Two-Hop Secure Communication Using an Untrusted Relay
© X. He and A. Yener. 2009
Received: 4 December 2008
Accepted: 8 October 2009
Published: 24 November 2009
We consider a source-destination pair that can only communicate through an untrusted intermediate relay node. The intermediate node is willing to employ a designated relaying scheme to facilitate reliable communication between the source and the destination. Yet, the information it relays needs to be kept secret from it. In this two-hop communication scenario, where the use of the untrusted relay node is essential, we find that a positive secrecy rate is achievable. The center piece of the achievability scheme is the help provided by either the destination node with transmission capability, or an external "good samaritan" node. In either case, the helper performs cooperative jamming that confuses the eavesdropping relay and disables it from being able to decipher what it is relaying. We next derive an upper bound on the secrecy rate for this system. We observe that the gap between the upper bound and the achievable rate vanishes as the power of the relay node goes to infinity. Overall, the paper presents a case for intentional interference, that is, cooperative jamming, as an enabler for secure communication.
Information theoretic security was proposed by Shannon . The idea of measuring secrecy using mutual information lends itself naturally to the investigation of how the channel can influence secrecy and further to the characterization of the fundamental limit of secure transmission rate. Wyner, in , defined the wiretap channel, and showed that secure communication from a transmitter to a "legitimate" receiver is possible when the signal received by the wiretapper (eavesdropper) is degraded with respect to that received by the legitimate receiver. Reference  identified the secrecy capacity of the general discrete memoryless wiretap channel. The secrecy capacity of the Gaussian wiretap channel is found in .
Recent progress in this area has extended classical information theory channel models to include secrecy constraints. Examples are the multiple access channel, the broadcast channel, the two-way channel, the three-node relay channel and the two-user interference channel [5–13]. These studies are beginning to lead to insights for designing secure wireless communication systems from the physical layer up. Prominent such examples include using multiple antennas to steer the transmitted signal away from an eavesdropper [14–16], transmitting with the intention of jamming the eavesdropper [8, 10, 17], and taking advantage of variations in channel state to provide secrecy [18–20].
The focus of this work is on a class of relay networks where the source and the destination have no direct link and thus can only communicate utilizing an intermediate relay node. This models the practical scenario where direct communication between the source and the destination is too "expensive" in terms of power consumption: direct communication may be used to send some very low rate control packages, for example to initialize the communication, but it is infeasible to sustain a nontrivial reliable communication rate due to the power constraint.
In such a scenario, the source-destination pair needs the relay to communicate. On the other hand, more often than not, this relay node may be "untrusted" . This does not mean the relay node is malicious, in fact quite the opposite, it may be part of the network and we will assume that it is willing to faithfully carry out the designated relaying scheme. The relay simply has a lower security clearance in the network and hence is not trusted with the confidential message it is relaying. Equivalently, we can assume the confidential message is one used for identification of the source node for authentication, which should never be revealed to a relay node in order not to be vulnerable to an impersonation attack. In all these cases, we must assume there is an eavesdropper colocated at the relay node when designing the system.
The "untrusted" relay model, or the eavesdropper being colocated with the relay node, was first studied in  for the general relay channel, with a rather pessimistic outlook, finding that for the degraded or the reversely degraded relay channel the relay node should not be deployed. More optimistic results for the relay channel with a colocated eavesdropper have been identified recently in [11, 21, 22]. Specifically, it has been shown that the cooperation from the relay may, in fact, be essential to achieving nonzero secrecy rate [11, 21]. The model is later extended to the more symmetric case in [23, 24] where the relay also has a confidential message of its own, which must be kept secret from the destination.
All these models assume that a direct link between the source and the destination is present including our previous work . In contrast, when there is no direct link, it is impossible for this network to convey a confidential message from the source to the destination while keeping it secret from the relay . This is because the destination can only receive signals from the relay resulting in a physically degraded relay channel . Therefore, the relay knows everything the destination knows regarding the confidential message, and the secrecy capacity is zero.
The differentiating feature of the model studied in this work from those described above including  is that the destination has transmission capability. This opens the possibility of the destination node to actively participate in ensuring the secrecy of the information it wants to obtain. In an effort to address a practical two-hop communication scenario, we shall consider each node to be half-duplex, which leads to a two-phase communication model. In addition, feedback to the source is not considered in the channel model. Interestingly, in this model, the transmission capability of the destination proves to be the enabler of secure communication. By recruiting the help of the destination to do "cooperative jamming", positive secrecy rate can be achieved that would not have been possible otherwise. We also remark that in case the transmission by the destination is not possible or desired, the help from an external cooperative jammer will do as well.
