## CryptoDB

### Takanori Isobe

#### Publications

**Year**

**Venue**

**Title**

2022

TOSC

New Low-Memory Algebraic Attacks on LowMC in the Picnic Setting
Abstract

The security of the post-quantum signature scheme Picnic is highly related to the difficulty of recovering the secret key of LowMC from a single plaintext-ciphertext pair. Since Picnic is one of the alternate third-round candidates in NIST post-quantum cryptography standardization process, it has become urgent and important to evaluate the security of LowMC in the Picnic setting. The best attacks on LowMC with full S-box layers used in Picnic3 were achieved with Dinur’s algorithm. For LowMC with partial nonlinear layers, e.g. 10 S-boxes per round adopted in Picnic2, the best attacks on LowMC were published by Banik et al. with the meet-in-the-middle (MITM) method.In this paper, we improve the attacks on LowMC in a model where memory consumption is costly. First, a new attack on 3-round LowMC with full S-box layers with negligible memory complexity is found, which can outperform Bouillaguet et al.’s fast exhaustive search attack and can achieve better time-memory tradeoffs than Dinur’s algorithm. Second, we extend the 3-round attack to 4 rounds to significantly reduce the memory complexity of Dinur’s algorithm at the sacrifice of a small factor of time complexity. For LowMC instances with 1 S-box per round, our attacks are shown to be much faster than the MITM attacks. For LowMC instances with 10 S-boxes per round, we can reduce the memory complexity from 32GB (238 bits) to only 256KB (221 bits) using our new algebraic attacks rather than the MITM attacks, while the time complexity of our attacks is about 23.2 ∼ 25 times higher than that of the MITM attacks. A notable feature of our new attacks (apart from the 4-round attack) is their simplicity. Specifically, only some basic linear algebra is required to understand them and they can be easily implemented.

2022

TOSC

New Cryptanalysis of ZUC-256 Initialization Using Modular Differences
Abstract

ZUC-256 is a stream cipher designed for 5G applications by the ZUC team. Together with AES-256 and SNOW-V, it is currently being under evaluation for standardized algorithms in 5G mobile telecommunications by Security Algorithms Group of Experts (SAGE). A notable feature of the round update function of ZUC-256 is that many operations are defined over different fields, which significantly increases the difficulty to analyze the algorithm.As a main contribution, with the tools of the modular difference, signed difference and XOR difference, we develop new techniques to carefully control the interactions between these operations defined over different fields. At first glance, our techniques are somewhat similar to those developed by Wang et al. for the MD-SHA hash family. However, as ZUC-256 is quite different from the MD-SHA hash family and its round function is much more complex, we are indeed dealing with different problems and overcoming new obstacles.As main results, by utilizing complex input differences, we can present the first distinguishing attacks on 31 out of 33 rounds of ZUC-256 and 30 out of 33 rounds of the new version of ZUC-256 called ZUC-256-v2 with low time and data complexities, respectively. These attacks target the initialization phase and work in the related-key model with weak keys. Moreover, with a novel IV-correcting technique, we show how to efficiently recover at least 16 key bits for 15-round ZUC-256 and 14-round ZUC-256-v2 in the related-key setting, respectively. It is unpredictable whether our attacks can be further extended to more rounds with more advanced techniques. Based on the current attacks, we believe that the full 33 initialization rounds provide marginal security.

2022

TOSC

Hybrid Code Lifting on Space-Hard Block Ciphers: Application to Yoroi and SPNbox
Abstract

There is a high demand for whitebox cryptography from the practical use of encryption in untrusted environments. It has been actively discussed for two decades since Chow et al. presented the whitebox implementation of DES and AES. The goal is to resist the key extraction from the encryption program and mitigate the code lifting of the program. At CCS2015, Bogdanov and Isobe proposed space-hard block ciphers as a dedicated design of whitebox block ciphers. It ensures that the key extraction is as difficult as the key recovery in the standard blackbox model. Moreover, to mitigate code lifting, they introduce space hardness, a kind of leakage-resilient security with the incompressibility of a huge program. For space-hard ciphers, code lifting (a partial leakage of the entire program) is useless to copy the functionality.In this paper, we consider a new attack model of space-hard block ciphers called hybrid code lifting. Space-hard block ciphers are intended to ensure security under a size-bounded leakage. However, they do not consider attackers (in the standard blackbox model) receiving the leakage by code lifting. If such attackers can recover the encryption program of a space-hard block cipher, such a cipher does not always satisfy the intention. We analyze Yoroi proposed in TCHES 2021. We introduce the canonical representation of Yoroi. Using the representation enables the recovery of the programs of Yoroi-16 and Yoroi-32 with 233 and 265.6 complexities, respectively, in spite of slight leakage. The canonical representation causes another attack against Yoroi. It breaks an authors’ security claim about the “longevity”. We additionally analyzed SPNbox proposed in Asiacrypt 2016. As a result, considering security on the hybrid code lifting, the original number of rounds is insufficient to achieve 128-bit security under quarter-size leakage.

