Own Idea vs Professors Project for Masters Thesis? I'm in a masters program where a thesis is required. I haven't officially started my thesis yet but I wrote a work-in-progress paper for an IEEE conference. The topic is on a mechanism I thought of to prevent unauthorised account use using AI. I found 2 professors to give me some feedback and they became co-authors on the paper. Though the feedback was mainly related to writing as neither had any background in security, though they are familiar with AI related topics.

Now that the work-in-progress paper has been submitted I received an email from one of the professors asking if I wanted to collaborate with him on a project. He mentioned he doesn't have funding for it now but he just applied. The topic is related adaptive learning systems making use of reinforcement learning. It seems many of the professors in my department are interested in how AI and computer systems can be used to improve education. Where as no one is working on anything security related.

I'm trying to decide if I should work on the project proposed by the professor for my thesis or the original idea I wrote about in my work-in-progress paper. I'm more interested in my own idea, but I'm thinking working on the other idea might give me access to research assistant funding (though this is not a sure thing). Is it normal to take on topics popular in your specific school's department?
Pursuing a topic beyond a school's core competencies (e.g., security) might result in worse supervision. For instance, 


Due to the inability of staff to provide the required guidance. That said, at your level, competent staff should be capable of supervising you. 
Poorly aligned student-supervisor interests. (Ill-alignment shouldn't matter in theory, but does in reality.)


These risks can be mitigated against, e.g., by finding an external supervisor/mentor.

Beyond your short-term goal (getting a degree), you should consider the longer-term: What do you want to do next? If you want to entertain the idea of pursuing a research career in security, then pursuing a thesis on the topic will help you decide whether that's the right path and will help you get on that path.