paper
title
New Features of Latin Dances: Analysis of Salsa, ChaCha, and Rumba.
authors
Jean-Philippe Aumasson, Simon Fischer, Shahram Khazaei, Willi Meier, and Christian Rechberger.
publication
In K. Nyberg, editor, Fast Software Encryption - FSE 2008, 15th International Workshop, Lausanne, Switzerland, February 10-13, 2008. Proceedings, volume 5086 of Lecture Notes in Computer Science, pages 470-488. Springer-Verlag, 2008.
awards
Best analysis of Rumba, the award comes with $1000 prize money.
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. In this paper, 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, and it allows us to present the first break 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, constructed 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 high-probability differential to reduce complexity of collision search from 2256 to 279 for 3-round Rumba. We give examples of collisions over three rounds for a version without feedforward, and near-collisions of weight 16 for three rounds of the original compression function, and of weight 129 for four rounds.
external links
[FSE 2008], [ePrint], [eSTREAM], [Salsa20 page], [ChaCha page], [Rumba page]