Secure Sockets Layer (SSL) Classes

The classes below provide support for secure network communication using the Secure Sockets Layer (SSL) protocol, using a native TLS backend, the OpenSSL Toolkit, or any appropriate TLS plugin to perform encryption and protocol handling.

From Qt version 5.15 onwards, the officially supported version for OpenSSL is 1.1.1 or later.

Enabling and Disabling SSL Support

When building Qt from source, Qt builds plugins for native TLS libraries that are supported for the operating system you are building for. For Windows this means https://docs.microsoft.com/en-us/windows/win32/com/schannel, while for macOS this is https://developer.apple.com/documentation/security/secure_transport.

On all platforms, the configuration system checks for the presence of the openssl/opensslv.h header provided by source or developer packages of OpenSSL. If found, it will enable and build the OpenSSL backend for Qt.

By default, an OpenSSL-enabled Qt library dynamically loads any installed OpenSSL library at run-time. However, it is possible to link against the library at compile-time by configuring Qt with the -openssl-linked option.

When building a version of Qt linked against OpenSSL, Qt's build system will use CMake's FindOpenSSL command to find OpenSSL in several standard locations. You can set the CMake variable OPENSSL_ROOT_DIR to force a specific location.

For example:

configure -openssl-linked -- -D OPENSSL_ROOT_DIR=<openssl_dir>

To disable SSL support in a Qt build, configure Qt with the -no-openssl option.

Considerations While Packaging Your Application

When you package your application, you may run a tool like windeployqt. This copies all the plugins for the libraries you use to the plugins/ folder. However, for TLS you only need one backend, and you may delete the other plugins before packaging your application. For example, if you're on Windows and don't require any of the extra features the OpenSSL backend provides, you can choose to forego shipping the qopensslbackend plugin as well as the OpenSSL library, and simply ship the qschannelbackend plugin.

However, shipping multiple backends is not a problem. Qt will attempt to load the backends in order (with OpenSSL attempted first) until one is successfully loaded. The other backends are then unused.

Datagram Transport Layer Security

Datagram Transport Layer Security (DTLS) is a protocol that enables security for datagram-based applications, providing them with protection against eavesdropping, tampering, or message forgery. The DTLS protocol is based on the stream-oriented Transport Layer Security (TLS) protocol. QtNetwork enables the use of DTLS with User Datagram Protocol (UDP), as defined by RFC 6347.

Import and Export Restrictions

Qt binary installers include the OpenSSL libraries used by QtNetwork. However, those are not automatically deployed with applications that are built with Qt. Import and export restrictions apply for some types of software, and for some parts of the world. Developers wishing to use SSL communication in their deployed applications should either ensure that their users have the appropriate libraries installed, or they should consult a suitably qualified legal professional to ensure that applications using code from the OpenSSL project are correctly certified for import and export in relevant regions of the world.