# Fundamental Limits of Covert Packet Insertion

## TL;DR

This paper investigates the fundamental limits of covert packet insertion in communication channels, establishing how many packets can be inserted without detection under various stochastic models of packet timing.

## Contribution

It derives the maximum number of packets that can be covertly inserted in renewal channels, extending previous results from Poisson processes to general renewal processes.

## Key findings

- In Poisson channels, Alice can insert up to O(√(λT)) packets covertly.
- In general renewal channels, Alice can insert up to O(√N) packets covertly.
- Inserting more than these limits leads to high probability of detection by Willie.

## Abstract

Covert communication conceals the existence of the transmission from a watchful adversary. We consider the fundamental limits for covert communications via packet insertion over packet channels whose packet timings are governed by a renewal process of rate $\lambda$. Authorized transmitter Jack sends packets to authorized receiver Steve, and covert transmitter Alice wishes to transmit packets to covert receiver Bob without being detected by watchful adversary Willie. Willie cannot authenticate the source of the packets. Hence, he looks for statistical anomalies in the packet stream from Jack to Steve to attempt detection of unauthorized packet insertion. First, we consider a special case where the packet timings are governed by a Poisson process and we show that Alice can covertly insert $\mathcal{O}(\sqrt{\lambda T})$ packets for Bob in a time interval of length $T$; conversely, if Alice inserts $\omega(\sqrt{\lambda T})$, she will be detected by Willie with high probability. Then, we extend our results to general renewal channels and show that in a stream of $N$ packets transmitted by Jack, Alice can covertly insert $\mathcal{O}(\sqrt{N})$ packets; if she inserts $\omega(\sqrt{N})$ packets, she will be detected by Willie with high probability.