PUFs in Security Protocols: Attack Models and Security Evaluations

In recent years, PUF-based schemes have not only been suggested for the basic security tasks of tamper sensitive key storage or system identification, but also for more complex cryptographic protocols like oblivious transfer (OT), bit commitment (BC), or key exchange (KE). In these works, so-called "Strong PUFs" are regarded as a new, fundamental cryptographic primitive of their own, comparable to the bounded storage model, quantum cryptography, or noisebased cryptography. This paper continues this line of research, investigating the correct adversarial attack model and the actual security of such protocols. In its first part, we define and compare different attack models. They reach from a clean, first setting termed the "stand-alone, good PUF model" to stronger scenarios like the "bad PUF model" and the "PUF re-use model". We argue why these attack models are realistic, and that existing protocols would be faced with them if used in practice. In the second part, we execute exemplary security analyses of existing schemes in the new attack models. The evaluated protocols include recent schemes from Brzuska et al. published at Crypto 2011 [1] and from Ostrovsky et al. [18]. While a number of protocols are certainly secure in their own, original attack models, the security of none of the considered protocols for OT, BC, or KE is maintained in all of the new, realistic scenarios. One consequence of our work is that the design of advanced cryptographic PUF protocols needs to be strongly reconsidered. Furthermore, it suggests that Strong PUFs require additional hardware properties in order to be broadly usable in such protocols: Firstly, they should ideally be "erasable", meaning that single PUF-responses can be erased without affecting other responses. If the area efficient implementation of this feature turns out to be difficult, new forms of Controlled PUFs [8] (such as Logically Erasable and Logically Reconfigurable PUFs [13]) may suffice in certain applications. Secondly, PUFs should be "certifiable", meaning that one can verify that the PUF has been produced faithfully and has not been manipulated in any way afterwards. The combined implementation of these features represents a pressing and challenging problem, which we pose to the PUF hardware community in this work.