LIZARD – A Lightweight Stream Cipher for Power-constrained Devices

Time-memory-data (TMD) tradeoff attacks limit the security level of many classical stream ciphers (like E0, A5/1, Trivium, Grain) to 1/2n, where n denotes the inner state length of the underlying keystream generator. In this paper, we present Lizard, a lightweight stream cipher for power-constrained devices like passive RFID tags. Its hardware efficiency results from combining a Grain-like design with the FP(1)-mode, a recently suggested construction principle for the state initialization of stream ciphers, which offers provable 2/3n-security against TMD tradeoff attacks aiming at key recovery. Lizard uses 120-bit keys, 64-bit IVs and has an inner state length of 121 bit. It is supposed to provide 80-bit security against key recovery attacks. Lizard allows to generate up to 218 keystream bits per key/IV pair, which would be sufficient for many existing communication scenarios like Bluetooth, WLAN or HTTPS

Provable security for lightweight message authentication and encryption

The birthday bound often limits the security of a cryptographic scheme to half of the block size or internal state size. This implies that cryptographic schemes require a block size or internal state size that is twice the security level, resulting in larger and more resource-intensive designs. In this thesis, we introduce abstract constructions for message authentication codes and stream ciphers that we demonstrate to be secure beyond the birthday bound. Our message authentication codes were inspired by previous work, specifically the message authentication code EWCDM by Cogliati and Seurin, as well as the work by Mennink and Neves, which demonstrates easy proofs of security for the sum of permutations and an improved bound for EWCDM. We enhance the sum of permutations by incorporating a hash value and a nonce in our stateful design, and in our stateless design, we utilize two hash values. One advantage over EWCDM is that the permutation calls, or block cipher calls, can be parallelized, whereas in EWCDM they must be performed sequentially. We demonstrate that our constructions provide a security level of 2n/3 bits in the nonce-respecting setting. Subsequently, this bound was further improved to 3n/4 bits of security. Additionally, it was later discovered that security degrades gracefully with nonce repetitions, unlike EWCDM, where the security drops to the birthday bound with a single nonce repetition. Contemporary stream cipher designs aim to minimize the hardware module's resource requirements by incorporating an externally available resource, all while maintaining a high level of security. The security level is typically measured in relation to the size of the volatile internal state, i.e., the state cells within the cipher's hardware module. Several designs have been proposed that continuously access the externally available non-volatile secret key during keystream generation. However, there exists a generic distinguishing attack with birthday bound complexity. We propose schemes that continuously access the externally available non-volatile initial value. For all constructions, conventional or contemporary, we provide proofs of security against generic attacks in the random oracle model. Notably, stream ciphers that use the non-volatile initial value during keystream generation offer security beyond the birthday bound. Based on these findings, we propose a new stream cipher design called DRACO

Lightweight cryptography on ultra-constrained RFID devices

Devices of extremely small computational power like RFID tags are used in practice to a rapidly growing extent, a trend commonly referred to as ubiquitous computing. Despite their severely constrained resources, the security burden which these devices have to carry is often enormous, as their fields of application range from everyday access control to human-implantable chips providing sensitive medical information about a person. Unfortunately, established cryptographic primitives such as AES are way to 'heavy' (e.g., in terms of circuit size or power consumption) to be used in corresponding RFID systems, calling for new solutions and thus initiating the research area of lightweight cryptography. In this thesis, we focus on the currently most restricted form of such devices and will refer to them as ultra-constrained RFIDs. To fill this notion with life and in order to create a profound basis for our subsequent cryptographic development, we start this work by providing a comprehensive summary of conditions that should be met by lightweight cryptographic schemes targeting ultra-constrained RFID devices. Building on these insights, we then turn towards the two main topics of this thesis: lightweight authentication and lightweight stream ciphers. To this end, we first provide a general introduction to the broad field of authentication and study existing (allegedly) lightweight approaches. Drawing on this, with the (n,k,L)^-protocol, we suggest our own lightweight authentication scheme and, on the basis of corresponding hardware implementations for FPGAs and ASICs, demonstrate its suitability for ultra-constrained RFIDs. Subsequently, we leave the path of searching for dedicated authentication protocols and turn towards stream cipher design, where we first revisit some prominent classical examples and, in particular, analyze their state initialization algorithms. Following this, we investigate the rather young area of small-state stream ciphers, which try to overcome the limit imposed by time-memory-data tradeoff (TMD-TO) attacks on the security of classical stream ciphers. Here, we present some new attacks, but also corresponding design ideas how to counter these. Paving the way for our own small-state stream cipher, we then propose and analyze the LIZARD-construction, which combines the explicit use of packet mode with a new type of state initialization algorithm. For corresponding keystream generator-based designs of inner state length n, we prove a tight (2n/3)-bound on the security against TMD-TO key recovery attacks. Building on these theoretical results, we finally present LIZARD, our new lightweight stream cipher for ultra-constrained RFIDs. Its hardware efficiency and security result from combining a Grain-like design with the LIZARD-construction. Most notably, besides lower area requirements, the estimated power consumption of LIZARD is also about 16 percent below that of Grain v1, making it particularly suitable for passive RFID tags, which obtain their energy exclusively through an electromagnetic field radiated by the reading device. The thesis is concluded by an extensive 'Future Research Directions' chapter, introducing various new ideas and thus showing that the search for lightweight cryptographic solutions is far from being completed

Stream ciphers: A Practical Solution for Efficient Homomorphic-Ciphertext Compression