2022

ASIACRYPT

Algebraic Meet-in-the-Middle Attack on LowMC
Abstract

By exploiting the feature of partial nonlinear layers, we propose a new technique called algebraic meet-in-the-middle (MITM) attack to analyze the security of LowMC, which can reduce the memory complexity of the simple difference enumeration attack over the state-of-the-art. Moreover, while an efficient algebraic technique to retrieve the full key from a differential trail of LowMC has been proposed at CRYPTO 2021, its time complexity is still exponential in the key size. In this work, we show how to reduce it to constant time when there are a sufficiently large number of active S-boxes in the trail. With the above new techniques, the attacks on LowMC and LowMC-M published at CRYPTO 2021 are further improved, and some LowMC instances could be broken for the first time. Our results seem to indicate that partial nonlinear layers are still not well-understood.

2022

ASIACRYPT

A Modular Approach to the Incompressibility of Block-Cipher-Based AEADs
Abstract

Incompressibility is one of the most fundamental security goals in white-box cryptography.
Given recent advances in the design of efficient and incompressible block ciphers such as SPACE, SPNbox and WhiteBlock,
we demonstrate the feasibility of reducing incompressible AEAD modes to incompressible block ciphers.
We first observe that several existing AEAD modes of operation, including CCM, GCM(-SIV), and OCB, would be all insecure against white-box adversaries even when used with an incompressble block cipher.
This motivates us to revisit and formalize incompressibility-based security definitions for AEAD schemes and for block ciphers, so that we become able to design modes and reduce their security to that of the underlying ciphers.
Our new security notion for AEAD, which we name whPRI, is an extension of the pseudo-random injection security in the black-box setting.
Similar security notions are also defined for other cryptosystems such as privacy-only encryption schemes.
We emphasize that whPRI ensures quite strong authenticity against white-box adversaries:
existential unforgeability beyond leakage.
This contrasts sharply with previous notions which have ensured either no authenticity or only universal unforgeability.
For the underlying ciphers we introduce a new notion of whPRP, which extends that of PRP in the black-box setting.
Interestingly, our incompressibility reductions follow from a variant of public indifferentiability.
In particular, we show that a practical whPRI-secure AEAD mode can be built from a whPRP-secure block cipher: We present a SIV-like composition of the sponge construction (utilizing a block cipher as its underlying primitive) with the counter mode and prove that such a construction is (in the variant sense) public indifferentiable from a random injection.
To instantiate such an AEAD scheme, we propose a 256-bit variant of SPACE, based on our conjecture that SPACE should be a whPRP-secure cipher.

2021

TOSC

Atom: A Stream Cipher with Double Key Filter
📺
Abstract

It has been common knowledge that for a stream cipher to be secure against generic TMD tradeoff attacks, the size of its internal state in bits needs to be at least twice the size of the length of its secret key. In FSE 2015, Armknecht and Mikhalev however proposed the stream cipher Sprout with a Grain-like architecture, whose internal state was equal in size with its secret key and yet resistant against TMD attacks. Although Sprout had other weaknesses, it germinated a sequence of stream cipher designs like Lizard and Plantlet with short internal states. Both these designs have had cryptanalytic results reported against them. In this paper, we propose the stream cipher Atom that has an internal state of 159 bits and offers a security of 128 bits. Atom uses two key filters simultaneously to thwart certain cryptanalytic attacks that have been recently reported against keystream generators. In addition, we found that our design is one of the smallest stream ciphers that offers this security level, and we prove in this paper that Atom resists all the attacks that have been proposed against stream ciphers so far in literature. On the face of it, Atom also builds on the basic structure of the Grain family of stream ciphers. However, we try to prove that by including the additional key filter in the architecture of Atom we can make it immune to all cryptanalytic advances proposed against stream ciphers in recent cryptographic literature.

2021

TOSC

Orthros: A Low-Latency PRF
📺
Abstract

We present Orthros, a 128-bit block pseudorandom function. It is designed with primary focus on latency of fully unrolled circuits. For this purpose, we adopt a parallel structure comprising two keyed permutations. The round function of each permutation is similar to Midori, a low-energy block cipher, however we thoroughly revise it to reduce latency, and introduce different rounds to significantly improve cryptographic strength in a small number of rounds. We provide a comprehensive, dedicated security analysis. For hardware implementation, Orthros achieves the lowest latency among the state-of-the-art low-latency primitives. For example, using the STM 90nm library, Orthros achieves a minimum latency of around 2.4 ns, while other constructions like PRINCE, Midori-128 and QARMA9-128- σ0 achieve 2.56 ns, 4.10 ns, 4.38 ns respectively.

