Open ecrypted PDF with C++/Qt (known passwords) - c++

Question

I am stuck with the following issue:
I need to open encrypted PDF files (with known passwords) from a simple C++/Qt program with the default PDF editor on the PC. I know the password to the PDFs, this is not the issue here.
Alternatively, it would be also ok, if the PDF is rendered within my Qt window, but I think that this would be even more complicated.
I know that I am able to open standard PDFs with fairly simple commands like
QDesktopServices::openUrl(QUrl("file:///home/.../fileName.pdf"));
But I do not know any option to insert a password here.
Does anybody know a way how to do this (preferably with mainly standard C++/Qt methods)?
Thank you very much and best regards

Answer 1

You may decrypt the original PDF file first, for instance with the qpdf library saving the output to a temporary file, and then open this temporary file with QDesktopServices::openUrl().
To decrypt a PDF file with the qpdf command line utility, the syntax is:
qpdf --password=KNOWNPASSWORD --decrypt input.pdf output.pdf
Here is a simple example of PDF decryption using the qpdf library from a Qt program, using the pkg-config utility to configure the dependency.
test.pro
QT = core
CONFIG += c++11 console link_pkgconfig
TEMPLATE = app
SOURCES += main.cpp
PKGCONFIG += libqpdf
main.cpp
#include <QCoreApplication>
#include <qpdf/QPDF.hh>
#include <qpdf/QPDFWriter.hh>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString inputFile = "input.pdf";
QString outputFile = "output.pdf";
QString userPassword = "USERPASS";
try
{
QPDF qpdf;
qpdf.processFile(inputFile.toLocal8Bit(), userPassword.toLocal8Bit());
QPDFWriter w(qpdf, outputFile.toLocal8Bit());
w.setPreserveEncryption(false);
w.write();
}
catch (std::exception& e)
{
qFatal("Error. %s", e.what());
}
}