Some software-related acts can be carried out without authorisation.
On the one hand, certain elements of a piece of software can be used by the designers of a different piece of software without authorisation, and on the other hand, certain uses can be made of the piece of software without authorisation.
The elements of software that are not protected
The underlying ideas of a piece of software
Copyright does not protect the ideas themselves. It only protects the way in which they are realised, the way in which they are arranged and translated into a program.
The ideas themselves, including the software functionalities, can therefore be used and realised differently by another piece of software.
For example, copyright does not prevent someone from creating word processing software or a spreadsheet, provided that there is no copy (or merely adaptation) of the way in which these functionalities have already been realised in an existing computer program.
Software elements that are the result of technical or functional constraints
Copyright only protects elements for which the designer had room for manoeuvre. The elements of the software that are dictated by the functionality of the software are not protected. So they may also be used to create a different piece of software without qualifying as counterfeiting.
The uses of a piece of software for which authorisation is not required
Software users may make a backup copy of a piece of software (unless such a copy was directly provided to the user).
The copies automatically made by the computer while the software was being used are, of course, authorised.
The observation, study and testing of the operation of the program
The user has the right to use the program in a way that allows him to understand the underlying ideas.
The decompilation of the program interfaces to allow its interoperability with another program
However, this exception is subject to many conditions. It is recommended to seek advice from a specialist if you want to ensure that a decompilation act is legal.