How (and Why) are nuke keys different than delete keys?

[Gavin Wahl <gavinwahl@gmail.com>]

I'm curious how delete and nuke permissions can be granted independently. Is there a separate HMAC key that just allows nukes (http://www.tarsnap.com/crypto.html states that there are only three HMAC keys, though).

Also, how is this feature expected to be useful? Is there some interesting security property you get by having a key with delete but not nuke, or vice versa?