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Anonymity is among the important properties of two-factor authentication schemes for wireless sensor networks (WSNs) to preserve user privacy. Though impressive efforts have been devoted to designing schemes with user anonymity by only using lightweight symmetric-key primitives such as hash functions and block ciphers, to the best of our knowledge none has succeeded so far. In this work, we take an initial step to shed light on the rationale underlying this prominent issue. Firstly, we scrutinize two previously-thought sound schemes, namely Fan et al.’s scheme and Xue et al.’s scheme, and demonstrate the major challenges in designing a scheme with user anonymity.

Secondly, using these two foremost schemes as case studies and on the basis of the work of Halevi–Krawczyk (1999) [44] and Impagliazzo–Rudich (1989) [43], we put forward a general principle: Public-key techniques are intrinsically …