Due to the legal reform on the protection of personal information in US/Japan and the enforcement of the General Data Protection Regulation (GDPR) in Europe, service providers are obliged to more securely manage the sensitive data stored in their server. In order to protect this kind of data, they generally employ a cryptographic encryption scheme and secure key management schemes such as a Hardware Security Module (HSM) and Trusted Platform Module (TPM). In this paper, we take a different approach based on the space-hard cipher. The space-hard cipher has an interesting property called the space hardness. Space hardness guarantees sufficient security against the adversary who gains a part of key data, e.g., 1/4 of key data. Combined with a simple network monitoring technique, we develop a practical leakage resilient scheme Virtual Vault, which is secure against the snapshot adversary who has full access to the memory in the server for a short period. Importantly, Virtual Vault is deployable by only a low-price device for network monitoring, e.g. L2 switch, and software of space-hard ciphers and packet analyzer, while typical solutions require a dedicated hardware for secure key managements such as HSM and TPM. Thus, Virtual Vault is easily added on the existing servers which do not have such dedicated hardware.

At the FSE conference of ToSC 2018, Kranz et al. presented their results on shortest linear programs for the linear layers of several well known block ciphers in literature. Shortest linear programs are essentially the minimum number of 2-input xor gates required to completely describe a linear system of equations. In the above paper the authors showed that the commonly used metrics like d-xor/s-xor count that are used to judge the “lightweightedness” do not represent the minimum number of xor gates required to describe a given MDS matrix. In fact they used heuristic based algorithms of Boyar-Peralta and Paar to find implementations of MDS matrices with even fewer xor gates than was previously known. They proved that the AES mixcolumn matrix can be implemented with as little as 97 xor gates. In this paper we show that the values reported in the above paper are not optimal. By suitably including random bits in the instances of the above algorithms we can achieve implementations of almost all matrices with lesser number of gates than were reported in the above paper. As a result we report an implementation of the AES mixcolumn matrix that uses only 95 xor gates. In FSE conference of ToSC 2019, Li et al. had tweaked the Boyar-Peralta algorithm to get low depth implementations of many matrices. We show that by introducing randomness in the tweaked algorithm, it is again possible to get low depth implementations with lesser number of gates than the above paper. As a result, we report a depth implementation of the AES mixcolumn matrix that uses only 103 xor gates, which is 2 gates less than the previous implementation. In the second part of the paper, we observe that most standard cell libraries contain both 2 and 3-input xor gates, with the silicon area of the 3-input xor gate being smaller than the sum of the areas of two 2-input xor gates. Hence when linear circuits are synthesized by logic compilers (with specific instructions to optimize for area), most of them would return a solution circuit containing both 2 and 3-input xor gates. Thus from a practical point of view, reducing circuit size in presence of these gates is no longer equivalent to solving the shortest linear program. In this paper we show that by adopting a graph based heuristic it is possible to convert a circuit constructed with 2-input xor gates to another functionally equivalent circuit that utilizes both 2 and 3-input xor gates and occupies less hardware area. As a result we obtain more lightweight implementations of all the matrices listed in the ToSC paper.

Troika is a recently proposed sponge-based hash function for IOTA's ternary architecture and platform, which is developed by CYBERCRYPT and is now used in IOTA's blockchain. In this paper, we introduce the preimage attack on 2/3 rounds of Troika with a divide-and-conquer approach. Firstly, we propose the equivalent conditions to determine whether a message is the preimage with an algebraic method. As a result, for the preimage attack on two-round Troika, we can search the preimage only in a valid smaller space and efficiently enumerate the messages which can satisfy most of the equivalent conditions with a guess-and-determine technique. Our experiments show that the time complexity of the preimage attack on 2-round Troika can be improved to 379 from 3243. For the preimage attack on 3-round Troika, the MILP-based method is applied to achieve the optimal time complexity, which is 327 times faster than brute force.

This paper investigates the security of KCipher-2 against differential attacks. We utilize an MILP-based method to evaluate the minimum number of active S-boxes in each round. We try to construct an accurate model to describe the 8-bit truncated difference propagation through the modular addition operation and the linear transformation of KCipher-2, respectively, which were omitted or simplified in the previous evaluation by Preneel et al. In our constructed model, the difference characteristics neglected in Preneel et al.'s evaluation can be taken into account and all valid differential characteristics can be covered. As a result, we reveal that the minimal number of active S-boxes is 25 over 15 rounds in the related IV setting and it is 17 over 24 rounds in the related IV-key setting. Therefore, this paper shows for the first time that KCipher-2 is secure against the related IV differential attack.

