StOMR: Stateful Oblivious Message Retrieval

Oblivious Message Retrieval (OMR) enables recipients to retrieve messages from an untrusted server without revealing which messages correspond to them. OMR is useful for privacy-preserving systems such as anonymous messaging and blockchains. Existing constructions are inherently built in the public-key setting and use Fully Homomorphic Encryption (FHE) to allow a detector to scan a public bulletin board and produce a compact encrypted digest containing only relevant messages. However, this design incurs a major practical limitation: sender-published signals are large (hundreds of kilobytes), leading to significant bandwidth, storage, and on-chain costs.

In this work, we explore the efficiency gains achievable by moving to a private, stateful setting, and introduce stateful OMR, a new model in which senders and receivers maintain lightweight private state to enable more efficient anonymous communication. Based on this model, we design StOMR, a protocol that significantly reduces the size of sender signals compared to prior approaches. To support the required shared state, we further introduce an OMR Key Encapsulation Mechanism (OMR-KEM), which allows users to privately establish shared state and instantiate stateful OMR without out-of-band channels.

We implement StOMR in C++ and evaluate it against the state-of-the-art systems PerfOMR (USENIX Security '24), SophOMR (USENIX Security '26), and InstantOMR (USENIX Security '26). Our results show that StOMR preserves detector performance while achieving a signal size approximately 550×, 650×, and 178× smaller than PerfOMR, SophOMR, and InstantOMR, respectively.