On optimal size in truncated differential attacks

Statistical attacks on block ciphers make use of a property of the cipher so that an event occurs with different probabilities depending on whether or not the correct key is used. For instance, differential cryptanalysis [3] and truncated differential cryptanalysis [5] consider characteristics or differentials which show that a particular output difference should be obtained with a relatively high probability when a particular input difference is used. Hence, when the correct key is used, the predicted differences occur more frequently.

Differential cryptanalysis is one of the very first attack proposed against block ciphers. This attack exploits the fact that some derivatives of the cipher (or of a reduced version of the cipher) have a nonrandom output distribution. Since this distribution highly depends on the behaviour of the derivatives of the nonlinear components of the cipher, Nyberg and Knudsen have introduced the notion of differential uniformity which measures the quality of an Sbox regarding its resistance to differential attacks. This notion is the starting point of many works, including the study and the construction of the so-called APN functions, which are the functions providing the best resistance against differential attacks. However, many new primitives have been proposed in the last five years, including the 64 hash functions submitted to the SHA-3 competition and a lot of lightweight block ciphers. Many of those new proposals have been attacked or evaluated by several sophisticated variants of the original differential attack. Some of them appear to be able to break some primitives which have been proved resistant against differential cryptanalysis. Those attacks include the cube attack, the rebound attack, the linear subspace attack and some meet-in-the-middle attacks. In this talk, we will study some properties of the building-blocks of a cipher and their impacts on these new attacks. In particular, we will investigate their connections with the classical notion of differential uniformity, and we will discuss the different criteria for choosing an appropriate nonlinear function when designing a new block cipher.

In this paper, we analyze the security of two recently proposed ultra-lightweight block ciphers, SLIM and LCB. SLIM is designed based on the Feistel paradigm, operating on 32-bit blocks and has an 80-bit key. The designers claim that SLIM is immune to differential cryptanalysis after they were only able to find a trail of up to 7 rounds by using a heuristic method. LCB is another ultra-lightweight block cipher with a 32-bit block and instead uses a 64-bit secret key. It was designed based on a hybrid of Feistel and substitution-permutation network structures. Although no concrete security analyses were performed, the designers claim that 10 rounds of the cipher is secure enough against various attacks including differential cryptanalysis. We verify these claims by proposing differential attacks on both ciphers. For SLIM, we first report optimal (i.e., having the best differential probability) trails for up to 32 rounds found using an SMT solver. We then propose practical key recovery attacks on up to 14 rounds that recover the final round key with time complexity \(2^{32}\). Next, a close inspection of LCB’s design revealed a lack of nonlinearity, whereby its S-box could be modelled as a permutation. As such, differential trails that hold with probability 1 can be trivially derived for any number of rounds of the cipher. A trivial distinguishing attack can be performed with just one known-ciphertext. We fix this flaw and go on to show that LCB is actually more secure (against differential cryptanalysis) than SLIM given the same number of rounds. To the best of our knowledge, these are the first third-party cryptanalysis attacks against both ciphers.KeywordsSLIMLCBDifferential cryptanalysisLightweight block cipherSMT

Impossible differential attack is a well-known mean to examine robustness of block ciphers. Using impossible differential cryptanalysis, we analyze security of a family of lightweight block ciphers, named Midori, that are designed considering low energy consumption. Midori state size can be either 64 bits for Midori64 or 128 bits for Midori128; however, both versions have key size equal to 128 bits. In this paper, we mainly study security of Midori64. To this end, we use various techniques such as early-abort, memory reallocation, miss-in-the-middle and turning to account the inadequate key schedule algorithm of Midori64. We first show two new 7round impossible differential characteristics which are, to the best of our knowledge, the longest impossible differential characteristics found for Midori64. Based on the new characteristics, we mount three impossible differential attacks for 10, 11, and 12 rounds on Midori64 with 2 87.7 , 2 90.63 , and 2 90.51 time complexity, respectively, to retrieve the master-key.

This paper presents an evaluation of the practical strength of the lightweight block cipher “Cypress” to the differential cryptanalysis, which is determined by the probability of the best found differential characteristic. The paper proposes a mathematical model for evaluating the block cipher “Cypress” to differential cryptanalysis and methods for searching for multi-round differential characteristics. The first method is based on the combination of highly probable one-round differential characteristics into multi-round ones, while the second method is based on the extension of one-round characteristics for several rounds. As a result of the application of the second search method to the block cipher Cypress-256, a 6-round differential characteristic was found. Since it was not found a differential characteristics for more than six rounds with a probability which is higher than the probability of a brute-force attack, the block cipher Cypress-256 is practically resistant to differential cryptanalysis.