In this paper, we revisit related-key security of TWINE block cipher with 80-bit and 128-bit keys. Using an MILP-aided automatic search algorithm, we point out the previous evaluation of TWINE with a 80-bit key is wrong, and give a corrected evaluation result. Besides, we show a first security evaluation of TWINE with a 128-bit key in the related-key setting, which was infeasible due to the high computation cost in the original proposal.

In this paper, we analyze the security of an end-to-end encryption scheme (E2EE) of LINE, a.k.a Letter Sealing. LINE is one of the most widely-deployed instant messaging applications, especially in East Asia. By a close inspection of their protocols, we give several attacks against the message integrity of Letter Sealing. Specifically, we propose forgery and impersonation attacks on the one-to-one message encryption and the group message encryption. All of our attacks are feasible with the help of an end-to-end adversary, who has access to the inside of the LINE server (e.g. service provider LINE themselves). We stress that the main purpose of E2EE is to provide a protection against the end-to-end adversary. In addition, we found some attacks that even do not need the help of E2E adversary, which shows a critical security flaw of the protocol. Our results reveal that the E2EE scheme of LINE do not sufficiently guarantee the integrity of messages compared to the state-of-the-art E2EE schemes such as Signal, which is used by WhatApp and Facebook Messenger. We also provide some countermeasures against our attacks. We have shared our findings with LINE corporation in advance. The LINE corporation has confirmed our attacks are valid as long as the E2E adversary is involved, and officially recognizes our results as a vulnerability of encryption break.

In this paper we present proofs for the new biases in RC4 which were experimentally found and listed out (without theoretical justifications and proofs) in a paper by Vanhoef et al. in USENIX 2015. Their purpose was to exploit the vulnerabilities of RC4 in TLS using the set of new biases found by them. We also show (and prove) new results on couple of very strong biases residing in the joint distribution of three consecutive output bytes of the RC4 stream cipher. These biases provides completely new distinguisher for RC4 taking roughly O(224) samples to distinguish streams of RC4 from a uniformly random stream. We also provide a list of new results with proofs relating to some conditional biases in the keystreams of the RC4 stream cipher.

HIGHT is a 64-bit block lightweight cipher, which adopts the ARX-based generalized Feistel network, and it accepts a 128-bit key. It is a standard encryption algorithm in South Korea and also is internationally standardized by ISO/IEC 18033-3. Therefore, many third-party cryptanalyses have been proposed against HIGHT. Impossible differential and integral attacks are applied to reduced-round HIGHT, and especially, the impossible differential attack causes the 27-round attack, which is the current best attack under the single-key setting. In this paper, we propose some improved integral attacks against HIGHT. We first apply the division property to HIGHT and find new 19-round integral characteristics, which are improved by two rounds compared with the previous best ones. We append 9-round key recovery to these characteristics and it enables us to attack 28-round HIGHT. Its time complexity is 2127.02 where 263 chosen plaintexts and 2117 memory are required. Moreover, we can attack 29-round HIGHT if the full codebook is used, where its time and memory complexities are 2126.07 and 2118, respectively. It improves by two rounds compared with the previous best attack.

In this paper, we explore the security of single-key Even-Mansour ciphers against key-recovery attacks. First, we introduce a simple key-recovery attack using key relations on an n-bit r-round single-key Even-Mansour cipher (r-SEM). This attack is feasible with queries of DTr=O(2rn) and $2^{rac{2r}{r + 1}n}$ memory accesses, which is $2^{rac{1}{r + 1}n}$ times smaller than the previous generic attacks on r-SEM, where D and T are the number of queries to the encryption function EK and the internal permutation P, respectively. Next, we further reduce the time complexity of the key recovery attack on 2-SEM by a start-in-the-middle approach. This is the first attack that is more efficient than an exhaustive key search while keeping the query bound of DT2=O(22n). Finally, we leverage the start-in-the-middle approach to directly improve the previous attacks on 2-SEM by Dinur et al., which exploit t-way collisions of the underlying function. Our improved attacks do not keep the bound of DT2=O(22n), but are the most time-efficient attacks among the existing ones. For n=64, 128 and 256, our attack is feasible with the time complexity of about $2^{n} cdot rac{1}{2 n}$ in the chosen-plaintext model, while Dinur et al.'s attack requires $2^{n} cdot rac{{ m log}(n)}{ n} $ in the known-plaintext model.