2021

TOSC

Exploiting Weak Diffusion of Gimli: Improved Distinguishers and Preimage Attacks
📺
Abstract

The Gimli permutation proposed in CHES 2017 was designed for cross-platform performance. One main strategy to achieve such a goal is to utilize a sparse linear layer (Small-Swap and Big-Swap), which occurs every two rounds. In addition, the round constant addition occurs every four rounds and only one 32-bit word is affected by it. The above two facts have been recently exploited to construct a distinguisher for the full Gimli permutation with time complexity 264. By utilizing a new property of the SP-box, we demonstrate that the time complexity of the full-round distinguisher can be further reduced to 252 while a significant bias still remains. Moreover, for the 18-round Gimli permutation, we could construct a distinguisher even with only 2 queries. Apart from the permutation itself, the weak diffusion can also be utilized to accelerate the preimage attacks on reduced Gimli-Hash and Gimli-XOF-128 with a divide-and-conquer method. As a consequence, the preimage attacks on reduced Gimli-Hash and Gimli-XOF-128 can reach up to 5 rounds and 9 rounds, respectively. Since Gimli is included in the second round candidates in NIST’s Lightweight Cryptography Standardization process, we expect that our analysis can further advance the understanding of Gimli. To the best of our knowledge, the distinguishing attacks and preimage attacks are the best so far.

2021

TOSC

Rocca: An Efficient AES-based Encryption Scheme for Beyond 5G
📺
Abstract

In this paper, we present an AES-based authenticated-encryption with associated-data scheme called Rocca, with the purpose to reach the requirements on the speed and security in 6G systems. To achieve ultra-fast software implementations, the basic design strategy is to take full advantage of the AES-NI and SIMD instructions as that of the AEGIS family and Tiaoxin-346. Although Jean and Nikolić have generalized the way to construct efficient round functions using only one round of AES (aesenc) and 128-bit XOR operation and have found several efficient candidates, there still seems to exist potential to further improve it regarding speed and state size. In order to minimize the critical path of one round, we remove the case of applying both aesenc and XOR in a cascade way for one round. By introducing a cost-free block permutation in the round function, we are able to search for candidates in a larger space without sacrificing the performance. Consequently, we obtain more efficient constructions with a smaller state size than candidates by Jean and Nikolić. Based on the newly-discovered round function, we carefully design the corresponding AEAD scheme with 256-bit security by taking several reported attacks on the AEGIS family and Tiaxion-346 into account. Our AEAD scheme can reach 138Gbps which is 4 times faster than the AEAD scheme of SNOW-V. Rocca is also much faster than other efficient schemes with 256-bit key length, e.g. AEGIS-256 and AES-256-GCM. As far as we know, Rocca is the first dedicated cryptographic algorithm targeting 6 systems, i.e., 256-bit key length and the speed of more than 100 Gbps.

2021

TOSC

Weak Keys in Reduced AEGIS and Tiaoxin
📺
Abstract

AEGIS-128 and Tiaoxin-346 (Tiaoxin for short) are two AES-based primitives submitted to the CAESAR competition. Among them, AEGIS-128 has been selected in the final portfolio for high-performance applications, while Tiaoxin is a third-round candidate. Although both primitives adopt a stream cipher based design, they are quite different from the well-known bit-oriented stream ciphers like Trivium and the Grain family. Their common feature consists in the round update function, where the state is divided into several 128-bit words and each word has the option to pass through an AES round or not. During the 6-year CAESAR competition, it is surprising that for both primitives there is no third-party cryptanalysis of the initialization phase. Due to the similarities in both primitives, we are motivated to investigate whether there is a common way to evaluate the security of their initialization phases. Our technical contribution is to write the expressions of the internal states in terms of the nonce and the key by treating a 128-bit word as a unit and then carefully study how to simplify these expressions by adding proper conditions. As a result, we find that there are several groups of weak keys with 296 keys each in 5-round AEGIS-128 and 8-round Tiaoxin, which allows us to construct integral distinguishers with time complexity 232 and data complexity 232. Based on the distinguisher, the time complexity to recover the weak key is 272 for 5-round AEGIS-128. However, the weak key recovery attack on 8-round Tiaoxin will require the usage of a weak constant occurring with probability 2−32. All the attacks reach half of the total number of initialization rounds. We expect that this work can advance the understanding of the designs similar to AEGIS and Tiaoxin.

