Private Communication Detection via Side-Channel Attacks

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Private communication detection (PCD) enables an ordinary network user to discover communication patterns (e.g., call time, length, frequency, and initiator) between two or more private parties. Analysis of communication patterns between private parties has historically been a powerful tool used by intelligence, military, law-enforcement and business organizations because it can reveal the strength of tie between these parties. Ordinary users are assumed to have neither eavesdropping capabilities (e.g., the network may employ strong anonymity measures) nor the legal authority (e.g. no ability to issue a warrant to network providers) to collect private-communication records. We show that PCD is possible by ordinary users merely by sending packets to various network end-nodes and analyzing the responses. Three approaches for PCD are proposed based on a new type of side channels caused by resource contention, and defenses are proposed. The Resource-Saturation PCD exploits the resource contention (e.g., a fixed-size buffer) by sending carefully designed packets and monitoring different responses. Its effectiveness has been demonstrated on three commercial closed-source VoIP phones. The Stochastic PCD shows that timing side channels in the form of probing responses, which are caused by distinct resource-contention responses when different applications run in end nodes, enable effective PCD despite network and proxy-generated noise (e.g., jitter, delays). It was applied to WiFi and Instant Messaging for resource contention in the radio channel and the keyboard, respectively. Similar analysis enables practical Sybil node detection. Finally, the Service-Priority PCD utilizes the fact that 3G/2G mobile communication systems give higher priority to voice service than data service. This allows detection of the busy status of smartphones, and then discovery of their call records by correlating the busy status. This approach was successfully applied to iPhone and Android phones in AT&T's network. An additional, unanticipated finding was that an Internet user could disable a 2G phone's voice service by probing it with short enough intervals (e.g., 1 second). PCD defenses can be traditional side-channel countermeasures or PCD-specific ones, e.g., monitoring and blocking suspicious periodic network traffic.