Generalized Feistel Network (GFN) is widely used in block ciphers. CLEFIA is one of the GFN type-2 block ciphers. CLEFIA employs Diffusion Switching Mechanism (DSM) in its diffusion layer. DSM improves CLEFIA's security by increasing its number of active S-boxes, which is an indicator of security against differential and linear cryptanalyses. However, two matrices in DSM increase implementational cost. In this paper, we pursue the research question whether it is possible to achieve the same security as original CLEFIA with only one matrix without overhead in hardware. Our idea to answer the research question is applying byte-shuffling technique to CLEFIA. Byte-shuffling is an operation to shuffle 8-bit bytes. On the other hand, traditional GFN ciphers rotate 32-bit or larger words in their permutation layer. Since implementation of byte-shuffling is considered as cost-free in hardware, it adds no overhead in comparison with word rotation. Byte-shuffling has numerous shuffle patterns whereas word rotation has a few patterns. In addition, security property varies among the shuffle patterns. So, we have to find the optimal shuffle pattern(s) on the way to pursue the research question. Although one way to find the optimal shuffle pattern is evaluating all possible shuffle patterns, it is impractical to evaluate them since the evaluation needs much time and computation. We utilize even-odd byte-shuffling technique to narrow the number of shuffle patterns to be searched. Among numerous shuffle patterns, we found 168 shuffle patterns as the optimal shuffle patterns. They achieved full diffusion in 5 rounds. This is the same security as original CLEFIA. They achieved enough security against differential and linear cryptanalyses at 13th and 14th round, respectively, by active S-box evaluations. It is just one and two rounds longer than original CLEFIA. However, it is three and two rounds earlier than CLEFIA without DSM.

In spite of the research for a linear layer of Type-2 Generalized Feistel Network (Type-2 GFN) over more than 10 years, finding a good 32-branch permutation for Type-2 GFN is still a very hard task due to a huge search space. In terms of the diffusion property, Suzaki and Minematsu investigated the required number of rounds to achieve the full diffusion when the branch number is up to 16. After that, Derbez et al. presented a class of 32-branch permutations that achieves the 9-round full diffusion and they prove that this is optimal. However, this class is not suitable to be used in Type-2 GFN because it requires a large number of rounds to ensure a sufficient number of active S-boxes. In this paper, we present how to find a good class of 32-branch permutations for Type-2 GFN. To achieve this goal, we convert Type-2 GFN into a LBlock-like structure, and then we evaluate the diffusion property and the resistance against major attacks, such as differential, linear, impossible differential and integral attacks by an MILP. As a result, we present a good class of 32-branch permutations that achieves the 10-round full diffusion, ensures differentially/linearly active S-boxes of 66 at 19 round, and has the 18/20-round impossible differential/integral distinguisher, respectively. The 32-branch permutation used in WARP was chosen among this class.

RC4 is a well-known stream cipher designed by Rivest. Due to considerable cryptanalysis efforts over past 20 years, several kinds of statistic biases in a key stream of RC4 have been observed so far. Finally, practical full plaintext recovery attacks on RC4 in SSL/TLS were independently proposed by AlFardan et al. and Isobe et al. in 2013. Responded to these attacks, usage of RC4 has drastically decreased in SSL/TLS. However, according to the research by Trustworthy Internet Movement, RC4 is still used by some websites for the encryption on SSL/TLS. In this paper, we shows a new plaintext recovery attack for RC4 under the assumption of HTTPS. We develop a method for exploiting single-byte and double-byte biases together to efficiently guess the target bytes, while previous attacks use either single-byte biases or double-byte biases. As a result, target plaintext bytes can be extracted with higher probability than previous best attacks given 229 ciphertexts encrypted by randomly-chosen keys. In the most efficient case, the success probability of our attack are more than twice compared to previous best attacks.

