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.

Abstract. KeeLoq is a lightweight block cipher with a 32-bit block size and a 64-bit key. Despite its short key size, it is widely used in remote keyless entry systems and other wireless authentication applications. For example, authentication protocols based on KeeLoq are supposedly used by various car manufacturers in anti-theft mechanisms. This paper presents a practical key recovery attack against KeeLoq that requires 2 16 known plaintexts and has a time complexity of 2 44.5 KeeLoq encryptions. It is based on the slide attack and a novel approach to meet-in-the-middle attacks. The fully implemented attack requires 65 minutes to obtain the required data and 7.8 days of calculations on 64 CPU cores. A variant which requires 2 16 chosen plaintexts needs only 3.4 days on 64 CPU cores. Using only 10 000 euro, an attacker can purchase a cluster of 50 dual core computers that will find the secret key in about two days. We investigated the way KeeLoq is intended to be used in practice and conclude that our attack can be used to subvert the security of real systems. An attacker can acquire chosen plaintexts in practice, and one of the two suggested key derivation schemes for KeeLoq allows to recover the master secret from a single key.

Abstract.We study the security of step-reduced but otherwise unmodified SHA-256. We show the first collision attacks on SHA-256 reduced to 23 and 24 steps with complexities 2 18 and 2 28.5 , respectively. We give example colliding message pairs for 23-step and 24-step SHA-256. The best previous, recently obtained result was a collision attack for up to 22 steps. We extend our attacks to 23 and 24-step reduced SHA-512 with respective complexities of 2 44.9 and 2 53.0 . Additionally, we show nonrandom behaviour of the SHA-256 compression function in the form of free-start near-collisions for up to 31 steps, which is 6 more steps than the recently obtained non-random behaviour in the form of a semi-freestart near-collision. Even though this represents a step forwards in terms of cryptanalytic techniques, the results do not threaten the security of applications using SHA-256.

Serpent is an SP Network block cipher submitted to the AES competition and chosen as one of its five finalists. The security of Serpent is widely acknowledged, especially as the best known attack so far is a differential-linear attack on only 11 rounds out of the 32 rounds of the cipher. In this paper we introduce a more accurate analysis of the differentiallinear attack on 11-round Serpent. The analysis involves both theoretical aspects as well as experimental results which suggest that previous attacks had overestimated complexities. Following our findings we are able to suggest an improved 11-round attack with a lower data complexity. Using the new results, we are able to devise the first known attack on 12-round Serpent.