AOT: Anonymization by Oblivious Transfer

Abstract

We introduce AOT, an anonymous communication system based on mix network architecture that uses oblivious transfer (OT) to deliver messages. Using OT to deliver messages helps AOT resist blending (n−1) attacks and helps AOT preserve receiver anonymity, even if a covert adversary controls all nodes in AOT. AOT comprises three levels of nodes, where nodes at each level perform a different function and can scale horizontally. The sender encrypts their payload and a tag-derived from a secret shared between the sender and receiver-with the public key of a Level-2 node and sends them to a Level-1 node. On a public bulletin board, Level-3 nodes publish tags associated with messages ready to be retrieved. Each receiver checks the bulletin board, identifies tags, and receives the associated messages using OT. A receiver can receive their messages even if the receiver is offline when messages are ready. Through what we call a "handshake" process, communicants can use the AOT protocol to establish shared secrets anonymously. Users play an active role in contributing to the unlinkability of messages: periodically, users initiate requests to AOT to receive dummy messages, such that an adversary cannot distinguish real and dummy requests.

A Survey on Anonymous Communication Systems With a Focus on Dining Cryptographers Networks

“…Indeed, OT is a cryptographic primitive that provides the capability for selecting and transferring data between two parties and can be used as a building block in different contexts where there is a requirement to hide or limit the information about data transfer [90]. This property of OT is used to design a delivery mechanism in a novel ACS called Anonymization by Oblivious Transfer (AOT) [95]. AOT is a protocol that uses OT to facilitate anonymous two-way communication and deliver the messages to the recipients.…”

“…Using OT hides which messages are received by users from a larger set of messages, hence a network adversary cannot link senders and receivers. In summary, the combination of OT and Mix networks increases the anonymity provided by the system, also users do not have to register with the system; any entity who knows the public key of any middle-layer node in the system can send messages with AOT [90], [95].…”