Tweakable block cipher (TBC) is an extension of conventional block cipher. We study how to build a TBC based on generalized Feistel structure (GFS), a classical block cipher construction. While known dedicated TBC proposals are based on substitution-permutation network (SPN), GFS has not been used for building TBC. In particular, we take 64-bit GFS block cipher TWINE and try to make it tweakable with a minimum change. To find a best one from a large number of candidates, we performed a comprehensive search with a help of mixed integer linear programming (MILP) solver. As a result, our proposal TWINE is quite efficient, has the same number of rounds as TWINE with extremely simple tweak schedule.

The stream cipher Sprout with a short internal state was proposed in FSE 2015. Although the construction guaranteed resistance to generic Time Memory Data Tradeoff attacks, there were some weaknesses in the design and the cipher was completely broken. In this paper we propose a family of stream ciphers LILLE in which the size of the internal state is half the size of the secret key. Our main goal is to develop robust lightweight stream cipher. To achieve it, our cipher based on the two-key Even Mansour construction and thus its security against key/state recovery attacks reduces to a well analyzed problem. We also prove that like Sprout, the construction is resistant to generic Time Memory Data Tradeoff attacks. Unlike Sprout, the construction of the cipher guarantees that there are no weak key-IV pairs which produce a keystream sequence with short period or which make the algebraic structure of the cipher weaker and easy to cryptanalyze. The reference implementations of all members of the LILLE family with standard cell libraries based on the STM 90nm and 65nm processes were also found to be smaller than Grain v1 while security of LILLE family depend on reliable problem in the symmetric cryptography.

Kreyvium is a NLFSR-based stream cipher which is oriented to homomorphic-ciphertext compression. This is a variant of Trivium with 128-bit security. Designers have evaluated the security of Kreyvium and concluded that the resistance of Kreyvium to the conditional differential cryptanalysis is at least the resistance of Trivium, and even better. However, we consider that this attack is effective for reduced Kreyvium due to the structure of it. This paper shows the conditional differential cryptanalysis for Kreyvium, and we propose distinguishing and key recovery attacks. We show how to arrange differences and conditions to obtain good higher-order conditional differential characteristics. We use two types of higher-order conditional differential characteristics to find a distinguisher, e.g. the bias of higher-order conditional differential characteristics of a keystream and the probabilistic bias of them. In the first one, we obtain the distinguisher on Kreyvium with 730 rounds from 20-th order characteristics. In the second one, we obtain the distinguisher on Kreyvium with 899 rounds from 25-th order conditional differential characteristics. Moreover, we show the key recovery attack on Kreyvium with 736 rounds from 20-th order characteristics. We experimentally confirm all our attacks. The second distinguisher shows that we can obtain the distinguisher on Kreyvium with more rounds than the distinguisher on Trivium. Therefore, Kreyvium has a smaller security margin than Trivium for the conditional differential cryptanalysis.

We propose new key recovery attacks on the two-round single-key n-bit Even-Mansour ciphers (2SEM) that are secure up to 22n/3 queries against distinguishing attacks proved by Chen et al. Our attacks are based on the meet-in-the-middle technique which can significantly reduce the data complexity. In particular, we introduce novel matching techniques which enable us to compute one of the two permutations without knowing a part of the key information. Moreover, we present two improvements of the proposed attack: one significantly reduces the data complexity and the other reduces the time complexity. Compared with the previously known attacks, our attack first breaks the birthday barrier on the data complexity although it requires chosen plaintexts. When the block size is 64 bits, our attack reduces the required data from 245 known plaintexts to 226 chosen plaintexts with keeping the time complexity required by the previous attacks. Furthermore, by increasing the time complexity up to 262, the required data is further reduced to 28, and DT=270, where DT is the product of data and time complexities. We show that our data-optimized attack requires DT=2n+6 in general cases. Since the proved lower bound on DT for the single-key one-round n-bit Even-Mansour ciphers is 2n, our results imply that adding one round to one-round constructions does not sufficiently improve the security against key recovery attacks. Finally, we propose a time-optimized attacks on 2SEM in which, we aim to minimize the number of the invocations of internal permutations.

