In this paper, we analyze the collision resistance of SHA-2 and provide the first results since the beginning of the NIST SHA-3 competition. We extend the previously best known semi-free-start collisions on SHA-256 from 24 to 32 (out of 64) steps and show a collision attack for 27 steps. All our attacks are practical and verified by colliding message pairs. We present the first automated tool for finding complex differential characteristics in SHA-2 and show that the techniques on SHA-1 cannot directly be applied to SHA-2. Due to the more complex structure of SHA-2 several new problems arise. Most importantly, a large amount of contradicting conditions occur which render most differential characteristics impossible. We show how to overcome these difficulties by including the search for conforming message pairs in the search for differential characteristics.

Abstract. The design of lightweight block ciphers has been a very active research topic over the last years. However, the lack of comparative source codes generally makes it hard to evaluate the extent to which implementations of different ciphers actually reach their low-cost goals on various platforms. This paper reports on an initiative aiming to relax this issue. First, we provide implementations of 12 block ciphers on an ATMEL AVR ATtiny45 8-bit microcontroller, and make the corresponding source code available on a web page. All implementations are made public under an open-source license. Common interfaces and design goals are followed by all designers to achieve comparable implementation results. Second, we evaluate performance figures of our implementations with respect to different metrics, including energy-consumption measurements and show our improvements compared to existing implementations.

In this paper, we focus on the construction of semi-free-start collisions for SHA-256, and show how to turn them into collisions. We present a collision attack on 28 steps of the hash function with practical complexity. Using a two-block approach we are able to turn a semifree-start collision into a collision for 31 steps with a complexity of at most 2 65.5. The main improvement of our work is to extend the size of the local collisions used in these attacks. To construct differential characteristics and confirming message pairs for longer local collisions, we had to improve the search strategy of our automated search tool. To test the limits of our techniques we present a semi-free-start collision for 38 steps.

In this paper, we analyze the security of RIPEMD-128 against collision attacks. The ISO/IEC standard RIPEMD-128 was proposed 15 years ago and may be used as a drop-in replacement for 128bit hash functions like MD5. Only few results have been published for RIPEMD-128, the best being a preimage attack for the first 33 steps of the hash function with complexity 2 124.5. In this work, we provide a new assessment of the security margin of RIPEMD-128 by showing attacks on up to 48 (out of 64) steps of the hash function. We present a collision attack reduced to 38 steps and a near-collisions attack for 44 steps, both with practical complexity. Furthermore, we show non-random properties for 48 steps of the RIPEMD-128 hash function, and provide an example for a collision on the compression function for 48 steps. For all attacks we use complex nonlinear differential characteristics. Due to the more complicated dual-stream structure of RIPEMD-128 compared to its predecessor, finding high-probability characteristics as well as conforming message pairs is nontrivial. Doing any of these steps by hand is almost impossible or at least, very time consuming. We present a general strategy to analyze dual-stream hash functions and use an automatic search tool for the two main steps of the attack. Our tool is able to find differential characteristics and perform advanced message modification simultaneously in the two streams.

In this work, we provide the first security analysis of reduced RIPEMD-160 regarding its collision resistance with practical complexity. The ISO/IEC standard RIPEMD-160 was proposed 15 years ago and may be used as a drop-in replacement for SHA-1 due to their same hash output length. Only few results have been published for RIPEMD-160 so far and most attacks have a complexity very close to the generic bound. In this paper, we present the first application of the attacks of Wang et al. on MD5 and SHA-1 to RIPEMD-160. Due to the dual-stream structure of RIPEMD-160 the application of these attacks is nontrivial and almost impossible without the use of automated tools. We present practical examples of semi-free-start near-collisions for the middle 48 steps (out of 80) and semi-free-start collisions for 36 steps of RIPEMD-160. Furthermore, our results show that the differential characteristics get very dense in RIPEMD-160 such that a full-round attack seems unlikely in the near future.