International audienceIn typical applications of homomorphic encryption, the first step consists for Alice to encrypt some plaintext m under Bob’s public key pk and to send the ciphertext c = HEpk(m) to some third-party evaluator Charlie. This paper specifically considers that first step, i.e. the problem of transmitting c as efficiently as possible from Alice to Charlie. As previously noted, a form of compression is achieved using hybrid encryption. Given a symmetric encryption scheme E, Alice picks a random key k and sends a much smaller ciphertext c′ = (HEpk(k), Ek(m)) that Charlie decompresses homomorphically into the original c using a decryption circuit CE−1 .In this paper, we revisit that paradigm in light of its concrete implemen- tation constraints; in particular E is chosen to be an additive IV-based stream cipher. We investigate the performances offered in this context by Trivium, which belongs to the eSTREAM portfolio, and we also pro- pose a variant with 128-bit security: Kreyvium. We show that Trivium, whose security has been firmly established for over a decade, and the new variant Kreyvium have an excellent performance

On designing secure small-state stream ciphers against time-memory-data tradeoff attacks

A new generation of stream ciphers, small-state stream ciphers (SSCs), was born in 2015 with the introduction of the Sprout cipher. The new generation is based on using key bits not only in the initialization but also continuously in the keystream generation phase. The new idea allowed designing stream ciphers with significantly smaller area size and low power consumption. A distinguishing time-memory-data tradeoff (TMDTO) attack was successfully applied against all SSCs in 2017 by Hamann et al. [1]. They suggested using not only key bits but also initial value (IV) bits continuously in the keystream generation phase to strengthen SSCs against TMDTO attacks. Then, Hamann and Krause [2] proposed a construction based on using only IV bits continuously in packet mode. They suggested an instantiation of an SSC and claimed that it is resistant to TMDTO attacks. We point out that storing IV bits imposes an overhead on cryptosystems that is not acceptable in many applications. More importantly, we show that the proposed SSC remains vulnerable to TMDTO attacks. To resolve security threat, the current paper proposes constructions, based on storing key or IV bits, that are the first to provide full security against TMDTO attacks. It is possible to obtain parameters for secure SSCs based on these suggested constructions. Our constructions are a fruitful research direction in stream ciphers

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)

Security of Ubiquitous Computing Systems

The chapters in this open access book arise out of the EU Cost Action project Cryptacus, the objective of which was to improve and adapt existent cryptanalysis methodologies and tools to the ubiquitous computing framework. The cryptanalysis implemented lies along four axes: cryptographic models, cryptanalysis of building blocks, hardware and software security engineering, and security assessment of real-world systems. The authors are top-class researchers in security and cryptography, and the contributions are of value to researchers and practitioners in these domains. This book is open access under a CC BY license

Multi-key Analysis of Tweakable Even-Mansour with Applications to Minalpher and OPP

The tweakable Even-Mansour construction generalizes the conventional Even-Mansour scheme through replacing round keys by strings derived from a master key and a tweak. Besides providing plenty of inherent variability, such a design builds a tweakable block cipher from some lower level primitive. In the present paper, we evaluate the multi-key security of TEM-1, one of the most commonly used one-round tweakable Even-Mansour schemes (formally introduced at CRYPTO 2015), which is constructed from a single n-bit permutation P and a function f(k, t) linear in k from some tweak space to {0, 1} n. Based on giant component theorem in random graph theory, we propose a collision-based multi-key attack on TEM-1 in the known-plaintext setting. Furthermore, inspired by the methodology of Fouque et al. presented at ASIACRYPT 2014, we devise a novel way of detecting collisions and eventually obtain a memory-efficient multi-key attack in the adaptive chosen-plaintext setting. As important applications, we utilize our techniques to analyze the authenticated encryption algorithms Minalpher (a second-round candidate of CAESAR) and OPP (proposed at EUROCRYPT 2016) in the multi-key setting. We describe knownplaintext attacks on Minalpher and OPP without nonce misuse, which enable us to recover almost all O(2n/3) independent masks by making O(2n/3) queries per key and costing O(22n/3) memory overall. After defining appropriate iterated functions and accordingly changing the mode of creating chains, we improve the basic blockwiseadaptive chosen-plaintext attack to make it also applicable for the nonce-respecting setting. While our attacks do not contradict the security proofs of Minalpher and OPP in the classical setting, nor pose an immediate threat to their uses, our results demonstrate their security margins in the multi-user setting should be carefully considered. We emphasize this is the very first third-party analysis on Minalpher and OPP

Ongoing Research Areas in Symmetric Cryptography

This report is a deliverable for the ECRYPT European network of excellence in cryptology. It gives a brief summary of some of the research trends in symmetric cryptography at the time of writing. 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 recently proposed 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)

On Cryptographic Properties of LFSR-based Pseudorandom Generators

Pseudorandom Generators (PRGs) werden in der modernen Kryptographie verwendet, um einen kleinen Startwert in eine lange Folge scheinbar zufälliger Bits umzuwandeln. Viele Designs für PRGs basieren auf linear feedback shift registers (LFSRs), die so gewählt sind, dass sie optimale statistische und periodische Eigenschaften besitzen. Diese Arbeit diskutiert Konstruktionsprinzipien und kryptanalytische Angriffe gegen LFSR-basierte PRGs. Nachdem wir einen vollständigen Überblick über existierende kryptanalytische Ergebnisse gegeben haben, führen wir den dynamic linear consistency test (DLCT) ein und analysieren ihn. Der DLCT ist eine suchbaum-basierte Methode, die den inneren Zustand eines PRGs rekonstruiert. Wir beschließen die Arbeit mit der Diskussion der erforderlichen Zustandsgröße für PRGs, geben untere Schranken an und Beispiele aus der Praxis, die veranschaulichen, welche Größe sichere PRGs haben müssen