Notions and relations for RKA-secure permutation and function families

The theory of designing block ciphers is mature, having seen signi¯cant progress since the early 1990s for over two decades, especially during the AES devel- opment e®ort. Nevertheless, interesting directions exist, in particular in the study of the provable security of block ciphers along similar veins as public-key primitives, i.e. the notion of pseudorandomness (PRP) and indistinguishability (IND). Furthermore, recent cryptanalytic progress has shown that block ciphers well designed against known cryptanalysis techniques including related-key attacks (RKA) may turn out to be less secure against related-key attacks than expected. The notion of provable security of block ciphers against related-key attacks was initiated by Bellare and Kohno, and sub- sequently treated by Lucks. Concrete block cipher constructions were proposed therein with provable security guarantees. In this paper, we are interested in the security no- tions for RKA-secure block ciphers

Related-Key Secure Pseudorandom Functions: The Case of Additive Attacks

In a related-key attack (RKA) an adversary attempts to break a cryptographic primitive by invoking the primitive with several secret keys which satisfy some known relation. The task of constructing provably RKA secure PRFs (for non-trivial relations) under a standard assumption has turned to be challenging. Currently, the only known provably-secure construction is due to Bellare and Cash (Crypto 2010). This important feasibility result is restricted, however, to linear relations over relatively complicated groups (e.g., $\Z^*_q$ where $q$ is a large prime) that arise from the algebraic structure of the underlying cryptographic assumption (DDH/DLIN). In contrast, applications typically require RKA-security with respect to simple additive relations such as XOR or addition modulo a power-of-two. In this paper, we partially fill this gap by showing that it is possible to deal with simple additive relations at the expense of relaxing the model of the attack. We introduce several natural relaxations of RKA-security, study the relations between these notions, and describe efficient constructions either under lattice assumptions or under general assumptions. Our results enrich the landscape of RKA security and suggest useful trade-offs between the attack model and the family of possible relations

Authenticated Encryption: How Reordering can Impact Performance

In this work, we look at authenticated encryption schemes from a new perspective. As opposed to focusing solely on the {\em ``security\u27\u27} implications of the different methods for constructing authenticated encryption schemes, we investigate the effect of the method used to construct an authenticated encryption scheme on the {\em ``performance\u27\u27} of the construction. We show that, as opposed to the current NIST standard, by performing the authentication operation before the encryption operation, the computational efficiency of the construction can be increased, without affecting the security of the overall construction. In fact, we show that the proposed construction is even more secure than standard authentication based on universal hashing in the sense that the hashing key is resilient to key recovery attacks

Analysis Design & Applications of Cryptographic Building Blocks

This thesis deals with the basic design and rigorous analysis of cryptographic schemes and primitives, especially of authenticated encryption schemes, hash functions, and password-hashing schemes. In the last decade, security issues such as the PS3 jailbreak demonstrate that common security notions are rather restrictive, and it seems that they do not model the real world adequately. As a result, in the first part of this work, we introduce a less restrictive security model that is closer to reality. In this model it turned out that existing (on-line) authenticated encryption schemes cannot longer beconsidered secure, i.e. they can guarantee neither data privacy nor data integrity. Therefore, we present two novel authenticated encryption scheme, namely COFFE and McOE, which are not only secure in the standard model but also reasonably secure in our generalized security model, i.e. both preserve full data inegrity. In addition, McOE preserves a resonable level of data privacy. The second part of this thesis starts with proposing the hash function Twister-Pi, a revised version of the accepted SHA-3 candidate Twister. We not only fixed all known security issues of Twister, but also increased the overall soundness of our hash-function design. Furthermore, we present some fundamental groundwork in the area of password-hashing schemes. This research was mainly inspired by the medial omnipresence of password-leakage incidences. We show that the password-hashing scheme scrypt is vulnerable against cache-timing attacks due to the existence of a password-dependent memory-access pattern. Finally, we introduce Catena the first password-hashing scheme that is both memory-consuming and resistant against cache-timing attacks

D.STVL.9 - Ongoing Research Areas in Symmetric Cryptography

This report gives a brief summary of some of the research trends in symmetric cryptography at the time of writing (2008). The following aspects of symmetric cryptography are investigated in this report: • the status of work with regards to different types of symmetric algorithms, including block ciphers, stream ciphers, hash functions and MAC algorithms (Section 1); • the algebraic attacks on symmetric primitives (Section 2); • the design criteria for symmetric ciphers (Section 3); • the provable properties of symmetric primitives (Section 4); • the major industrial needs in the area of symmetric cryptography (Section 5)