Abstract
Functional encryption (FE for short) can be used to calculate a function output of a message, without revealing other information about the message. There are mainly two types of security definitions for FE, exactly simulation-based security (SIM-security) and indistinguishability-based security (IND-security). Both of them have some limitations: FE with SIM-security supporting all circuits cannot be constructed for unbounded number of ciphertext and/or key queries, while IND-security is sometimes not enough: there are examples where an FE scheme is IND-secure but not intuitively secure. In this paper, we present a new security definition which can avoid the drawbacks of both SIM-security and IND-security, called indistinguishability-based security against probabilistic queries (pIND-security for short), and we give an FE construction for all circuits which is secure for unbounded key/ciphertext queries under this new security definition. We prove that this new security definition is strictly between SIM-security and IND-security, and provide new applications for FE which were not known to be constructed from IND-secure or SIM-secure FE.
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Notes
- 1.
We also note that a trapdoor may not only be hidden in the function, but also in the messages. We slightly modify the function in Example 1.2, and let f be defined as: \(f(pk,m,r)=\mathsf {PKE.Enc}(pk,m;r)\), and \(M_b=\{pk\}\times \{m_b\}\times \mathcal {R}\), so \(f(M_0)\) and \(f(M_1)\) are distributions with trapdoor, and the trapdoor is hidden in the message distribution, not the function.
- 2.
Here we allows different distributions \(\bar{M'}_b^j\) to include the same randomness \(\bar{r}^*\), which means that there is a shared inner state between these sampling algorithms. We note that SIM-secure FE implies pIND-secure FE even considering stateful ciphertext queries like this, so it will not affect the validity of the proof.
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Acknowledgement
This work is partially supported by the National Key R &D Program of China (No. 2020YFA0712300) and the National Natural Science Foundation of China (No. 62202294, No. 62272294).
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Wang, G., Sun, SF., Wang, Z., Gu, D. (2023). Functional Encryption Against Probabilistic Queries: Definition, Construction and Applications. In: Boldyreva, A., Kolesnikov, V. (eds) Public-Key Cryptography – PKC 2023. PKC 2023. Lecture Notes in Computer Science, vol 13941. Springer, Cham. https://doi.org/10.1007/978-3-031-31371-4_15
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