Security Implications of Cooperative Communications in Wireless Networks Salik Makda † , Ankur Choudhary ⋆ , Naveen Raman † , Thanasis Korakis † , Zhifeng Tao ⋄ , Shivendra Panwar † † Department of Electrical and Computer Engineering, Polytechnic University, Brooklyn, NY 11201 ⋆ Department of Computer Science and Engineering, Indian Institute of Technology, Kanpur, India ⋄ Mitsubishi Electric Research Laboratories, Cambridge, MA 02139 e-mail: smagda@poly.edu, ankurc@iitk.ac.in, nraman01@students.poly.edu, korakis@poly.edu, tao@merl.com, panwar@catt.poly.edu Abstract—Cooperative communications is an innovative tech- nique that is expected to change the behavior of wireless networks in the near feature. In the MAC layer, this technique defines new protocols by enabling additional collaboration from stations that otherwise will not directly participate in the transmission. A typical example of such a protocol is CoopMAC [1], a cooperative MAC protocol that involves an intermediate station or helper in the communication between a transmitter and a receiver. Under this scheme, the transmitter sends its packets to the receiver by forwarding them through the helper. In this way the protocol takes advantage of spatial diversity and faster two- hop transmission, significantly improving the performance of the network. In such an environment, where the sender relies on an intermediate helper to forward its packets to the original destination, numerous security issues may arise. The present security schemes need to be adapted in order to support end- to-end security in the source-helper-destination communication model. In this paper we discuss the potential security issues that cooperation may raise and propose two new security schemes to address those concerns. To evaluate the feasibility of the pro- posed algorithms, we implement them using open source drivers platform, which is explained in the paper in detail. Moreover, the paper also discusses the design challenges encounterd and share the experience and insights gained during implementation. Our implementations of the suggested techniques allow the WEP, WPA and WPA2 (802.11i) security protocols to successfully operate in the new cooperative environment. Index Terms—Wireless security, cooperative communications, cooperative MAC protocols, open source implementation I. I NTRODUCTION Cooperative communications consists of schemes and tech- niques that take advantage of spatial diversity among neighbor- ing stations in a wireless network. These schemes, by enabling additional collaboration from stations that otherwise will not directly participate in the transmission, achieve tremendous improvement in the overall performance of the network. The innovation of cooperative communications was initially con- fined to the physical layer. However, in order for the network to take advantage of cooperative diversity, new higher layer protocols should be devised accordingly. Towards this end, several MAC layer protocols have been proposed to take advantage of the notion of cooperation. A typical example is a protocol called CoopMAC [1], which illustrates how the legacy IEEE 802.11 [2] can be enhanced with minimal mod- ifications to maximize the benefits of cooperative diversity. Under this protocol, a station that experiences a bad channel with an intended receiver, instead of transmitting directly to a receiver at a low transmission rate, can use an intermediate station (usually called a helper) to forward its packets to the receiver. Due to the fact that the helper experiences a good channel with both stations involved in the communication, the two hop transmission is done at high rates. Thus the overal communication consumes significantly less time than the original one. As a result, CoopMAC can substantially improve the performance of the network. The interested reader can find more information about the details of the protocol in [1]. An implementation of the cooperative MAC scheme in the HostAP Linux wireless driver has been discussed in [3]. Al- though cooperative communications enables high performance in wireless networks, its reliance on the premises that some third-party station has to be involved in the communication raises serious concern of potential security compromise. More specifically, the fact that the helper receives the packet and retransmits it to the receiver has the potential of opening holes in the security of the network and hence must be thoroughly investigated. In this paper we first study the potential security issues that may arise in such a cooperative network. We then propose two new schemes that adapt today’s security techniques (i.e., WEP, WPA, WPA2-802.11i) [2] [4] to the new cooperative environment. In order to prove the feasibility of the proposed schemes we implemented them in MADWIFI version number 0.9.2, which is the most recent version of an open source Linux driver platform for Atheros chipsets. In the paper, we describe these efforts as well as the challenges we have faced and the experience gained during this process. The rest of the paper is organized as follows: In section II we briefly introduce cooperative MAC protocols and enu- merate the possible security concerns that the protocol may raise. In section III we provide a brief overview of the 802.11i security framework. In section IV we propose the solutions to the security problems described earlier in section II. In section V we discuss the details of the implementation of the proposed security schemes. Finally, we conclude in section VI by highlighting the experience and insights we have gained during the implementation process.