Many people may think they have the man in the iron mask thing figured out. I on the other hand approach things knowing that I know nothing at all. On that note, here goes my theory for who I believe is the man in the mask.
Towards the end of last season we see the flash traveling through the wormhole to try and save his mother. When he takes that trip, among many things we see through that trip, is barry in a, what looks to be Iron Heights prison suit.
Here's my theory. Among all the other things that zoom has done, we find out that he's been lying about lots of things in the way he tells his stories or simply leaving things out to jumble the contexts. I believe since zoom has been running around in timelines its not too much of a venture to believe he's also been through other universes/earths along the way. I believe Barry in some form was in a prison and for some reason which I can't quite work out yet zoom found him, either in the future or alternate earth.
Last reason, if you look at the picture i posted with the article, you'll notice how the man in the mask and Barry match up size wise and i think it was a clever foreshadowing job from the producers how they have them mirror and also how the mans mask kind of projects over Barry to give more hints.
Well there is my first article on moviepilot, don't be shy and hit me with the hard news in the comments and also let me know what your theories are on the whole thing and if you think I'm just a crazy tin foil hat wielder of the flash iron mask man theories.