"The world just witnessed the surge of a new and powerful attacker, which was able to coerce operators and acquire the necessary keys to break the privacy of users. Once the encryption key is exposed, the only viable measure to preserve data confidentiality is to limit the adversary's access to the cipher-text. This may be achieved, for example, using multi-cloud storage systems. These systems spread data across multiple servers in different administrative domains, to cater for availability and fault tolerance. If the adversary can only compromise a subset of these domains, multi-cloud storage systems may prevent the adversary from accessing the entire cipher-text."