Secure lightweight block ciphers have become an important aspect due to the fact that they are a popular choice for providing security in ubiquitous devices. Two of the most important attacks on block ciphers are differential cryptanalysis [1] and linear cryptanalysis [2]. Calculating the number of active S-boxes is one of the method to examine the security of block ciphers against differential attack. In this paper, we count the minimum number of active S-boxes for several rounds of the lightweight ciphers namely KLEIN, LED and AES. We utilized the method proposed in [9], where calculation of the minimum number of active S-boxes is formulated as a Mixed Integer Linear Programming (MILP) problem. The objective function is to minimize the number of active S-boxes, subject to the constraints imposed by the differential propagation of the cipher. The experimental results are presented in this paper and found to be encouraging.

Differential cryptanalysis is one of the fundamental cryptanalysis techniques to evaluate the security of the block cipher. In many cases, resistance to differential cryptanalysis is proven through the upper bound of the differential characteristic probability, not the differential probability. Since the attacker uses a differential rather than a differential characteristic, resistance based on a differential characteristic tends to overestimate the security level of the block cipher. Such an overestimation is notably observed in lightweight block ciphers SKINNY, Midori, and CRAFT. In this paper, we examine the gap between the differential characteristics and the differential probability of lightweight block ciphers. We present practical methods for computing differential probability using a multistage graph. Using these methods, we count the exact number of maximum differential characteristics with fixed plaintext/ciphertext difference and activity pattern. By the exact number of maximum differential characteristics, we can calculate the probability that is closer to the real differential probability. In addition, by modifying the method, we compute a more accurate differential probability by considering the characteristics of the lower probability. We find differential distinguishers of 9-round Midori64 with probability 2−61.58, 9-round SKINNY64 with 2−58.67 and 14-round CRAFT with 2−60.32. Furthermore, we find a related-tweakey differential distinguisher of 11-round SKINNY64-64 with 2−55.93 and a related-tweak differential distinguisher of 17-round CRAFT with probability 2−63.37. Finally, we explain why these gaps are notable in Midori64, SKINNY64 and CRAFT by relating the S-box differential distribution table.

Mixed integer linear programming (MILP) model was presented by Sun et al. at Asiacrypt 2014 to search for differential characteristics of block ciphers. Based on this model, it is easy to assess block ciphers against differential attack. In this paper, the MILP model is improved to search for differential trails of Midori64 which is a family of lightweight block ciphers provided by Banik et al. at Asiacrypt 2015. We find the best 5-round differential characteristics of Midori64 with MILP-based model, and the probabilities are 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-52</sup> and 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-58</sup> respectively. Based on these distinguishers, we give key recovery attacks on the 11-round reduced Midori64 with data complexities of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55.6</sup> and 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">61.2</sup> , and time complexities of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">109.35</sup> and 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100.26</sup> .

Substitution boxes with resilient cryptographic possessions are normally utilized in block ciphers to give the substantial property of nonlinearity. They are important to resist standard attacks such as linear and differential cryptanalysis. A cryptographically robust S-box must be sound with respect to cryptographic properties like nonlinearity, bit independent criteria, strict avalanche criteria, linear and differential approximation probability. In this paper, we have developed an innovative construction scheme of nonlinear component of block cipher based on the action of projective linear groups on the projective line, and the permutation triangle groups. This nonlinear component, namely S-box, is responsible for making the relation between plaintext and ciphertext intractable which is one of the most important requirements of any modern block ciphers. By widening the scope of the proposed S-boxes, we have applied these lightweight nonlinear components in watermarking scheme.

Nowadays, there is a strong demand for increasing the protection of resource-constrained devices such as Radio frequency identification (RFID) systems. Current cryptographic algorithms are sufficient for high-resource desktop computers. RFID systems are commonly used in high-security applications such as access control systems, transaction banking systems, and payment systems. The attacker attempts to mislead RFIDs for unauthorized access to services without payment or to circumvent security mechanisms by detecting a secret password. The biggest challenge in RFID systems is how to ensure successful protection against such infringements. Lightweight cryptography can provide security assurance for protecting RFID systems. This article presents a new ultra-lightweight cryptography algorithm for RFID systems called SLIM. SLIM is a 32-bit block cipher based on the Feistel structure since block ciphers are the most used cryptographic and provide very tight protection for IoT devices. The key challenge in designing a lightweight block cipher is to cope with performance, cost, and security. SLIM, like all symmetric block cipher, uses the same key for encryption and decryption. The proposed algorithm has an excellent performance in both hardware and software environments, with a limited implementation area, an acceptable cost/security for RFID systems, and an energy-efficient behaviour. SLIM has demonstrated high immunity against the most effective linear and differential cryptanalysis attacks and has a sufficient margin of defence against these attacks.

This paper analyzes the security of the RC5 encryption algorithm against differential and linear cryptanalysis. RC5 is a new block cipher recently designed by Ron Rivest. It has a variable word size, a variable number of rounds, and a variable-length secret key. In RC5, the secret key is used to fill an expanded key table which is then used in encryption. Both our differential and linear attacks on RC5 recover every bit of the expanded key table without any exhaustive search. However, the plaintext requirement is strongly dependent on the number of rounds. For 64-bit block size, our differential attack on nine-round RC5 uses 245 chosen plaintext pairs (about the same as DES), while 262 pairs are needed for 12-round RC5. Similarly, our linear attack on live-round RC5 uses 247 known plaintexts (about the same as DES), and the plaintext requirement is impractical for more than six rounds. We conjecture that the linear approximations used in our linear cryptanalysis are optimal. Thus, we conclude that Rivest's suggested use of 12 rounds is sufficient to make differential and linear cryptanalysis of RC5 impractical.

