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/*!
    \example secureudpserver
    \title DTLS server
    \ingroup examples-network
    \brief This examples demonstrates how to implement a simple DTLS server.

    \image secureudpserver-example.png Screenshot of the DTLS server example.

    \note The DTLS server example is intended to be run alongside the \l{secureudpclient}{DTLS client} example.

    The server is implemented by the DtlsServer class. It uses QUdpSocket,
    QDtlsClientVerifier, and QDtls to test each client's reachability, complete a handshake,
    and read and write encrypted messages.

    \snippet secureudpserver/server.h 0

    The constructor connects the QUdpSocket::readyRead() signal to its
    readyRead() slot and sets the minimal needed TLS configuration:

    \snippet secureudpserver/server.cpp 1

    \note The server is not using a certificate and is relying on Pre-Shared
    Key (PSK) handshake.

    listen() binds QUdpSocket:

    \snippet secureudpserver/server.cpp 2

    The readyRead() slot processes incoming datagrams:

    \dots
    \snippet secureudpserver/server.cpp 3
    \dots

    After extracting an address and a port number, the server first tests
    if it's a datagram from an already known peer:

    \dots
    \snippet secureudpserver/server.cpp 4
    \dots

    If it is a new, unknown address and port, the datagram is processed as a
    potential ClientHello message, sent by a DTLS client:

    \dots
    \snippet secureudpserver/server.cpp 5
    \dots

    If it's a known DTLS client, the server either decrypts the datagram:

    \dots
    \snippet secureudpserver/server.cpp 6
    \dots

    or continues a handshake with this peer:

    \dots
    \snippet secureudpserver/server.cpp 7
    \dots

    handleNewConnection() verifies it's a reachable DTLS client, or sends a
    HelloVerifyRequest:

    \snippet secureudpserver/server.cpp 8
    \dots

    If the new client was verified to be a reachable DTLS client, the server creates
    and configures a new QDtls object, and starts a server-side handshake:

    \dots
    \snippet secureudpserver/server.cpp 9
    \dots

    doHandshake() progresses through the handshake phase:

    \snippet secureudpserver/server.cpp 11

    During the handshake phase, the QDtls::pskRequired() signal is emitted and
    the pskRequired() slot provides the preshared key:

    \snippet secureudpserver/server.cpp 13

    \note For the sake of brevity, the definition of pskRequired() is oversimplified.
    The documentation for the QSslPreSharedKeyAuthenticator class explains in detail
    how this slot can be properly implemented.

    After the handshake is completed for the network peer, an encrypted DTLS
    connection is considered to be established and the server decrypts subsequent
    datagrams, sent by the peer, by calling decryptDatagram(). The server also
    sends an encrypted response to the peer:

    \snippet secureudpserver/server.cpp 12

    The server closes its DTLS connections by calling QDtls::shutdown():

    \snippet secureudpserver/server.cpp 14

    During its operation, the server reports errors, informational messages, and
    decrypted datagrams, by emitting signals errorMessage(), warningMessage(),
    infoMessage(), and datagramReceived(). These messages are logged by the server's
    UI:

    \snippet secureudpserver/mainwindow.cpp 0
*/