Revisiting Single-Server Algorithms for Outsourcing Modular Exponentiation

  • Jothi Rangasamy
  • Lakshmi Kuppusamy
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 11356)


We investigate the problem of securely outsourcing modular exponentiations to a single, malicious computational resource. We revisit recently proposed schemes using single server and analyse them against two fundamental security properties, namely privacy of inputs and verifiability of outputs. Interestingly, we observe that the chosen schemes do not appear to meet both the security properties. In fact we present a simple polynomial-time attack on each algorithm, allowing the malicious server either to recover a secret input or to convincingly fool the client with wrong outputs. Then we provide a fix to the identified problem in the ExpSOS scheme. With our fix and without pre-processing, the improved scheme becomes the best to-date outsourcing scheme for single-server case. Finally we present the first precomputation-free single-server algorithm, \(\pi \)ExpSOS for simultaneous exponentiations, thereby solving an important problem formulated in [6].


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© Springer Nature Switzerland AG 2018

Authors and Affiliations

  1. 1.Society for Electronic Transactions and Security (SETS)ChennaiIndia

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