In my research field, software tools are considered a research outcome, and therefore it's normal that researchers build tools and make them publicly available. However, in many cases, the authors only make the executable for a tool available, without sharing the source code used to build the tool.
This makes things complicated for people who want to use or extend the tool: for example, they might be unable to run the tool, because it has a dependency to an outdated, flawed, or expensive commercial "baseline" tool. While they might have the programming skills to modify the tool so it can be run, this usually requires changes to the source code.
However, there is a kind of tool called decompiler which allows you to restore the original source code (or something similar) from the given executable. In some cases, you can work with the restored source code as good as if the authors had made the code available in the first place.
Assuming I want to use a particular tool for my research, and I have already unsuccessfully contacted the authors to obtain the source code in a consensual way.
Is it OK if I decompile the code to do my research?Yes
If yes, then how do I deal with the fact that I decompiled the tool in my paper? Would it be considered rude/sketchy to say that I did it?
Can I make the modified tool available to other researchers (and yes, as executables and/or source code)?No
Edit: Thanks everyone. For questions 1 and 3, the correct answers for my case are now clear to me (see above). I'm still not sure about question 2, though. Are there previous examples in literature where people did this? Would it be acceptable to not address the fact that I decompiled the source code, by treating it like an irrelevant detail? Reimplementing the complete tool is not an option in my case.