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 onward, the officially supported version for OpenSSL is 1.1.1 or later.

Qt version 5.15.1 onward is also compatible with OpenSSL 3.

QDtls

This class provides encryption for UDP sockets

QDtlsClientVerifier

This class implements server-side DTLS cookie generation and verification

QDtlsClientVerifier::GeneratorParameters

This class defines parameters for DTLS cookie generator

QOcspResponse

This class represents Online Certificate Status Protocol response

QSsl

Declares enums common to all SSL classes in Qt Network

QSslCertificate

Convenient API for an X509 certificate

QSslCertificateExtension

API for accessing the extensions of an X509 certificate

QSslCipher

Represents an SSL cryptographic cipher

QSslConfiguration

Holds the configuration and state of an SSL connection

QSslDiffieHellmanParameters

Interface for Diffie-Hellman parameters for servers

QSslEllipticCurve

Represents an elliptic curve for use by elliptic-curve cipher algorithms

QSslError

SSL error

QSslKey

Interface for private and public keys

QSslPreSharedKeyAuthenticator

Authentication data for pre shared keys (PSK) ciphersuites

QSslServer

Implements an encrypted, secure TCP server over TLS

QSslSocket

SSL encrypted socket for both clients and servers

QSslSocket::AlertLevel

Describes the level of an alert message

QSslSocket::AlertType

Enumerates possible codes that an alert message can have

QDtls::HandshakeState

Describes the current state of DTLS handshake

QSslSocket::ImplementedClass

Enumerates classes that a TLS backend implements

QDtls::QDtlsError

Describes errors that can be found by QDtls and QDtlsClientVerifier

QOcspResponse::QOcspCertificateStatus

Describes the Online Certificate Status

QOcspResponse::QOcspRevocationReason

Describes the reason for revocation

QSslSocket::SupportedFeature

Enumerates possible features that a TLS backend supports

For Android applications see Adding OpenSSL Support for Android.

Enabling and Disabling SSL Support when Building Qt from Source

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 Schannel, while for macOS this is 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.

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