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Abstract. Despite the fact that we evidently have very good block ciphers at hand today, some fundamental questions on their security are still unsolved. One such fundamental problem is to precisely assess the security of a given block cipher with respect to linear cryptanalysis. In by far most of the cases we have to make (clearly wrong) assumptions, e.g., assume independent round-keys. Besides being unsatisfactory from a scientific perspective, the lack of fundamental understanding might have an impact on the performance of the ciphers we use. As we do not understand the security sufficiently enough, we often tend to embed a security margin -from an efficiency perspective nothing else than wasted performance. The aim of this paper is to stimulate research on these foundations of block ciphers. We do this by presenting three examples of ciphers that behave differently to what is normally assumed. Thus, on the one hand these examples serve as counter examples to common beliefs and on the other hand serve as a guideline for future work.

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Aim:The aim of the present study was to evaluate the shear bond strength of zirconia, stabilised with 5% yttria, luted to enamel and to evaluate the fracture pattern at loss of retention. Methods: A total of 53 test specimen were manufactured from two partially stabilised zirconia materials, Zirkonzahn Prettau Anterior (ZPA) (n ¼ 16) and Whitepeaks CopraSmile Symphony 5 layer (WCS) (n ¼ 18), and a lithium disilicate (Ivoclar e.Max Press) (n ¼ 19) acting as control. All test specimens were cemented to human enamel with Variolink Esthetic DC and then subjected to a shear bond strength test. Fracture and surface analysis were performed using light and scanning electron microscope. Results: No significant differences in shear bond strength were detected when analysing the three groups. Dividing them according to the fracture pattern significant difference in shear bond strength between the two zirconia groups could be seen analysing test bodies with failure of adhesion to the test body, but not to enamel. The ZPA had higher shear bond strength (23.68 MPa) than WCS (13.00 MPa). No significant differences were seen compared to the control group (19.02 MPa).Conclusion: Partially stabilised zirconia shows potential as a material to be used where macro mechanical bonding is not possible, although this study does not reveal how or if the bonding deteriorates over time. ARTICLE HISTORY

Abstract. The hardware-attractive block cipher family KTANTAN was studied by Bogdanov and Rechberger who identified flaws in the key schedule and gave a meet-in-the-middle attack. We revisit their result before investigating how to exploit the weakest key bits. We then develop several related-key attacks, e.g., one on KTANTAN32 which finds 28 key bits in time equivalent to 2 3.0 calls to the full KTANTAN32 encryption. The main result is a related-key attack requiring 2 28.44 time (half a minute on a current CPU) to recover the full 80-bit key. For KTANTAN48, we find three key bits in the time of one encryption, and give several other attacks, including full key recovery. For KTANTAN64, the attacks are only slightly more expensive, requiring 2 10.71 time to find 38 key bits, and 2 32.28 for the entire key. For all attacks, the requirements on related-key material are modest as in the forward and backward directions, we only need to flip a single key bit. All attacks succeed with probability one. Our attacks directly contradict the designers' claims. We discuss why this is, and what can be learnt from this.

PRINTcipher is a recent lightweight block cipher designed by Knudsen et al. Some noteworthy characteristics are a burnt-in key, a key-dependent permutation layer and identical round keys. Independent work on PRINTcipher has identified weak key classes that allow for a key recovery-the obvious countermeasure is to avoid these weak keys at the cost of a small loss of key entropy. This paper identifies several larger classes of weak keys. We show how to distinguish classes of keys and give a 28-round linear attack applicable to half the keys. We show that there are several similar attacks, each focusing on a specific class of keys. We also observe how some specific properties of PRINTcipher allow us to collect several samples from each plaintext-ciphertext pair. We use this property to construct an attack on 29-round PRINTcipher applicable to a fraction 2 −5 of the keys.

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