2021

CRYPTO

Cryptanalysis of Full LowMC and LowMC-M with Algebraic Techniques
📺
Abstract

In this paper, we revisit the difference enumeration techniques for LowMC and develop new algebraic techniques to achieve efficient key-recovery attacks with negligible memory complexity. \mbox{Benefiting} from our technique to reduce the memory complexity, we could significantly improve the attacks on LowMC when the block size is much larger than the key size and even break LowMC with such a kind of parameter. On the other hand, with our new key-recovery technique, we could significantly improve the time to retrieve the full key if given only a single pair of input and output messages together with the difference trail that they take, which was stated as an interesting question by Rechberger et al. in ToSC 2018. Combining both the techniques, with only 2 chosen plaintexts, we could break 4 rounds of LowMC adopting a full S-Box layer with block size of 129, 192 and 255 bits, respectively, which are the 3 recommended parameters for Picnic3, an alternative \mbox{third-round} candidate in NIST's Post-Quantum Cryptography competition. We have to emphasize that our attacks do not indicate that Picnic3 is broken as the Picnic use-case is very different and an attacker cannot even freely choose 2 plaintexts to encrypt for a concrete LowMC instance. However, such parameters are deemed as secure in the latest LowMC. Moreover, much more rounds of seven instances of the backdoor cipher \mbox{LowMC-M} as proposed by Peyrin and Wang in CRYPTO 2020 can be broken without finding the backdoor by making full use of the allowed $2^{64}$ data. The above mentioned attacks are all achieved with negligible memory.

2021

TCHES

Yoroi: Updatable Whitebox Cryptography
📺
Abstract

Whitebox cryptography aims to provide security in the whitebox setting where the adversary has unlimited access to the implementation and its environment. In order to ensure security in the whitebox setting, it should prevent key extraction attacks and code-lifting attacks, in which the adversary steals the original cryptographic implementation instead of the key, and utilizes it as a big key. Although recent published ciphers such as SPACE, SPNbox, and Whiteblock successfully achieve security against the key extraction attacks, they only provide mitigation of codelifting attack by the so-called space hardness and incompressibility properties of the underlying tables as the space-hard/incompressible table might be eventually stolen by continuous leakage. The complete prevention of such attacks may need to periodically update the secret key. However, that entails high costs and might introduce an additional vulnerability into the system due to the necessity for the reencryption of all data by the updated key. In this paper, we introduce a new property, denominated longevity, for whitebox cryptography. This property enhances security against code-lifting attacks with continuous leakage by updating incompressible tables instead of the secret key. We propose a family of new whitebox-secure block ciphers Yoroi that has the longevity property in addition to the space hardness. By updating its implementation periodically, Yoroi provides constant security against code-lifting attacks without key updating. Moreover, the performance of Yoroi is competitive with existing ciphers implementations in the blackbox and whitebox context.

2021

ASIACRYPT

Algebraic Attacks on Rasta and Dasta Using Low-Degree Equations
📺
Abstract

Rasta and Dasta are two fully homomorphic encryption friendly symmetric-key primitives proposed at CRYPTO 2018 and ToSC 2020, respectively. We point out that the designers of Rasta and Dasta neglected an important property of the $\chi$ operation. Combined with the special structure of Rasta and Dasta, this property directly leads to significantly improved algebraic cryptanalysis. Especially, it enables us to theoretically break 2 out of 3 instances of full Agrasta, which is the aggressive version of Rasta with the block size only slightly larger than the security level in bits. We further reveal that Dasta is more vulnerable against our attacks than Rasta for its usage of a linear layer composed of an ever-changing bit permutation and a deterministic linear transform. Based on our cryptanalysis, the security margins of Dasta and Rasta parameterized with $(n,\kappa,r)\in\{(327,80,4),(1877,128,4),(3545,256,5)\}$ are reduced to only 1 round, where $n$, $\kappa$ and $r$ denote the block size, the claimed security level and the number of rounds, respectively. These parameters are of particular interest as the corresponding ANDdepth is the lowest among those that can be implemented in reasonable time and target the same claimed security level.

2021

TOSC

Perfect Trees: Designing Energy-Optimal Symmetric Encryption Primitives
📺
Abstract

Energy efficiency is critical in battery-driven devices, and designing energyoptimal symmetric-key ciphers is one of the goals for the use of ciphers in such environments. In the paper by Banik et al. (IACR ToSC 2018), stream ciphers were identified as ideal candidates for low-energy solutions. One of the main conclusions of this paper was that Trivium, when implemented in an unrolled fashion, was by far the most energy-efficient way of encrypting larger quantity of data. In fact, it was shown that as soon as the number of databits to be encrypted exceeded 320 bits, Trivium consumed the least amount of energy on STM 90 nm ASIC circuits and outperformed the Midori family of block ciphers even in the least energy hungry ECB mode (Midori was designed specifically for energy efficiency).In this work, we devise the first heuristic energy model in the realm of stream ciphers that links the underlying algebraic topology of the state update function to the consumptive behaviour. The model is then used to derive a metric that exhibits a heavy negative correlation with the energy consumption of a broad range of stream cipher architectures, i.e., the families of Trivium-like, Grain-like and Subterranean-like constructions. We demonstrate that this correlation is especially pronounced for Trivium-like ciphers which leads us to establish a link between the energy consumption and the security guarantees that makes it possible to find several alternative energy-optimal versions of Trivium that meet the requirements but consume less energy. We present two such designs Trivium-LE(F) and Trivium-LE(S) that consume around 15% and 25% less energy respectively making them the to date most energy-efficient encryption primitives. They inherit the same security level as Trivium, i.e., 80-bit security. We further present Triad-LE as an energy-efficient variant satisfying a higher security level. The simplicity and wide applicability of our model has direct consequences for the conception of future hardware-targeted stream ciphers as for the first time it is possible to optimize for energy during the design phase. Moreover, we extend the reach of our model beyond plain encryption primitives and propose a novel energy-efficient message authentication code Trivium-LE-MAC.

