SSL Verification with Qt and a custom CA certificate

So I wanted to make my application updater for guitone SSL-aware the other day. The server setup was an easy job: Add the new domain (guitone.thomaskeller.biz) to cacert.org, create a new certificate request with the new SubjectAltName (and all the other, existing alternative names – a procedure where this script becomes handy), upload to CAcert, sign it there, download and install the new cert on my server, setup a SSL vhost for the domain – done!

Now, on Qt’s side of things using SSL is rather easy as well, the only thing you have to do is give the setHost method another parameter:

QHttp * con = new QHttp();
con->setHost("some.host.com", QHttp::ConnectionModeHttps);
con->get("/index.html");
// connect to QHttp's done() signal and read the response
This should actually work for all legit SSL setups if Qt (or, to be more precise, the underlying openssl setup) knows about the root certificate with which your server certificate has been signed. Unfortunately, CAcert’s root certificate is not installed in most cases, so you basically have two options:

  1. Connect to QHttp’s sslErrors(...) signal to the QHttp::ignoreSslErrors() slot. This, of course, pretty much defeats the whole purpose of an SSL connection, because the user is not warned on any SSL error, so also legit errors (certificate expired or malicious) are just ignored. (*)
  2. Make the root certificate of CAcert known to the local setup, so the verification process can proceed properly.

I decided to do the latter thing. This is how the code should now look like:

QHttp * con = new QHttp();
QFile certFile("path/to/root.crt");
Q_ASSERT(certFile.open(QIODevice::ReadOnly));
QSslCertificate cert(&certFile, QSsl::Pem);
// this replaces the internal QTcpSocket QHttp uses; unfortunately
// we cannot reuse that one because Qt does not provide an accessor
// for it
QSslSocket * sslSocket = new QSslSocket(this);
sslSocket->addCaCertificate(cert);
httpConnection->setSocket(sslSocket);
con->setHost("some.host.com", QHttp::ConnectionModeHttps);
con->get("/index.html");
// connect to QHttp's done() signal and read the response
Particularily interesting to note here is that the QIODevice (in my case the QFile instance) has to be opened explicitely before it is given to QSslCertificate. I did not do this previously, Qt neither gave me a warning nor an error, but simply refused to verify my server certificate, just because it didn’t load the root certificate properly.

(*) One could, of course, check the exact triggered SSL error from QSslError::error(), in our case this could be f.e. QSslError::UnableToGetLocalIssuerCertificate, but this is rather hacky and could certainly be abused by a man in the middle as well.