  We consider the problem of logical data erasure, contrasting with physical
erasure in the same way that end-to-end information flow control contrasts with
access control. We present a semantic hierarchy for erasure policies, using a
possibilistic knowledge-based semantics to define policy satisfaction such that
there is an intuitively clear upper bound on what information an erasure policy
permits to be retained. Our hierarchy allows a rich class of erasure policies
to be expressed, taking account of the power of the attacker, how much
information may be retained, and under what conditions it may be retained.
While our main aim is to specify erasure policies, the semantic framework
allows quite general information-flow policies to be formulated for a variety
of semantic notions of secrecy.
