[Resource Topic] 2023/217: Indifferentiability of the Sponge Construction with a Restricted Number of Message Blocks

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Title:
Indifferentiability of the Sponge Construction with a Restricted Number of Message Blocks

Authors: Charlotte Lefevre

Abstract:

The sponge construction is a popular method for hashing. Quickly after its introduction, the sponge was proven to be tightly indifferentiable from a random oracle up to \approx 2^{c/2} queries, where c is the capacity. However, this bound is not tight when the number of message blocks absorbed is restricted to \ell <\lceil \frac{c}{2(b-c)}\rceil + 1 (but still an arbitrary number of blocks can be squeezed). In this work, we show that this restriction leads to indifferentiability from a random oracle up to \approx \min \left\{2^{b/2}, \max\left\{2^{c/2}, 2^{b- \ell \times (b-c)} \right\}\right\} queries, where b>c is the permutation size. Depending on the parameters chosen, this result allows to have enhanced security or to absorb at a larger rate for applications that require a fixed-length input hash function.

ePrint: https://eprint.iacr.org/2023/217

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