User blog comment:Godisme/HOT HOT HEAT/@comment-1268838-20121205140742

Well, technically only the Oken's location is passed down to the captain commander. In other words, I think we can assume that what Ichigo was just told is the secret that Yamamoto died with. Which makes the key pretty foolproof, all things considered. It certainly makes sense that Aizen had to go so far as to make his own if the only way to get in is with a Royal Guard's consent.