Stratara 4.6.0

An erasure shreds every key that names the subject, and nothing stopped the next encryption for that subject
from creating a new one: a key store asked for a scope it does not hold creates a first version. A job, a request
in flight or a second process that encrypted for an erased subject afterwards got a fresh key, and what it wrote
stayed readable for good. The key store now records an erased subject and refuses to create a key for it again.

Added

• IKeyStore.MarkSubjectErasedAsync(KeySubject) records that a tenant or user has been erased. It has a
  default implementation that throws NotSupportedException, so an existing key store still compiles.
• KeySubject names the tenant or user, and KeySubject.Covers(KeyScope) states which keys belong to its
  erasure: for a tenant, a key of any level that names it; for a user, a user-level key that names it.
• KeyScopeErasedException (an InvalidOperationException) is what a key store throws for such a scope. It
  carries the refused Scope.

Changed

• An encryption for an erased subject fails. After a subject's erasure, GetOrCreateCurrentKeyAsync and
  RotateAsync throw KeyScopeErasedException for every scope the subject covers, whether or not it ever held a
  key, and create nothing. The blob encryptor, the encrypting settings store and the field-encrypting serializer
  pass it through. Code that erased a subject and then wrote encrypted data for it again now sees the exception;
  the mark is permanent and cannot be lifted. Revoking every version of a scope is not an erasure and marks nothing.
• ISubjectEraser marks the subject before it sweeps any plane, so an encryption racing the erasure
  either creates its key first, and that key is shredded with the rest, or is refused. A key store that cannot mark
  a subject leaves the erasure as before and logs SubjectNotMarkable (112009).
• The file-backed key store writes format version 2, which holds the marks, and refuses to open a file in a
  newer format than it knows. InMemoryKeyStore refuses an erased subject as the file store does; DummyKeyStore
  refuses to mark one, as it refuses every erasure operation.

Upgrading:

• Upgrade every process that shares a key store file in one step. A process on an earlier release reads a
  version-2 file, ignores the marks, creates keys for an erased subject and drops the marks the next time it writes
  the file. A version-1 file needs no migration. Rolling back is possible but loses the marks on the first write.
• Map KeyScopeErasedException to your answer for data that is gone, if a request can reach an encryption for
  a subject after its erasure.
• A key store of your own implements MarkSubjectErasedAsync, refusing in GetOrCreateCurrentKeyAsync and
  RotateAsync every scope a marked subject Covers. Without it the erasure warns and the gap stays open.