2020

TOSC

Cube-Based Cryptanalysis of Subterranean-SAE
📺
Abstract

Subterranean 2.0 designed by Daemen, Massolino and Rotella is a Round 2 candidate of the NIST Lightweight Cryptography Standardization process. In the official document of Subterranean 2.0, the designers have analyzed the state collisions in unkeyed absorbing by reducing the number of rounds to absorb the message from 2 to 1. However, little cryptanalysis of the authenticated encryption scheme Subterranean-SAE is made. For Subterranean-SAE, the designers introduce 8 blank rounds to separate the controllable input and output, and expect that 8 blank rounds can achieve a sufficient diffusion. Therefore, it is meaningful to investigate the security by reducing the number of blank rounds. Moreover, the designers make no security claim but expect a non-trivial effort to achieve full-state recovery in a nonce-misuse scenario. In this paper, we present the first practical full-state recovery attack in a nonce-misuse scenario with data complexity of 213 32-bit blocks. In addition, in a nonce-respecting scenario and if the number of blank rounds is reduced to 4, we can mount a key-recovery attack with 2122 calls to the internal permutation of Subterranean-SAE and 269.5 32-bit blocks. A distinguishing attack with 233 calls to the internal permutation of Subterranean-SAE and 233 32-bit blocks is achieved as well. Our cryptanalysis does not threaten the security claim for Subterranean-SAE and we hope it can enhance the understanding of Subterranean-SAE.

2020

CRYPTO

Automatic Verification of Differential Characteristics: Application to Reduced Gimli
📺
Abstract

Since Keccak was selected as the SHA-3 standard, more and more permutation-based primitives have been proposed. Different from block ciphers, there is no round key in the underlying permutation for permutation-based primitives. Therefore, there is a higher risk for a differential characteristic of the underlying permutation to become incompatible when considering the dependency of difference transitions over different rounds. However, in most of the MILP or SAT based models to search for differential characteristics, only the difference transitions are involved and are treated as independent in different rounds, which may cause that an invalid one is found for the underlying permutation. To overcome this obstacle, we are motivated to design a model which automatically avoids the inconsistency in the search for differential characteristics. Our technique is to involve both the difference transitions and value transitions in the constructed model. Such an idea is inspired by the algorithm to find SHA-2 characteristics as proposed by Mendel et al. in ASIACRYPT 2011, where the differential characteristic and the conforming message pair are simultaneously searched. As a first attempt, our new technique will be applied to the Gimli permutation, which was proposed in CHES 2017. As a result, we reveal that some existing differential characteristics of reduced Gimli are indeed incompatible, one of which is found in the Gimli document. In addition, since only the permutation is analyzed in the Gimli document, we are lead to carry out a comprehensive study, covering the proposed hash scheme and the authenticated encryption (AE) scheme specified for Gimli, which has become a second round candidate of the NIST lightweight cryptography standardization process. For the hash scheme, a semi-free-start (SFS) collision attack can reach up to 8 rounds starting from an intermediate round. For the AE scheme, a state recovery attack is demonstrated to achieve up to 9 rounds. It should be emphasized that our analysis does not threaten the security of Gimli.

2019

CRYPTO

Efficient Collision Attack Frameworks for RIPEMD-160
📺
Abstract

RIPEMD-160 is an ISO/IEC standard and has been applied to generate the Bitcoin address with SHA-256. Due to the complex dual-stream structure, the first collision attack on reduced RIPEMD-160 presented by Liu, Mendel and Wang at Asiacrypt 2017 only reaches 30 steps, having a time complexity of $$2^{70}$$. Apart from that, several semi-free-start collision attacks have been published for reduced RIPEMD-160 with the start-from-the-middle method. Inspired from such start-from-the middle structures, we propose two novel efficient collision attack frameworks for reduced RIPEMD-160 by making full use of the weakness of its message expansion. Those two frameworks are called dense-left-and-sparse-right (DLSR) framework and sparse-left-and-dense-right (SLDR) framework. As it turns out, the DLSR framework is more efficient than SLDR framework since one more step can be fully controlled, though with extra $$2^{32}$$ memory complexity. To construct the best differential characteristics for the DLSR framework, we carefully build the linearized part of the characteristics and then solve the corresponding nonlinear part using a guess-and-determine approach. Based on the newly discovered differential characteristics, we provide colliding messages pairs for the first practical collision attacks on 30 and 31 (out of 80) steps of RIPEMD-160 with time complexity $$2^{35.9}$$ and $$2^{41.5}$$ respectively. In addition, benefiting from the partial calculation, we can attack 33 and 34 (out of 80) steps of RIPEMD-160 with time complexity $$2^{67.1}$$ and $$2^{74.3}$$ respectively. When applying the SLDR framework to the differential characteristic used in the Asiacrypt 2017 paper, we significantly improve the time complexity by a factor of $$2^{13}$$. However, it still cannot compete with the results obtained from the DLSR framework. To the best of our knowledge, these are the best collision attacks on reduced RIPEMD-160 with respect to the number of steps, including the first colliding message pairs for 30 and 31 steps of RIPEMD-160.

