# Dziembowski, Pietrzak: “Leakage-Resilient Cryptography”

## 10.07.2019, 10:00 – 11:00

10.07.2019 10:00-11:00

Speaker: Sogol Mazaheri, Cryptoplexity Group, TU Darmstadt | Location: Mornewegstraße 32 (S4|14), Room 5.3.01, Darmstadt

Organizer: Christian Janson

**Abstract**

This talk is the fourth one in the seminar series „Reading the Crypto Classics“ for the summer term 2019. The idea of this seminar is to jointly read classical milestone papers in the area of cryptography, to discuss their impact and understand their relevance for current research areas. The seminar is running as an Oberseminar, but at the same time meant to be a joint reading group seminar of the CROSSING Special Interest Group on Advanced Cryptography with all interested CROSSING members being invited to participate.

This issue will cover the paper

Dziembowski, Pietrzak: “Leakage-Resilient Cryptography” (FOCS 2008), DOI: 10.1109/FOCS.2008.56

with the following abstract:

„We construct a stream-cipher S whose implementation is secure even if a bounded amount of arbitrary (adversarially chosen) information on the internal state of S is leaked during computation. This captures all possible side-channel attacks on S where the amount of information leaked in a given period is bounded, but overall can be arbitrary large. The only other assumption we make on the implementation of S is that only data that is accessed during computation leaks information. The stream-cipher S generates its output in chunks K1, K2, . . . and arbitrary but bounded information leakage is modeled by allowing the adversary to adaptively chose a function fl : {0,1}^* --> {0,1}^lambda before Kl is computed, she then gets fl(tau_l) where tau_l is the internal state of S that is accessed during the computation of Kg. One notion of security we prove for S is that Kg is indistinguishable from random when given K1,…, K1-1,f1(tau_1),…, fl-1(tau_l-1) and also the complete internal state of S after Kg has been computed (i.e. S is forward-secure). The construction is based on alternating extraction (used in the intrusion-resilient secret-sharing scheme from FOCS'07). We move this concept to the computational setting by proving a lemma that states that the output of any PRG has high HILL pseudoentropy (i.e. is indistinguishable from some distribution with high min-entropy) even if arbitrary information about the seed is leaked. The amount of leakage lambda that we can tolerate in each step depends on the strength of the underlying PRG, it is at least logarithmic, but can be as large as a constant fraction of the internal state of S if the PRG is exponentially hard.“**Further information**