If it asks on iOS, it may well be that the whatever it is you tried to open it with, doesn't know how to handle .rar files and as result assumes it's password protected.
Contrary to what the previous poster said, changing extension from rar <-> zip doesn't make a zip from a rar or a rar from a zip. On Windows, if you're using anything else than the default compressed folder explorer add-on, that'll very likely support multiple formats and just ignores the extension. FYI, a CBR (Comic Book Rar) is actually a true RAR file. Like a CBZ (Comic Book Zip) is actually a true ZIP file. Some programs use either RAR or ZIP as dedicated default filetype, but to prevent auto-launching, reading and scanning when clicking on a RAR or ZIP, they also use a dedicated extension to distinguish _their_ respective files from the generic ones.