The idea of using a helpful jammer goes back to [17, 26, 27] and has since been used in many different models. Besides the multiple access, two-way  and relay wiretap channels , other recent results that use "cooperative jamming" as the part of the achievability scheme, include [28–30]. In , a separate jammer is added to the classical Gaussian wiretap channel model. The jamming signal is revealed to the legitimate receiver via a wired link so that an advantage over the eavesdropper is gained. Reference  does not assume the wired connection, and employs a scheme tantamount to the two user multiple access channel with an external eavesdropper where one of the users perform cooperative jamming. Reference  considers the case where both the eavesdropper and the legitimate receiver observes a modulus channel and the destination carries out the jamming. We note that all these works deal with an external eavesdropper, in contrast to the focus of this work, which is an untrusted (but legitimate) node in the network.
In general, the optimality of recruiting a helpful jammer remains open as the converse results are limited. For the Gaussian case, the main difficulty is to find a upper bound for which the optimal input distribution can be found and evaluated. Doing so usually involves the introduction of genie information, as in the converse for the Gaussian wiretap channel , MIMO wiretap channel  and MAC wiretap channel . The proofs then typically invoke the entropy power inequality, as in , or the generalized entropy power inequality, as in .
In this work, we derive a computable upper bound for the model in consideration by first introducing a second eavesdropper, an approach first used for a three-node relay channel in . Next, after several steps of genie arguments, the channel is transformed into a wiretap channel with a helpful jammer, whose outer bound is then evaluated. The resulting bound is nontrivial in the sense that it is strictly tighter than the bound for the same channel without secrecy constraints. We also prove that it is tighter than an upper bound derived using the generalized entropy power inequality following a similar approach to , when the maximum sum received SNR at the relay is greater than dB. We show that the gap between the bound and the achievable rates converges to zero when the power of the relay goes to infinity.
The paper is organized as follows. Section 2 presents the channel model and the two-phase protocol that utilizes cooperative jamming. In Section 3, we derive the achievable rates. Section 4 presents our upper bound and compares with other known upper bounds. Section 5 presents the numerical results. Finally, Section 6 presents the conclusion.
2. Channel Model
The channel alternates between these two phases according to a random or deterministic schedule, which is generated by a global controller independently from the signals associated with the channel model. Hence here the term "schedule" is simply a finite number of channel uses which are either marked as phase one or phase two. We use to denote the number of channel uses marked as phase one, and to denote the number of channel uses marked as phase two. It should be noted that in general the channel uses of phase one are not consecutive. Neither are the channel uses of phase two. We assume the schedule is stable, in the sense that the following limit exists:
For a given , we use to denote a sequence of schedules with increasing number of channel uses such that (3) holds. According to this definition, becomes the limit of the time sharing factor of phase one in the schedule as .
is the total number of channel uses.
After a number of phases, the destination node (node ) decodes a message from the signals it transmitted during the periods of phase one and the signals it received during the periods of phase two. For reliable communication, should equal the message from the source node with high probability. Hence we have the following requirement:
Therefore, the secrecy constraint can be expressed as
such that (9) and (12) are fulfilled. When deriving achievable rate, we will focus on a specific sequence of schedules , and maximize the secrecy rate over . When deriving the upper bound, we will consider all possible sequences of .
An apparent vulnerability of the described two phase protocol is that the destination may not be aware that the source has initiated its transmission. In this case, without the protection of the jamming signal from the destination, the message from the source would be revealed to the relay node and hence compromised. To prevent this from happening, proper initialization of the protocol is necessary.
3. Achievable Rate
In this section, we derive the achievable secrecy rate with the following sequence of deterministic periodic schedules.
With this sequence of schedules, we have the following theorem.
The proof is given in the appendix.
It can be seen from (17) that, for any fixed time sharing factor the relay should always transmit at maximum power . However, the optimal transmission power of the source may be less than . This can be seen as follows: For a given jamming power , the achievable rate is not a monotonically increasing function of . This is because, if or , , indicating that even if the source power budget is , the optimal transmission power is actually finite. Let this value be . may or may not fall into the interval , which is the range of power consumption allowed for phase one. If it does, then the source should transmit with power rather than . If not, then the corresponding optimal value needs to be determined.
4. Upper Bound
In this section, we derive an upper bound for the secrecy rate.
