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Computer security, cyber security, digital security or information technology security (IT security) is the protection of computer systems and networks from attacks by malicious actors that may result in unauthorized information disclosure, theft of, or damage to hardware, software, or data, as well as from the disruption or misdirection of the services they provide. The field is significant due to the expanded reliance on computer systems, the Internet, and wireless network standards such as Bluetooth and Wi-Fi. Also, due to the growth of smart devices, including smartphones, televisions, and the various devices that constitute the Internet of things (IoT). Cybersecurity is one of the most significant challenges of the contemporary world, due to both the complexity of information systems and the societies they support. Security is of especially high importance for systems that govern large-scale systems with far-reaching physical effects, such as power distribution, elections, and

Advanced Differential-Style Cryptanalysis of the NSA's Skipjack Block Cipher

Skipjack is a block cipher designed by the NSA for use in US government phones, and commercial mobile and wireless products by ATT. Among its initial implementations in hardware were the Clipper chip and Fortezza PC cards, which have since influenced the private communications market to be compatible with this technology. For instance, the Fortezza card comes in PCMCIA interface and is a very easy plug-n-play device to add on to mobile and wireless systems to provide encryption for wireless transmissions. Initially classified when it was first proposed, Skipjack was declassified in 1998, and it sparked numerous security analyses from security researchers worldwide because it provides insight into the state-of-the-art security design techniques used by a highly secretive government intelligence agency such as the NSA. In this paper, commemorating a decade since Skipjack's public revelation, we revisit the security of Skipjack, in particular its resistance to advanced differential-style distinguishers. In contrast to previous work that considered conventional and impossible differential distinguishers, we concentrate our attention on the more recent advanced differential-style and related-key distinguishers that were most likely not considered in the original design objectives of the NSA. In particular, we construct first-known related-key impossible differential, rectangle and related-key rectangle distinguishers of Skipjack. Our related-key attacks (i.e., related-key miss-in-the-middle and related-key rectangle attacks) are better than all the previous related-key attacks on Skipjack. Finally, we characterize the strength of Skipjack against all these attacks and motivate reasons why, influenced by the Skipjack structure, some attacks fare better. What is intriguing about Skipjack is its simple key schedule and a structure that is a cross between conventional Feistel design principles and the unconventional use of different round types. This work complements past results on the security analysis of Skipjack and is hoped to provide further insight into the security of an NSA-designed block cipher; the only one publicly known to date.

2009

Power Yoga: Variable-Stretch Security of CCM for Energy-Efficient Lightweight IoT

Emiljano Gjiriti, Reza Reyhanitabar, Damian Vizár

The currently ongoing NIST LWC project aims at identifying new standardization targets for lightweight authenticated encryption with associated data (AEAD) and (optionally) lightweight cryptographic hashing. NIST has deemed it important for performance and cost to be optimized on relevant platforms, especially for short messages. Reyhanitabar, Vaudenay and Vizar (Asiacrypt 2016) gave a formal treatment for security of nonce-based AEAD with variable stretch, i.e., when the length of the authentication tag is changed between encryptions without changing the key. They argued that AEAD supporting variable stretch is of practical interest for constrained applications, especially low-power devices operated by battery, due to the ability to flexibly trade communication overhead and level of integrity. In this work, we investigate this hypothesis with affirmative results. We present vCCM, a variable-stretch variant of the standard CCM and prove it is secure when used with variable stretch. We then experimentally measure the energy consumption of a real-world wireless sensor node when encrypting and sending messages with vCCM and CCM, respectively. Our projections show that the flexible trade of integrity level and ciphertext expansion can lead up to 21% overall energy consumption reduction in certain scenarios. As vCCM is obtained from the widely-used CCM by a black-box transformation, allowing any existing CCM implementations to be reused as-is, our results can be immediately put to use in practice. vCCM is all the more relevant because neither the NIST LWC project, nor any of the candidates give a consideration for the support of variable stretch and the related integrity-overhead trade-off.

RUHR-UNIV BOCHUM, HORST GORTZ INST IT-SICHERHEIT2021

Cryptographic Hash Functions in Groups and Provable Properties

Juraj Sarinay

We consider several "provably secure" hash functions that compute simple sums in a well chosen group (G,*). Security properties of such functions provably translate in a natural way to computational problems in G that are simple to define and possibly also hard to solve. Given k disjoint lists Li of group elements, the k-sum problem asks for gi ∊ Li such that g1 * g2 ... gk = 1G. Hardness of the problem in the respective groups follows from some "standard" assumptions used in public-key cryptology such as hardness of integer factoring, discrete logarithms, lattice reduction and syndrome decoding. We point out evidence that the k-sum problem may even be harder than the above problems. Two hash functions based on the group k-sum problem, SWIFFTX and FSB, were submitted to NIST as candidates for the future SHA-3 standard. Both submissions were supported by some sort of a security proof. We show that the assessment of security levels provided in the proposals is not related to the proofs included. The main claims on security are supported exclusively by considerations about available attacks. By introducing "second-order" bounds on bounds on security, we expose the limits of such an approach to provable security. A problem with the way security is quantified does not necessarily mean a problem with security itself. Although FSB does have a history of failures, recent versions of the two above functions have resisted cryptanalytic efforts well. This evidence, as well as the several connections to more standard problems, suggests that the k-sum problem in some groups may be considered hard on its own, and possibly lead to provable bounds on security. Complexity of the non-trivial tree algorithm is becoming a standard tool for measuring the associated hardness. We propose modifications to the multiplicative Very Smooth Hash and derive security from multiplicative k-sums in contrast to the original reductions that related to factoring or discrete logarithms. Although the original reductions remain valid, we measure security in a new, more aggressive way. This allows us to relax the parameters and hash faster. We obtain a function that is only three times slower compared to SHA-256 and is estimated to offer at least equivalent collision resistance. The speed can be doubled by the use of a special modulus, such a modified function is supported exclusively by the hardness of multiplicative k-sums modulo a power of two. Our efforts culminate in a new multiplicative k-sum function in finite fields that further generalizes the design of Very Smooth Hash. In contrast to the previous variants, the memory requirements of the new function are negligible. The fastest instance of the function expected to offer 128-bit collision resistance runs at 24 cycles per byte on an Intel Core i7 processor and approaches the 17.4 figure of SHA-256. The new functions proposed in this thesis do not provably achieve a usual security property such as preimage or collision resistance from a well-established assumption. They do however enjoy unconditional provable separation of inputs that collide. Changes in input that are small with respect to a well defined measure never lead to identical output in the compression function.

EPFL2011

Advanced Differential-Style Cryptanalysis of the NSA's Skipjack Block Cipher

2009

Cryptographic Hash Functions in Groups and Provable Properties

Juraj Sarinay

EPFL2011