I want to create a program, which can encrypt and decrypt a complete file with an individual password. Is there any way to manage this in Qt and/or C++ and how?
Neither Qt nor the C++ standard library have encryption built-in. You'll need another external library to handle encryption.
www.cryptopp.com is a very complete C++ library with implementations of most algorithms.
The actual program (select file, read, obtain key, encrypt etc) should be piece of cake.
I've never used it myself, but I've heard great things about QCA. It's cross platfrom, uses a Qt-style API and Qt datatypes.
Old, I know, but try Botan. It's actually used internally by Qt Creator 2.0. If you download the Qt Creator 2.0 sources you can find a copy of Botan 1.8.8 all set up for the Qt build system (qmake).
Qt doesn't provide functionality to encrypt/decrypt.
QCryptographicHash only generates hashes. It is not what you are looking for.