Abstract. The stream cipher Salsa20 was introduced by Bernstein in 2005 as a candidate in the eSTREAM project, accompanied by the reduced versions Salsa20/8 and Salsa20/12. ChaCha is a variant of Salsa20 aiming at bringing better diffusion for similar performance. Variants of Salsa20 with up to 7 rounds (instead of 20) have been broken by differential cryptanalysis, while ChaCha has not been analyzed yet. We introduce a novel method for differential cryptanalysis of Salsa20 and ChaCha, inspired by correlation attacks and related to the notion of neutral bits. This is the first application of neutral bits in stream cipher cryptanalysis. It allows us to break the 256-bit version of Salsa20/8, to bring faster attacks on the 7-round variant, and to break 6-and 7-round ChaCha. In a second part, we analyze the compression function Rumba, built as the XOR of four Salsa20 instances and returning a 512-bit output. We find collision and preimage attacks for two simplified variants, then we discuss differential attacks on the original version, and exploit a highprobability differential to reduce complexity of collision search from 2 256 to 2 79 for 3-round Rumba. To prove the correctness of our approach we provide examples of collisions and near-collisions on simplified versions.
Abstract. A recent framework for chosen IV statistical distinguishing analysis of stream ciphers is exploited and formalized to provide new methods for key recovery attacks. As an application, a key recovery attack on simplified versions of two eSTREAM Phase 3 candidates is given: For Grain-128 with IV initialization reduced to up to 180 of its 256 iterations, and for Trivium with IV initialization reduced to up to 672 of its 1152 iterations, it is experimentally demonstrated how to deduce a few key bits. Evidence is given that the present analysis is not applicable on Grain-128 or Trivium with full IV initialization.
We study mix-nets with randomized partial checking (RPC) as proposed by Jakobsson, Juels, and Rivest (2002). RPC is a technique to verify the correctness of an execution both for Chaumian and homomorphic mix-nets. The idea is to relax the correctness and privacy requirements to achieve a more efficient mix-net.We identify serious issues in the original description of mix-nets with RPC and show how to exploit these to break both correctness and privacy, both for Chaumian and homomorphic mix-nets. Our attacks are practical and applicable to real world mix-net implementations, e.g., the Civitas and the Scantegrity voting systems.
Abstract. We construct a provably secure mix-net from any CCA2 secure cryptosystem. The mix-net is secure against active adversaries that statically corrupt less than λ out of k mix-servers, where λ is a threshold parameter, and it is robust provided that at most min(λ − 1, k − λ) mix-servers are corrupted.The main component of our construction is a mix-net that outputs the correct result if all mix-servers behaved honestly, and aborts with probability 1 − O(H −(t−1) ) otherwise (without disclosing anything about the inputs), where t is an auxiliary security parameter and H is the number of honest parties. The running time of this protocol for long messages is roughly 3tc, where c is the running time of Chaum's mix-net (1981).
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