We first need to determine the optimal schedule. It turns out that it is easy to find: We simply let the first channel uses be phase one, and the remaining channel uses be phase two. The optimality of this schedule can be proved as follows.
Suppose a different schedule is used. Since the signals received in the past are not used for encoding purposes at node and , we can always move the channel uses of phase one to the front without affecting the signals transmitted by these two nodes. On the other hand, we notice that the relay can only use signals received in the past to compute its transmission signals. However, during phase one, the relay only receives signals. Since moving phase one ahead only means the relay could receive signals sooner, doing so will not limit the capability of the relay to calculate its transmitted signals. Consequently, we observe that no matter what schedule is used to achieve a secrecy rate, we can always modify this schedule such that all channel uses of phase one are ahead of those of phase two and still achieve the same secrecy rate. Hence in the following we only consider the optimal schedule.
From (15), this means
Hence the message is kept secret from the second eavesdropper. This means, for a given coding scheme that achieves secrecy rate in Figure 3, the same secrecy rate is achievable with the introduction of this additional eavesdropper.
It can be verified that, for any fixed , equation (37) is an increasing function of and . Therefore, the upper bound is maximized with maximum average power. Equation (38) can then be tightened by minimizing it over . The optimal is given below:
As a result, we have the following theorem.
The second term in (45) is always negative.
4.1. Comparison with the Bound Derived with Generalized Entropy Power Inequality
Recently the generalized entropy power inequality  was used to derive a computable upper bound for the Gaussian multiple access channel with secrecy constraints . Here the same technique is applicable and another computable upper bound for the model in Figure 1 can be derived. It is of interest to know which bound is tighter. Next, we prove that as long as , this upper bound is always looser than the bound given by (38)-(39).
First, we briefly describe the derivation of the bound based on the approach in :
Hence (52) can be upper bounded with
Since the gap between the achievable rates and the bound given by (55) is bounded by bit/channel use when , the gap between the achievable rates and the bound given by (41) is also bounded by bit/channel use when and . Note that since when we have , the condition is equivalent to .
5. Numerical Results
Also shown in each figure is the cut-set bound without secrecy constraints. The improvement provided by the new bounds depends on the power budget. In general, the improvement is small if the power of the jammer is large. Note that since we have normalized all channel gains and included them into the power constraint, the power budget difference can be considered a consequence of the difference in link gains.
Figure 9 also illustrates the power control problem described in Remark 4. Without power control at the source node, the achievable rate will eventually decrease to zero. Note that this behavior crystallizes only when the relay's power is limited.
In this paper, we have considered a relay network without a direct link, where relaying is essential for the source and the destination to communicate despite the fact that the relay node is untrusted. Imposing secrecy constraints at the relay node, contrary to the previous work, we have shown that a nonzero secrecy rate is indeed achievable. This is accomplished by enlisting the help of the destination (or another dedicated node) who transmits to jam the relay, and uses the jamming signal as side information. We have derived an upper bound for the secrecy rate with the assumption that no feedback is used for encoding at the source or destination. The new upper bound is strictly tighter than the upper bound without secrecy constraints. We have also proved that it is tighter than an upper bound derived from generalized entropy power inequality when the maximum sum received SNR at the relay is greater than dB. The gap between the bound and the achievable rates converges to when the power of the transmitter, the relay and the jammer goes to . Numerical results show that our upper bound is in general close to the achievable rate, and is indistinguishable from it for a fixed time sharing factor with a relay whose power is in abundance.
In this work, we considered the case where the source or the jammer does not make use of the relay transmission for encoding purposes. An upper bound for the secrecy rate when feedback is used is recently found in . A gap exists between the upper bound and the achievable rate in , which is bounded by bit per channel use but does not vanish when the power of the transmitter, the relay and the jammer goes to infinity. By comparison, the bound presented in this work is asymptotically tighter in this case.
We conclude by reiterating that our findings in this paper presents cooperative jamming as an enabler for secrecy from an internal eavesdropper, and motivates further investigation of such cooperation ideas in more general settings including those in larger networks. We also comment whether and when cooperative jamming actually yields the secrecy capacity (region) for various multiuser channels remain open problems in information theory.
This work was presented in part at the IEEE Globecom Conference, December 2008. This work is supported in part by the National Science Foundation with Grants CCR-0237727, CCF-051483, CNS-0716325, CNS-0721445, and the DARPA ITMANET Program with Grant W911NF-07-1-0028.
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