QSslDiffieHellmanParameters: simplify defaultParameters() construction

This commit simplifies defaultParameters() to simply construct an empty
QSslDiffieHellmanParameters and assigning the DER-form of the DH parameters
to QSslDiffieHellmanParametersPrivate's derData field.

This creates a valid QSslDiffieHellmanParameters instance, but skips any
potentially expensive verification steps.

The previous implementation of defaultParameters() would use the public
fromEncoded() method to construct an instance of the default parameters.
This triggers a verification of the passed-in data, which can be expensive.

To ensure our defaultParameters() QSslDiffieHellmanParameters instance does
pass verification, this commit adds an autotest to verify that.

Fixes QTBUG-57815.

Change-Id: I6b1d9dbbfde526b232c319195ddbad42326be27c
Task-number: QTBUG-57815
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/ssl/qssldiffiehellmanparameters.cpp

@@ -68,6 +68,12 @@
 
 QT_BEGIN_NAMESPACE
 
+// The 1024-bit MODP group from RFC 2459 (Second Oakley Group)
+Q_AUTOTEST_EXPORT const char *qssl_dhparams_default_base64 =
+    "MIGHAoGBAP//////////yQ/aoiFowjTExmKLgNwc0SkCTgiKZ8x0Agu+pjsTmyJR"
+    "Sgh5jjQE3e+VGbPNOkMbMCsKbfJfFDdP4TVtbVHCReSFtXZiXn7G9ExC6aY37WsL"
+    "/1y29Aa37e44a/taiZ+lrp8kEXxLH+ZJKGZR7OZTgf//////////AgEC";
+
 /*!
     Returns the default QSslDiffieHellmanParameters used by QSslSocket.
 
@@ -76,15 +82,9 @@ QT_BEGIN_NAMESPACE
 */
 QSslDiffieHellmanParameters QSslDiffieHellmanParameters::defaultParameters()
 {
-    // The 1024-bit MODP group from RFC 2459 (Second Oakley Group)
-    return fromEncoded(
-        QByteArray::fromBase64(QByteArrayLiteral(
-            "MIGHAoGBAP//////////yQ/aoiFowjTExmKLgNwc0SkCTgiKZ8x0Agu+pjsTmyJR"
-            "Sgh5jjQE3e+VGbPNOkMbMCsKbfJfFDdP4TVtbVHCReSFtXZiXn7G9ExC6aY37WsL"
-            "/1y29Aa37e44a/taiZ+lrp8kEXxLH+ZJKGZR7OZTgf//////////AgEC"
-        )),
-        QSsl::Der
-    );
+    QSslDiffieHellmanParameters def;
+    def.d->derData = QByteArray::fromBase64(QByteArray(qssl_dhparams_default_base64));
+    return def;
 }
 
 /*!

tests/auto/network/ssl/qssldiffiehellmanparameters/tst_qssldiffiehellmanparameters.cpp

@@ -42,6 +42,13 @@
 #include <QSslSocket>
 #include <QByteArray>
 
+// Default DH parameters, exported by qssldiffiehellmanparameters.cpp.
+QT_BEGIN_NAMESPACE
+extern Q_AUTOTEST_EXPORT const char *qssl_dhparams_default_base64;
+QT_END_NAMESPACE
+
+QT_USE_NAMESPACE
+
 class tst_QSslDiffieHellmanParameters : public QObject
 {
     Q_OBJECT
@@ -54,6 +61,7 @@ private Q_SLOTS:
     void constructionPEM();
     void unsafe512Bits();
     void unsafeNonPrime();
+    void defaultIsValid();
 #endif
 };
 
@@ -157,6 +165,33 @@ void tst_QSslDiffieHellmanParameters::unsafeNonPrime()
 #endif
 }
 
+void tst_QSslDiffieHellmanParameters::defaultIsValid()
+{
+    // The QSslDiffieHellmanParameters::defaultParameters() method takes a shortcut,
+    // by not verifying the passed-in parameters. Instead, it simply assigns the default
+    // DH parameters to the derData field of QSslDiffieHellmanParametersPrivate.
+    //
+    // This test ensures that our default parameters pass the internal verification tests
+    // by constructing, using fromEncoded(), a QSslDiffieHellmanParameters instance that
+    // we expect to be equivalent to the one returned by defaultParameters(). By using
+    // fromEncoded() we go through the internal verification mechanisms. Finally, to ensure
+    // the two instances are equivalent, we compare them.
+
+    const auto dh = QSslDiffieHellmanParameters::fromEncoded(
+        QByteArray::fromBase64(QByteArray(qssl_dhparams_default_base64)),
+        QSsl::Der
+    );
+
+    const auto defaultdh = QSslDiffieHellmanParameters::defaultParameters();
+
+#ifndef QT_NO_OPENSSL
+    QCOMPARE(dh.isEmpty(), false);
+    QCOMPARE(dh.isValid(), true);
+    QCOMPARE(dh.error(), QSslDiffieHellmanParameters::NoError);
+    QCOMPARE(dh, defaultdh);
+#endif
+}
+
 #endif // QT_NO_SSL
 
 QTEST_MAIN(tst_QSslDiffieHellmanParameters)