Covert Communications Despite Traffic Data Retention

We show that Alice and Bob can communicate covertly and anonymously, despite Eve having access to the traffic data of most machines on the Internet. Our protocols take advantage of small amounts of shared state that exist in many TCP/IP stacks, and use them to construct a covert channel. Techniques inspired from Direct Sequence Spread Spectrum (DSSS) are used to make sure that the communication is covert and resistant to noise. We implement a prototype based on ICMP Echo (ping) to illustrate the practicality of our approach and discuss how a more complex protocol would modulate information through the use of TCP features to make communication detection very difficult. The feasibility of covert communications despite stringent traffic data retention, has far reaching policy consequences.