2019

TOSC

Cryptanalysis of Plantlet
📺
Abstract

Plantlet is a lightweight stream cipher designed by Mikhalev, Armknecht and Müller in IACR ToSC 2017. It has a Grain-like structure with two state registers of size 40 and 61 bits. In spite of this, the cipher does not seem to lose in security against generic Time-Memory-Data Tradeoff attacks due to the novelty of its design. The cipher uses a 80-bit secret key and a 90-bit IV. In this paper, we first present a key recovery attack on Plantlet that requires around 276.26 Plantlet encryptions. The attack leverages the fact that two internal states of Plantlet that differ in the 43rd LFSR location are guaranteed to produce keystream that are either equal or unequal in 45 locations with probability 1. Thus an attacker can with some probability guess that when 2 segments of keystream blocks possess the 45 bit difference just mentioned, they have been produced by two internal states that differ only in the 43rd LFSR location. Thereafter by solving a system of polynomial equations representing the keystream bits, the attacker can find the secret key if his guess was indeed correct, or reach some kind of contradiction if his guess was incorrect. In the latter event, he would repeat the procedure for other keystream blocks with the given difference. We show that the process when repeated a finite number of times, does indeed yield the value of the secret key.
In the second part of the paper, we observe that the previous attack was limited to internal state differences that occurred at time instances that were congruent to 0 mod 80. We further observe that by generalizing the attack to include internal state differences that are congruent to all equivalence classed modulo 80, we lower the total number of keystream bits required to perform the attack and in the process reduce the attack complexity to 269.98 Plantlet encryptions.

2019

TOSC

New Semi-Free-Start Collision Attack Framework for Reduced RIPEMD-160
📺
Abstract

RIPEMD-160 is a hash function published in 1996, which shares similarities with other hash functions designed in this time-period like MD4, MD5 and SHA-1. However, for RIPEMD-160, no (semi-free-start) collision attacks on the full number of steps are known. Hence, it is still used, e.g., to generate Bitcoin addresses together with SHA-256, and is an ISO/IEC standard. Due to its dual-stream structure, even semifree- start collision attacks starting from the first step only reach 36 steps, which were firstly shown by Mendel et al. at Asiacrypt 2013 and later improved by Liu, Mendel and Wang at Asiacrypt 2017. Both of the attacks are based on a similar freedom degree utilization technique as proposed by Landelle and Peyrin at Eurocrypt 2013. However, the best known semi-free-start collision attack on 36 steps of RIPEMD-160 presented at Asiacrypt 2017 still requires 255.1 time and 232 memory. Consequently, a practical semi-free-start collision attack for the first 36 steps of RIPEMD-160 still requires a significant amount of resources. Considering the structure of these previous semi-free-start collision attacks for 36 steps of RIPEMD-160, it seems hard to extend it to more steps. Thus, we develop a different semi-free-start collision attack framework for reduced RIPEMD-160 by carefully investigating the message expansion of RIPEMD-160. Our new framework has several advantages. First of all, it allows to extend the attacks to more steps. Second, the memory complexity of the attacks is negligible. Hence, we were able to mount semi-free-start collision attacks on 36 and 37 steps of RIPEMD-160 with practical time complexity 241 and 249 respectively. Additionally, we describe semi-free-start collision attacks on 38 and 40 (out of 80) steps of RIPEMD-160 with time complexity 252 and 274.6, respectively. To the best of our knowledge, these are the best semi-free-start collision attacks for RIPEMD-160 starting from the first step with respect to the number of steps, including the first practical colliding message pairs for 36 and 37 steps of RIPEMD-160.

2018

CRYPTO

Fast Correlation Attack Revisited
📺
Abstract

A fast correlation attack (FCA) is a well-known cryptanalysis technique for LFSR-based stream ciphers. The correlation between the initial state of an LFSR and corresponding key stream is exploited, and the goal is to recover the initial state of the LFSR. In this paper, we revisit the FCA from a new point of view based on a finite field, and it brings a new property for the FCA when there are multiple linear approximations. Moreover, we propose a novel algorithm based on the new property, which enables us to reduce both time and data complexities. We finally apply this technique to the Grain family, which is a well-analyzed class of stream ciphers. There are three stream ciphers, Grain-128a, Grain-128, and Grain-v1 in the Grain family, and Grain-v1 is in the eSTREAM portfolio and Grain-128a is standardized by ISO/IEC. As a result, we break them all, and especially for Grain-128a, the cryptanalysis on its full version is reported for the first time.

