Relations among notions of complete non-malleability: indistinguishability characterisation and efficient construction without random oracles

  • Authors:
  • Manuel Barbosa;Pooya Farshim

  • Affiliations:
  • CCTC, Departamento de Informática, Universidade do Minho, Braga, Portugal;Royal Holloway, University of London, Egham, Surrey, United Kingdom

  • Venue:
  • ACISP'10 Proceedings of the 15th Australasian conference on Information security and privacy
  • Year:
  • 2010

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Abstract

We study relations among various notions of complete nonmalleability, where an adversary can tamper with both ciphertexts and public-keys, and ciphertext indistinguishability. We follow the pattern of relations previously established for standard non-malleability. To this end, we propose a more convenient and conceptually simpler indistinguishability-based security model to analyse completely non-malleable schemes. Our model is based on strong decryption oracles, which provide decryptions under arbitrarily chosen public keys.We give the first precise definition of a strong decryption oracle, pointing out the subtleties in different approaches that can be taken. We construct the first efficient scheme, which is fully secure against strong chosen-ciphertext attacks, and therefore completely non-malleable, without random oracles.