Oblivious Revocable Functions and Encrypted Indexing

Many online applications, such as online file backup services, support the sharing of indexed data between a set of devices. These systems may offer client-side encryption of the data, so that the stored data is inaccessible to the online host. A potentially desirable goal in this setting would be to protect not just the contents of the backed-up files, but also their identifiers. However, as these identifiers are typically used for indexing, a deterministic consistent mapping across devices is necessary. Additionally, in a multi-device setting, it may be desirable to maintain an ability to revoke a device’s access—e.g. through rotating encryption keys for new data. We present a new primitive, called the Oblivious Revocable Function (ORF), which operates in the above setting and allows identifiers to be obliviously mapped to a consistent value across multiple devices, while enabling the server to permanently remove an individual device’s ability to map values. This permits a stronger threat model against metadata, in which metadata cannot be derived from identifiers by a revoked device colluding with the service provider, so long as the service provider was honest at the instant of revocation. We describe a simple Diffie- Hellman-based construction that achieves ORFs and provide a proof of security under the UC framework

On Ends-to-Ends Encryption: Asynchronous Group Messaging with Strong Security Guarantees

In the past few years secure messaging has become mainstream, with over a billion active users of end-to-end encryption protocols through apps such as WhatsApp, Signal, Facebook Messenger, Google Allo, Wire and many more. While these users\u27 two-party communications now enjoy very strong security guarantees, it turns out that many of these apps provide, without notifying the users, a weaker property for group messaging: an adversary who compromises a single group member can intercept communications indefinitely. One reason for this discrepancy in security guarantees is that most existing group messaging protocols are fundamentally synchronous, and thus cannot be used in the asynchronous world of mobile communications. In this paper we show that this is not necessary, presenting a design for a tree-based group key exchange protocol in which no two parties ever need to be online at the same time, which we call Asynchronous Ratcheting Tree (ART). ART achieves strong security guarantees, in particular including post-compromise security. We give a computational security proof for ART\u27s core design as well as a proof-of-concept implementation, showing that ART scales efficiently even to large groups. Our results show that strong security guarantees for group messaging are achievable even in the modern, asynchronous setting, without resorting to using inefficient point-to-point communications for large groups. By building on standard and well-studied constructions, our hope is that many existing solutions can be applied while still respecting the practical constraints of mobile devices

On Ends-to-Ends Encryption: Asynchronous Group Messaging with Strong Security Guarantees

In the past few years secure messaging has become mainstream, with over a billion active users of end-to-end encryption protocols such as Signal. The Signal Protocol provides a strong property called post-compromise security to its users. However, it turns out that many of its implementations provide, without notification, a weaker property for group messaging: an adversary who compromises a single group member can read and inject messages indefinitely. We show for the first time that post-compromise security can be achieved in realistic, asynchronous group messaging systems. We present a design called Asynchronous Ratcheting Trees (ART), which uses tree-based Diffie-Hellman key exchange to allow a group of users to derive a shared symmetric key even if no two are ever online at the same time. ART scales to groups containing thousands of members, while still providing provable security guarantees. It has seen significant interest from industry, and forms the basis for two draft IETF RFCs and a chartered working group. Our results show that strong security guarantees for group messaging are practically achievable in a modern setting