ASIACRYPT 2013: Advances in Cryptology - ASIACRYPT 2013 pp 401-420

# On Continual Leakage of Discrete Log Representations

• Shweta Agrawal
• Yevgeniy Dodis
• Vinod Vaikuntanathan
• Daniel Wichs
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 8270)

## Abstract

Let $$\mathbb{G}$$ be a group of prime order q, and let g 1,…,g n be random elements of $$\mathbb{G}$$. We say that a vector x = $$(x_1,\ldots,x_n)\in \mathbb{Z}_q^n$$ is a discrete log representation of some some element $$y\in\mathbb{G}$$ (with respect to g 1,…,g n ) if $$g_1^{x_1}\cdots g_n^{x_n} = y$$. Any element y has many discrete log representations, forming an affine subspace of $$\mathbb{Z}_q^n$$. We show that these representations have a nice continuous leakage-resilience property as follows. Assume some attacker $$\mathcal{A}(g_1,\ldots,g_n,y)$$ can repeatedly learn L bits of information on arbitrarily many random representations of y. That is, $$\mathcal{A}$$ adaptively chooses polynomially many leakage functions $$f_i:\mathbb{Z}_q^n\rightarrow \{0,1\}^L$$, and learns the value f i (x i ), where x i is a fresh and random discrete log representation of y. $$\mathcal{A}$$ wins the game if it eventually outputs a valid discrete log representation x* of y. We show that if the discrete log assumption holds in $$\mathbb{G}$$, then no polynomially bounded $$\mathcal{A}$$ can win this game with non-negligible probability, as long as the leakage on each representation is bounded by $$L\approx (n-2)\log q = (1-\frac{2}{n})\cdot$$ |x|.

As direct extensions of this property, we design very simple continuous leakage-resilient (CLR) one-way function (OWF) and public-key encryption (PKE) schemes in the so called “invisible key update” model introduced by Alwen et al. at CRYPTO’09. Our CLR-OWF is based on the standard Discrete Log assumption and our CLR-PKE is based on the standard Decisional Diffie-Hellman assumption. Prior to our work, such schemes could only be constructed in groups with a bilinear pairing.

As another surprising application, we show how to design the first leakage-resilient traitor tracing scheme, where no attacker, getting the secret keys of a small subset of decoders (called “traitors”) and bounded leakage on the secret keys of all other decoders, can create a valid decryption key which will not be traced back to at least one of the traitors.

## Keywords

Encryption Scheme Signature Scheme Security Parameter Negligible Function Semantic Security
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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