2018

CRYPTO

Improved Division Property Based Cube Attacks Exploiting Algebraic Properties of Superpoly
📺
Abstract

The cube attack is an important technique for the cryptanalysis of symmetric key primitives, especially for stream ciphers. Aiming at recovering some secret key bits, the adversary reconstructs a superpoly with the secret key bits involved, by summing over a set of the plaintexts/IV which is called a cube. Traditional cube attack only exploits linear/quadratic superpolies. Moreover, for a long time after its proposal, the size of the cubes has been largely confined to an experimental range, e.g., typically 40. These limits were first overcome by the division property based cube attacks proposed by Todo et al. at CRYPTO 2017. Based on MILP modelled division property, for a cube (index set) I, they identify the small (index) subset J of the secret key bits involved in the resultant superpoly. During the precomputation phase which dominates the complexity of the cube attacks, $$2^{|I|+|J|}$$2|I|+|J| encryptions are required to recover the superpoly. Therefore, their attacks can only be available when the restriction $$|I|+|J|<n$$|I|+|J|<n is met.In this paper, we introduced several techniques to improve the division property based cube attacks by exploiting various algebraic properties of the superpoly.
1.We propose the “flag” technique to enhance the preciseness of MILP models so that the proper non-cube IV assignments can be identified to obtain a non-constant superpoly.2.A degree evaluation algorithm is presented to upper bound the degree of the superpoly. With the knowledge of its degree, the superpoly can be recovered without constructing its whole truth table. This enables us to explore larger cubes I’s even if $$|I|+|J|\ge n$$|I|+|J|≥n.3.We provide a term enumeration algorithm for finding the monomials of the superpoly, so that the complexity of many attacks can be further reduced.
As an illustration, we apply our techniques to attack the initialization of several ciphers. To be specific, our key recovery attacks have mounted to 839-round Trivium, 891-round Kreyvium, 184-round Grain-128a and 750-round Acornrespectively.

2018

TOSC

Towards Low Energy Stream Ciphers
📺
Abstract

Energy optimization is an important design aspect of lightweight cryptography. Since low energy ciphers drain less battery, they are invaluable components of devices that operate on a tight energy budget such as handheld devices or RFID tags. At Asiacrypt 2015, Banik et al. presented the block cipher family Midori which was designed to optimize the energy consumed per encryption and which reduces the energy consumption by more than 30% compared to previous block ciphers. However, if one has to encrypt/decrypt longer streams of data, i.e. for bulk data encryption/decryption, it is expected that a stream cipher should perform even better than block ciphers in terms of energy required to encrypt. In this paper, we address the question of designing low energy stream ciphers. To this end, we analyze for common stream cipher design components their impact on the energy consumption. Based on this, we give arguments why indeed stream ciphers allow for encrypting long data streams with less energy than block ciphers and validate our findings by implementations. Afterwards, we use the analysis results to identify energy minimizing design principles for stream ciphers.

2018

TOSC

ShiftRows Alternatives for AES-like Ciphers and Optimal Cell Permutations for Midori and Skinny
📺
Abstract

We study possible alternatives for ShiftRows to be used as cell permutations in AES-like ciphers. As observed during the design process of the block cipher Midori, when using a matrix with a non-optimal branch number for the MixColumns operation, the choice of the cell permutation, i.e., an alternative for ShiftRows, can actually improve the security of the primitive. In contrast, when using an MDS matrix it is known that one cannot increase the minimum number of active S-boxes by deviating from the ShiftRows-type permutation. However, finding the optimal choice for the cell permutation for a given, non-optimal, MixColumns operation is a highly non-trivial problem. In this work, we propose techniques to speed up the search for the optimal cell permutations significantly. As case studies, we apply those techniques to Midori and Skinny and provide possible alternatives for their cell permutations. We finally state an easy-to-verify sufficient condition on a cell permutation, to be used as an alternative in Midori, that attains a high number of active S-boxes and thus provides good resistance against differential and linear attacks.

