A Authentication Protocol As described previously the strong PUF receives a

A authentication protocol as described previously the

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A. Authentication Protocol As described previously, the strong PUF receives a challenge and generates a response. However, the require- ments of a strong PUF state that an adversary provided with polynomial CRPs should not be able to predict the response to a new challenge. Although this is a desirable property, it also presents a usage problem. Since the PUF acts as a ‘‘black box,’’ even the authentication server only has access to previously ob- served CRPs and, therefore, also cannot predict the re- sponse to a new challenge. Therefore, the protocol for using PUFs is significantly different than most public/private key cryptographic sys- tems. Consider a server authenticating a client. 1) PUF is manufactured. 2) Server obtains access to PUF and generates a table of CRPs. These pairs are stored in internal secret storage. 3) PUF is given to the client. 4) The client submits a request to the server to authenticate. 5) Server picks a known CRP and submits the chal- lenge to the client. 6) The client runs the challenge on the PUF, returns the response to the server. 7) Server checks to see that the response is correct and marks the CRP as used. Because the server cannot predict the PUF behavior, it must internally store CRPs to be used later. Each CRP must be used only once. Therefore, the server must either store enough CRPs so that it will not run out, or it must pe- riodically ‘‘recharge’’ the table by establishing secure com- munication with an authenticated client and requesting responses to new challenges. To address the CRP table scalability problem, newer protocols based on storage of a compact model for PUF have emerged. A brief discussion of these protocols is included in Section VII-A. Note that each client PUF will have unique CRPs, and therefore can be individually authenticated. In addition, the server must store tables of CRPs for each of the clients to be authenticated. B. Arbiter PUF Topologies The initial implementation of silicon PUFs had known security issues due to the fact that the delays were linearly added to produce the resultant response bit [8]. As a result, they could be learned with relative ease. This issue naturally led to the introduction of other ‘‘nonlinear’’ effects to make such modeling attacks more difficult. These efforts included xor arbiter PUFs, lightweight secure PUFs, and feedforward arbiter PUFs [8], [16], [17], [22], [39]. In a xor arbiter, multiple arbiter PUF outputs are xor ’ed to form a single response bit. This is shown in Fig. 2. These structures have shown greater resilience against machine learning attacks [24], [35]. Recent studies have demonstrated the vulnerability of the xor arbiters to a combination of machine learning and side- channel attacks [19], [36]. Developing methods to suppress the side channels could help in alleviating this vulnerability.
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