LiCi algorithm is a newly lightweight block cipher. Due to its new design idea adopted by Patil et al, it has the advantages of compact design, low energy consumption and less chip area, thus is is especially suitable for resource-constrained environments. Currently, its security receives extensively attention, and Patil et al. claimed that the 16-round reduced LiCi can sufficiently resist both differential attack and linear attack. In this paper, a new 10-round impossible differential distinguisher is constructed based on the differential characteristics of the S-box and the meet-in-the-middle technique. Moreover, on the basis of this distinguisher, a 16-round impossible differential attack on LiCi is proposed by respectively extending 3-round forward and backward via the key scheduling scheme. This attack requires a time complexity of about 283.08 16-round encryptions, a data complexity of about 259.76 chosen plaintexts, and a memory complexity of 276.76 data blocks, which illustrates that the 16-round LiCi cipher can not resist impossible differential attack.

In today's world of the internet, billions of computer systems are connected to one another in a global network. The internet provides an unsecured channel in which hundreds of terabytes of data is being transmitted daily. Computer and software systems rely on encryption algorithms such as block ciphers to ensure that sensitive data remains confidential and secure. However, adversaries can leverage the statistical behavior of underlying ciphers to recover encryption keys. Accurate evaluation of the security margins of these encryption algorithms remains to be a big challenge. In this paper, we tackle this issue by introducing several searching strategies based on differential cryptanalysis. By clustering differential paths, the searching algorithm derives more accurate distinguishers as compared to examining individual paths, which in turn provides a more accurate estimation of cipher security margins. We verify the effectiveness of this technique on ciphers with the generalized Feistel and SPN structures, whereby the best distinguishers for each of the investigated ciphers were obtained by discovering clusters with thousands of paths. With the KATAN block cipher family as a test case, we also show how to apply the searching algorithm alongside other cryptanalysis techniques such as the boomerang attack and related-key model to obtain the best cryptanalytic results. This also depicts the flexibility of the proposed searching scheme, which can be tailored to improve upon other differential attack variants. In short, the proposed searching strategy realizes an automated security evaluation tool with higher accuracy compared to previous techniques. In addition, it is applicable to a wide range of encryption schemes which makes it a flexible tool for both academic research and industrial purposes.

Today's headlines abound with the promised explosion of the Internet of things. Studies claim that there will be the existence of billions of internet connected devices in coming years. The small size and limited processing power of many connected devices could inhibit encryption and other robust security measure. Here cryptography plays a pivotal role. Also most of the devices available in the market today are resource constrained. Hence cryptographic solutions must be easy to implement and have high performance on a wide range of severely constrained devices. The relatively new field of lightweight cryptography provides significant advantages over existing algorithms when addressing security issues for highly constrained devices. In this project a detailed study of LBlock block cipher is done which is a lightweight cipher in both hardware and 8-bit platforms and an IP core is developed. The block size of LBlock is 64-bit and the key size is 80-bit and it can achieve competitive hardware and software performances when compared with other known lightweight block ciphers. The security evaluation shows that LBlock can achieve enough security margin against known attacks such as differential cryptanalysis, linear cryptanalysis, impossible differential cryptanalysis and related-key attacks etc. LBlock cryptocore is designed, build and characterized for the efficient implementation in low resource devices. It can serve as a benchmark for the hardware design engineers to model devices that utilizes lightweight characteristics. LBlock cipher is implemented on Xilinx Spartan-6 FPGA (XC6LX16-CS324) and its performance metrics were obtained.

With the widespread use of RFID technology and the rapid development of Internet of Things, the research of lightweight block cipher has become one of the hot issues in cryptography research. In recent years, lightweight block ciphers have emerged and are widely used, and their security is also crucial. Skinny-64/192 can be used to protect data security such as the applications of wireless multimedia and wireless sensor networks. In this paper, we use the new method to verify the security of Skinny-64/192. The method is called mixed-integer linear programming (MILP) which can characterize precisely the linear operation and nonlinear operation in a round function. By applying MILP program, we can automatically find a 11-round differential characteristic for Skinny-64/192 with the minimum number of active s-boxes. The probability of differential trail is 2-147, that is, far greater than 2-192 which is the probability of success for an exhaustive search. In addition, comparing this method with the one proposed by Sun et al., we also have a great improvement; that is, no new variables will be added in ShiftRows operation. It can reduce greatly the number of variables and improve the running speed of the computer. Besides, the experimental result proves that Skinny-64/192 is safe on 11-round differential analysis and validates the effectiveness of the MILP method.