In this paper, we propose an effective key recovery attack on stream ciphers Py and Pypy with chosen IVs. Our method uses an internal-state correlation based on the vulnerability that the randomization of the internal state in the KSA is inadequate, and it improves two previous attacks proposed by Wu and Preneel (a WP-1 attack and a WP-2 attack). For a 128-bit key and a 128-bit IV, the WP-1 attack can recover a key with 223 chosen IVs and time complexity 272. First, we improve the WP-1 attack by using the internal-state correlation (called a P-1 attack). For a 128-bit key and a 128-bit IV, the P-1 attack can recover a key with 223 chosen IVs and time complexity 248, which is 1/224 of that of the WP-1 attack. The WP-2 attack is another improvement on the WP-1 attack, and it has been known as the best previous attack against Py and Pypy. For a 128-bit key and a 128-bit IV, the WP-2 attack can recover a key with 223 chosen IVs and time complexity 224. Second, we improve the WP-2 attack by using the internal-state correlation as well as the P-1 attack (called a P-2 attack). For a 128-bit key and a 128-bit IV, the P-2 attack can recover a key with 223 chosen IVs and time complexity 224, which is the same capability as that of the WP-2 attack. However, when the IV size is from 64 bits to 120 bits, the P-2 attack is more effective than the WP-2 attack. Thus, the P-2 attack is the known best attack against Py and Pypy.

In this paper, we explore the security against integral attacks on well-known stream ciphers SNOW 3G and KCipher-2. SNOW 3G is the core of the 3GPP confidentiality and integrity algorithms UEA2 and UIA2, and KCipher-2 is a standard algorithm of ISO/IEC 18033-4 and CRYPTREC. Specifically, we investigate the propagation of the division property inside SNOW 3G and KCipher-2 by the Mixed-Integer Linear Programming to efficiently find an integral distinguisher. As a result, we present a 7-round integral distinguisher with 23 chosen IVs for KCipher-2. As far as we know, this is the first attack on a reduced variant of KCipher-2 by the third party. In addition, we present a 13-round integral distinguisher with 27 chosen IVs for SNOW 3G, whose time/data complexity is half of the previous best attack by Biryukov et al.

Authenticated-Encryption with Associated-Data (AEAD) plays an important role in guaranteeing confidentiality, integrity, and authenticity in network communications. To meet the requirements of high-performance applications, several AEADs make use of AES New Instructions (AES-NI), which can conduct operations of AES encryption and decryption dramatically fast by hardware accelerations. At SAC 2013, Wu and Preneel proposed an AES-based AEAD scheme called AEGIS-128/128L/256, to achieve high-speed software implementation. At FSE 2016, Jean and Nikolić generalized the construction of AEGIS and proposed more efficient round functions. At ToSC 2021, Sakamoto et al. further improved the constructions of Jean and Nikolić, and proposed an AEAD scheme called Rocca for beyond 5G. In this study, we first evaluate the security of the initialization phases of Rocca and AEGIS family against differential and integral attacks using MILP (Mixed Integer Linear Programming) tools. Specifically, according to the evaluation based on the lower bounds for the number of active S-boxes, the initialization phases of AEGIS-128/128L/256 are secure against differential attacks after 4/3/6 rounds, respectively. Regarding integral attacks, we present the integral distinguisher on 6 rounds and 6/5/7 rounds in the initialization phases of Rocca and AEGIS-128/128L/256, respectively. Besides, we evaluate the round function of Rocca and those of Jean and Nikolić as cryptographic permutations against differential, impossible differential, and integral attacks. Our results indicate that, for differential attacks, the growth rate of increasing the number of active S-boxes in Rocca is faster than those of Jean and Nikolić. For impossible differential and integral attacks, we show that the round function of Rocca achieves the sufficient level of the security against these attacks in smaller number of rounds than those of Jean and Nikolić.

Proof of Work (PoW), which is a consensus algorithm for blockchain, entails a large number of meaningless hash calculations and wastage of electric power and computational resources. In 2021, it is estimated that the PoW of Bitcoin consumes as much electricity as Pakistan's annual power consumption (91TWh). This is a serious problem against sustainable development goals. To solve this problem, this study proposes Meaningful-PoW (mPoW), which involves a meaningful calculation, namely the application of a genetic algorithm (GA) to PoW. Specifically, by using the intermediate values that are periodically generated through GA calculations as an input to the Hashcash used in Bitcoin, it is possible to make this scheme a meaningful calculation (GA optimization problem) while maintaining the properties required for PoW. Furthermore, by applying a device-binding technology, mPoW can be ASIC resistant without the requirement of a large memory. Thus, we show that mPoW can reduce the excessive consumption of both power and computational resources.