Use The Unarchiver from the Mac App Store. Standard Apple "Archive Utility" often fails to prompt for the password correctly [1].
Depending on when the ROM archive was uploaded or updated on the platform, one of two primary lowercase keys will unlock the archive: romspure password new
Even with the right password, you might run into errors. Here is how to fix them: Avoid Copy-Pasting Use The Unarchiver from the Mac App Store