Hourglass schemes
Marten van Dijk, Ari Juels, Alina Oprea, Ronald L. Rivest, Emil Stefanov, Nikos Triandopoulos · 2012
We consider the following challenge: How can a cloud storage provider prove to a tenant that it's encrypting files at rest, when the provider itself holds the corresponding encryption keys? Such proofs demonstrate sound encryption policies and file confidentiality. (Cheating, cost-cutting, or misconfigured providers may bypass the computation/management burdens of encryption and store plaintext only.)