Abstract
Securely managing encrypted data on an untrusted party is a challenging problem that has motivated the study of a wide variety of cryptographic primitives. A special class of such primitives allows an untrusted party to transform a ciphertext encrypted under one key to a ciphertext under another key, using some auxiliary information that does not leak the underlying data. Prominent examples of such primitives in the symmetric setting are key-homomorphic (weak) PRFs, updatable encryption, and proxy re-encryption. Although these primitives differ significantly in terms of their constructions and security requirements, they share two important properties: (a) they have secrets with structure or extra functionality, and (b) all known constructions of these primitives satisfying reasonably strong definitions of security are based on concrete public-key assumptions, e.g., DDH and LWE.
This raises the question of whether these objects inherently belong to the world of public-key primitives, or they can potentially be built from simple symmetric-key objects such as pseudorandom functions. In this work, we show that the latter possibility is unlikely. More specifically, we show that:
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Any (bounded) key-homomorphic weak PRF with an abelian output group implies a (bounded) input-homomorphic weak PRF, which has recently been shown to imply not only public-key encryption but also a variety of primitives such as PIR, lossy TDFs, and even IBE.
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Any ciphertext-independent updatable encryption scheme that is forward and post-compromise secure implies PKE. Moreover, any symmetric-key proxy re-encryption scheme with reasonably strong security guarantees implies a forward and post-compromise secure ciphertext-independent updatable encryption, and hence PKE.
In addition, we show that unbounded (or exact) key-homomorphic weak PRFs over abelian groups are impossible in the quantum world. In other words, over abelian groups, bounded key-homomorphism is the best that we can hope for in terms of post-quantum security. Our attack also works over other structured primitives with abelian groups and exact homomorphisms, including homomorphic one-way functions and input-homomorphic weak PRFs.
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Notes
- 1.
We note that this equality can be relaxed to achieve approximate key-homomorphic PRFs, and these (approximate) key-homomorphic PRFs can be built from lattice-based assumptions, like LWE.
- 2.
A weak PRF is a PRF for which the pseudorandomness guarantee holds when the inputs are sampled uniformly at random.
- 3.
This was originally envisioned by the authors of [NPR99] as a PRF in the random oracle model, but we note that it is equivalent to a weak PRF in the standard model.
- 4.
We use the term “subset sum” loosely to essentially indicate subset group-operation over the output space of the KHwPRF. Depending on whether the group is additive or multiplicative, we perform either subset sums or subset products.
- 5.
We remark that known algorithms to solve systems of linear equations over abelian groups need an explicit representation of the group, see [GR02] for more details. For example, such an explicit representation is not known to an adversary against a DDH-hard group \(\mathbb {G}\).
- 6.
Notice that for the correctness of key exchange, we require the group \(\mathcal {Y}\) to be abelian.
- 7.
Notice that it is almost always the case that \(\mathcal {Y}\) is an efficiently samplable group. By Theorem 5 of [AGKP14], a set of uniform elements with size forms a generating set for \(\mathcal {Y}\) with an overwhelming probability.
- 8.
Notice that each component may live in a different cyclic group.
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Alamati, N., Montgomery, H., Patranabis, S. (2019). Symmetric Primitives with Structured Secrets. In: Boldyreva, A., Micciancio, D. (eds) Advances in Cryptology – CRYPTO 2019. CRYPTO 2019. Lecture Notes in Computer Science(), vol 11692. Springer, Cham. https://doi.org/10.1007/978-3-030-26948-7_23
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