Clear SSL key data as soon as possible when move-assigning

Move-assign uses qSwap to exchange the private pointer and thus can
extend the lifetime of sensitive data. The move assignment operator
is changed so it releases the private data as soon as possible.

[ChangeLog][QtNetwork][QSslKey] Key data is cleared as soon as
possible when move-assigning.

Change-Id: Iebd029bf657acfe000417ce648e3b3829948c0e5
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>

src/network/ssl/qsslkey.h

@@ -71,9 +71,8 @@ public:
             const QByteArray &passPhrase = QByteArray());
     explicit QSslKey(Qt::HANDLE handle, QSsl::KeyType type = QSsl::PrivateKey);
     QSslKey(const QSslKey &other);
-#ifdef Q_COMPILER_RVALUE_REFS
-    QSslKey &operator=(QSslKey &&other) noexcept { swap(other); return *this; }
-#endif
+    QSslKey(QSslKey &&other) noexcept;
+    QSslKey &operator=(QSslKey &&other) noexcept;
     QSslKey &operator=(const QSslKey &other);
     ~QSslKey();
 

src/network/ssl/qsslkey_p.cpp

@@ -385,6 +385,24 @@ QSslKey::QSslKey(const QSslKey &other) : d(other.d)
 {
 }
 
+QSslKey::QSslKey(QSslKey &&other) noexcept
+    : d(nullptr)
+{
+    qSwap(d, other.d);
+}
+
+QSslKey &QSslKey::operator=(QSslKey &&other) noexcept
+{
+    if (this == &other)
+        return *this;
+
+    // If no one else is referencing the key data we want to make sure
+    // before we swap the d-ptr that it is not left in memory.
+    d.reset();
+    qSwap(d, other.d);
+    return *this;
+}
+
 /*!
     Destroys the QSslKey object.
 */

src/network/ssl/qsslkey_qt.cpp

@@ -48,6 +48,8 @@
 
 #include <QtNetwork/qpassworddigestor.h>
 
+#include <cstring>
+
 QT_USE_NAMESPACE
 
 static const quint8 bits_table[256] = {
@@ -186,8 +188,9 @@ static QByteArray deriveKey(QSslKeyPrivate::Cipher cipher, const QByteArray &pas
 
 void QSslKeyPrivate::clear(bool deep)
 {
-    Q_UNUSED(deep);
     isNull = true;
+    if (deep)
+        std::memset(derData.data(), 0, derData.size());
     derData.clear();
     keyLength = -1;
 }