(serverKey, serverCert);
    server.listen(443, true);

The final code can be something like:

void startWebServer()
  server.setSslInitHandler([](Ssl::Session& session) {
    session.keyCert.assign(serverKey, serverCert);
  server.listen(443, true);
  server.paths.set("/", onIndex);

This is also demonstrated for secure MQTT in the MQTT Client Hello sample.

Security Considerations

Does it really make sense to use SSL for an HttpServer on an ESP8266 device?

The certificate/private key pair should make it impossible for an external user to decrypt your traffic so that the things that you sent are kept private, but there are some complications with this:

  • The private key will not stay private for long. The private key should be kept encrypted on the flash memory, to prevent casual reading. But even with decryption there is a high probability that someone will be able to disassemble your application and figure out how to decrypt the key.

  • Costs for certificate. Let’s imagine that you have overcome the first issue. Then comes the second issue - if you want your users to accept the certificate it has to be signed by one of the trusted certificate authorities. And that costs money. And if you want to use a unique certificate/private key pair for every device than it will make things worse, moneywise. Note: Free SSL certificates are now available, for example These will expire if not kept up to date so adds additional complexity to your application.

  • You can handle up to 2 or maximum 3 connections. SSL needs 16K of memory to make the initial handshake. The memory consumption after a successful handshake can decrease to 4K, just for the SSL, per request. But realistically this means that you will end up with a server that can handle maximum 2 or 3 simultaneous connections before the heap memory is consumed and has to be released.

Therefore, in our humble opinion, it would be better to rely on the WIFI security that your Access Point (AP) provides and make this AP accessible only for your IoT devices.

Configuration Variables

  • 0 (default): SSL requires lots of RAM and some intensive processing, so to conserve resources it is disabled by default.

  • 1: to enable the default SSL adapter. At the moment that is Axtls.

  • Axtls: to enable SSL support using the AXTLS 8266 component.

  • Bearssl: to enable SSL support using the Bear SSL component.

API Documentation


Used by

Environment Variables

SoC support

  • esp32

  • esp32c2

  • esp32c3

  • esp32s2

  • esp32s3

  • esp8266

  • host

  • rp2040