The project that Latitude[1] was talking about. I am not saying you are legally obligated to do something (IANAL). I am just saying it is common courtesy.
Trust me. The logo does not make or break your program.
As far as we know, said project only exists in latitude's mind, and the only public information about it is that he may or may not still have plans for a "secure chat" program. I'd be hard-pressed to call that an "existing project", since for all practical purposes, it isn't.
You are asserting a made-up argument. Having an active public project behind a logo is not a necessary condition for not re-purposing the logo for another project. This particular concept just happens to be taken.
Every good logo is trivial in retrospect, it's finding a strong concept that's a bitch. I looked at /gd/ thread and virtually all of it revolves around shaping a keyhole this way or that way. Then suddenly someone says - "Here, done". Turning an empty space into a chat bubble requires a step up, it's an altogether stronger and more complex concept. It is possible that another person came up with it independently, but also consider that with all the variety of secure messaging apps no one had thought of it before 2012. It's not an easy concept to stumble upon.
(edit) This is getting meta. It is plenty obvious that Tox fellas think they are in the clear and it's OK to recycle an existing logo. I think that it's not. That's hard to reconcile.
>Re-purposing the logo
>recycle an existing logo
No-one is re-purposing the logo. You are going out of your way to make it seem as if they have literally stolen your logo, when in reality, they have a similar design that is not a copy.
