## CryptoDB

### Sruthi Sekar

#### Publications

**Year**

**Venue**

**Title**

2021

CRYPTO

Adaptive Extractors and their Application to Leakage Resilient Secret Sharing
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Abstract

We introduce Adaptive Extractors, which unlike traditional randomness extractors, guarantee security even when an adversary obtains leakage on the source \textit{after} observing the extractor output. We make a compelling case for the study of such extractors by demonstrating their use in obtaining adaptive leakage in secret sharing schemes.
Specifically, at FOCS 2020, Chattopadhyay, Goodman, Goyal, Kumar, Li, Meka, Zuckerman, built an adaptively secure leakage resilient secret sharing scheme (LRSS) with both rate and leakage rate being $\mathcal{O}(1/n)$, where $n$ is the number of parties. In this work, we build an adaptively secure LRSS that offers an interesting trade-off between rate, leakage rate, and the total number of shares from which an adversary can obtain leakage. As a special case, when considering $t$-out-of-$n$ secret sharing schemes for threshold $t = \alpha n$ (constant $0<\alpha<1$), we build a scheme with constant rate, constant leakage rate, and allow the adversary leakage from all but $t-1$ of the shares, while giving her the remaining $t-1$ shares completely in the clear. (Prior to this, constant rate LRSS scheme tolerating adaptive leakage was unknown for \textit{any} threshold.)
Finally, we show applications of our techniques to both non-malleable secret sharing and secure message transmission.

2019

PKC

Registration-Based Encryption from Standard Assumptions
Abstract

The notion of Registration-Based Encryption (RBE) was recently introduced by Garg, Hajiabadi, Mahmoody, and Rahimi [TCC’18] with the goal of removing the private-key generator (PKG) from IBE. Specifically, RBE allows encrypting to identities using a (compact) master public key, like how IBE is used, with the benefit that the PKG is substituted with a weaker entity called “key curator” who has no knowledge of any secret keys. Here individuals generate their secret keys on their own and then publicly register their identities and their corresponding public keys to the key curator. Finally, individuals obtain “rare” decryption-key updates from the key curator as the population grows. In their work, they gave a construction of RBE schemes based on the combination of indistinguishability obfuscation and somewhere statistically binding hash functions. However, they left open the problem of constructing RBE schemes based on standard assumptions.In this work, we resolve the above problem and construct RBE schemes based on standard assumptions (e.g., CDH or LWE). Furthermore, we show a new application of RBE in a novel context. In particular, we show that anonymous variants of RBE (which we also construct under standard assumptions) can be used for realizing abstracts forms of anonymous messaging tasks in simple scenarios in which the parties communicate by writing messages on a shared board in a synchronized way.

2019

JOFC

Four-State Non-malleable Codes with Explicit Constant Rate
Abstract

Non-malleable codes (NMCs), introduced by Dziembowski, Pietrzak and Wichs (ITCS 2010), provide a powerful guarantee in scenarios where the classical notion of error-correcting codes cannot provide any guarantee: a decoded message is either the same or completely independent of the underlying message, regardless of the number of errors introduced into the codeword. Informally, NMCs are defined with respect to a family of tampering functions $$\mathcal {F}$$ F and guarantee that any tampered codeword decodes either to the same message or to an independent message, so long as it is tampered using a function $$f \in \mathcal {F}$$ f ∈ F . One of the well-studied tampering families for NMCs is the t -split-state family, where the adversary tampers each of the t “states” of a codeword, arbitrarily but independently. Cheraghchi and Guruswami (TCC 2014) obtain a rate-1 non-malleable code for the case where $$t = \mathcal {O}(n)$$ t = O ( n ) with n being the codeword length and, in (ITCS 2014), show an upper bound of $$1-1/t$$ 1 - 1 / t on the best achievable rate for any t -split state NMC. For $$t=10$$ t = 10 , Chattopadhyay and Zuckerman (FOCS 2014) achieve a constant-rate construction where the constant is unknown. In summary, there is no known construction of an NMC with an explicit constant rate for any $$t= o(n)$$ t = o ( n ) , let alone one that comes close to matching Cheraghchi and Guruswami’s lowerbound! In this work, we construct an efficient non-malleable code in the t -split-state model, for $$t=4$$ t = 4 , that achieves a constant rate of $$\frac{1}{3+\zeta }$$ 1 3 + ζ , for any constant $$\zeta > 0$$ ζ > 0 , and error $$2^{-\varOmega (\ell / log^{c+1} \ell )}$$ 2 - Ω ( ℓ / l o g c + 1 ℓ ) , where $$\ell $$ ℓ is the length of the message and $$c > 0$$ c > 0 is a constant.

#### Coauthors

- Nishanth Chandran (1)
- Sanjam Garg (1)
- Mohammad Hajiabadi (1)
- Bhavana Kanukurthi (4)
- Mohammad Mahmoody (1)
- Sai Lakshmi Bhavana Obbattu (4)
- Ahmadreza Rahimi (1)