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With applications in secure messaging, Updatable Public Key Encryption (UPKE) was proposed by Jost et al. (EUROCRYPT ’19) and Alwen et al. (CRYPTO ’20). It is a natural relaxation of forward-secure public-key encryption. In UPKE, we can update secret keys by using update ciphertexts which any sender can generate. The UPKE schemes proposed so far that satisfy the strong CCA security are Haidar et al.’s concrete construction (CCS ’22) and Dodis et al.’s generic construction that use Non-Interactive Zero-Knowledge (NIZK) arguments. Yet, even despite the aid of random oracles, their concrete efficiency, except for the recent independent work (EUROCRYPT ’24), is quite far from the most efficient CPA-secure scheme. In this paper, we first demonstrate a simple and efficient attack against Dodis et al.’s strongly CCA-secure scheme, and show how to fix it. Then, based on the observation from the attack and fix, we propose a new strongly CCA-secure generic construction for a UPKE scheme with random oracles and show that its instantiation is almost as concretely efficient as the most efficient CPA-secure one. Our construction does not rely on either NIZKs, the AGM, or GGM.

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Updatable Public Key Encryption with Strong CCA Security: Security Analysis and Efficient Generic Construction

  • 会议论文

  • 发表日期：

    2025年

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  • Kyoichi Asano,
  • Yohei Watanabe

摘要

With applications in secure messaging, Updatable Public Key Encryption (UPKE) was proposed by Jost et al. (EUROCRYPT ’19) and Alwen et al. (CRYPTO ’20). It is a natural relaxation of forward-secure public-key encryption. In UPKE, we can update secret keys by using update ciphertexts which any sender can generate. The UPKE schemes proposed so far that satisfy the strong CCA security are Haidar et al.’s concrete construction (CCS ’22) and Dodis et al.’s generic construction that use Non-Interactive Zero-Knowledge (NIZK) arguments. Yet, even despite the aid of random oracles, their concrete efficiency, except for the recent independent work (EUROCRYPT ’24), is quite far from the most efficient CPA-secure scheme. In this paper, we first demonstrate a simple and efficient attack against Dodis et al.’s strongly CCA-secure scheme, and show how to fix it. Then, based on the observation from the attack and fix, we propose a new strongly CCA-secure generic construction for a UPKE scheme with random oracles and show that its instantiation is almost as concretely efficient as the most efficient CPA-secure one. Our construction does not rely on either NIZKs, the AGM, or GGM.