Friendly CryptoJam: a mechanism for securing physical-layer attributes

The broadcast nature of wireless communications exposes various "transmission attributes," such as the packet size, the inter-packet times, and the modulation scheme. These attributes can be exploited by an adversary to launch passive or active attacks. A passive attacker threatens user's privacy and confidentiality by performing traffic analysis and classification, whereas an active attacker exploits captured attributes to launch selective jamming/dropping attacks. This so-called PHY-layer security problem is present even when the payload is encrypted. For example, by determining the modulation scheme, the attacker can estimate the data rate, and hence the payload size, and later use it to launch traffic classification or selective rate-adaptation attacks.

In this paper, we propose Friendly CryptoJam, a novel approach that combines analog-domain friendly jamming and modulation-level encryption. Friendly CryptoJam decorrelates the payload's modulation scheme from other transmission attributes by always "upgrading" it to the highest-order modulation scheme supported by the system (a concept we refer to as (modulation unification) using a secret pseudo-random sequence. Such upgrade is a form of transmitter-based friendly jamming. At the same time, modulation symbols are encrypted to protect unencrypted PHY-layer fields (modulation encryption). To generate and sync the secret sequence, an efficient message embedding technique based on Barker sequences is proposed, which exploits the structure of the preamble and overlays a frame-specific seed on it. We study the implications of the scheme on PHY-layer functions through simulations and USRP-based experiments. The results confirm that Friendly CryptoJam is quite successful in hiding the targeted attributes, at the cost of a small increase in the transmission power.