2017

TOSC

Analysis of Software Countermeasures for Whitebox Encryption
Abstract

Whitebox cryptography aims to ensure the security of cryptographic algorithms in the whitebox model where the adversary has full access to the execution environment. To attain security in this setting is a challenging problem: Indeed, all published whitebox implementations of standard symmetric-key algorithms such as AES to date have been practically broken. However, as far as we know, no whitebox implementation in real-world products has suffered from a key recovery attack. This is due to the fact that commercial products deploy additional software protection mechanisms on top of the whitebox implementation. This makes practical attacks much less feasible in real-world applications. There are numerous software protection mechanisms which protect against standard whitebox attacks. One such technique is control flow obfuscation which randomizes the order of table lookups for each execution of the whitebox encryption module. Another technique is randomizing the locations of the various Look up tables (LUTs) in the memory address space. In this paper we investigate the effectiveness of these countermeasures against two attack paradigms. The first known as Differential Computational Analysis (DCA) attack was developed by Bos, Hubain, Michiels and Teuwen in CHES 2016. The attack passively collects software execution traces for several plaintext encryptions and uses the collected data to perform an analysis similar to the well known differential power attacks (DPA) to recover the secret key. Since the software execution traces contain time demarcated physical addresses of memory locations being read/written into, they essentially leak the values of the inputs to the various LUTs accessed during the whitebox encryption operation, which as it turns out leaks sufficient information to perform the power attack. We found that if in addition to control flow obfuscation, one were to randomize the locations of the LUTs in the memory, then it is very difficult to perform the DCA on the resultant system using such table inputs and extract the secret key in reasonable time. As an alternative, we investigate the version of the DCA attack which uses the outputs of the tables instead of the inputs to mount the power analysis attack. This modified DCA is able to extract the secret key from the flow obfuscated and location randomized versions of several whitebox binaries available in crypto literature. We develop another attack called the Zero Difference Enumeration (ZDE) attack. The attack records software traces for several pairs of strategically selected plaintexts and performs a simple statistical test on the effective difference of the traces to extract the secret key. We show that ZDE is able to recover the keys of whitebox systems. Finally we propose a new countermeasure for protecting whitebox binaries based on insertion of random delays which aims to make both the ZDE and DCA attackspractically difficult by adding random noise in the information leaked to the attacker.

2017

TOSC

Some cryptanalytic results on Lizard
Abstract

Lizard is a lightweight stream cipher proposed by Hamann, Krause and Meier in IACR ToSC 2017. It has a Grain-like structure with two state registers of size 90 and 31 bits. The cipher uses a 120-bit secret key and a 64-bit IV. The authors claim that Lizard provides 80-bit security against key recovery attacks and a 60-bit security against distinguishing attacks. In this paper, we present an assortment of results and observations on Lizard. First, we show that by doing 258 random trials it is possible to find a set of 264 triplets (K, IV0, IV1) such that the Key-IV pairs (K, IV0) and (K, IV1) produce identical keystream bits. Second, we show that by performing only around 228 random trials it is possible to obtain 264 Key-IV pairs (K0, IV0) and (K1, IV1) that produce identical keystream bits. Thereafter, we show that one can construct a distinguisher for Lizard based on IVs that produce shifted keystream sequences. The process takes around 251.5 random IV encryptions (with encryption required to produce 218 keystream bits) and around 276.6 bits of memory. Next, we propose a key recovery attack on a version of Lizard with the number of initialization rounds reduced to 223 (out of 256) based on IV collisions. We then outline a method to extend our attack to 226 rounds. Our results do not affect the security claims of the designers.

2012

FSE

#### Program Committees

- FSE 2022
- FSE 2020
- Asiacrypt 2020
- FSE 2019
- Asiacrypt 2019
- Asiacrypt 2018
- FSE 2016

#### Coauthors

- Toru Akishita (2)
- Gianira N. Alfarano (1)
- Kazumaro Aoki (1)
- Frederik Armknecht (1)
- Subhadeep Banik (9)
- Khashayar Barooti (1)
- Christof Beierle (1)
- Andrey Bogdanov (5)
- Andrea Caforio (2)
- Zhenfu Cao (2)
- Tingting Cui (1)
- Christoph Dobraunig (2)
- Jian Guo (1)
- Yonglin Hao (2)
- Harunaga Hiwatari (2)
- Akinori Hosoyamada (1)
- Ryoma Ito (1)
- Martin Bjerregaard Jepsen (1)
- Shinsaku Kiyomoto (1)
- Yuji Koike (1)
- Stefan Kölbl (1)
- Gregor Leander (1)
- Chaoyun Li (1)
- Ji Li (1)
- Fukang Liu (15)
- Willi Meier (15)
- Florian Mendel (2)
- Vasily Mikhalev (1)
- Kazuhiko Minematsu (1)
- Atsushi Mitsuda (1)
- Masakatu Morii (1)
- Yuto Nakano (1)
- Toshihiro Ohigashi (1)
- Francesco Regazzoni (2)
- Kosei Sakamoto (4)
- Santanu Sarkar (5)
- Kyoji Shibutani (7)
- Taizo Shirai (1)
- Elmar Tischhauser (1)
- Yosuke Todo (6)
- Qingju Wang (1)
- Gaoli Wang (4)
- Yuhei Watanabe (2)
- Kan Yasuda (1)
- Bin Zhang (2)