International Association
for Cryptologic Research

The idea of hybrid homomorphic encryption (HHE) is to drastically reduce bandwidth requirements when using homomorphic encryption (HE) at the cost of more expensive computations in the encrypted domain. To this end, various dedicated schemes for symmetric encryption have already been proposed. However, it is still unclear if those ideas are already practically useful, because (1) no cost-benefit analysis was done for use cases and (2) very few implementations are publicly available. We address this situation in several ways. We build an open-source benchmarking r framework, we explore properties of the respective HHE proposals. It turns out that even medium-sized use cases are infeasible, especially when involving integer arithmetic. Next, we propose Pasta, a cipher thoroughly optimized for integer HHE use cases. Pasta is designed to minimize the multiplicative depth, while also leveraging the structure of two state-of-the-art integer HE schemes (BFV and BGV) to minimize the homomorphic evaluation latency. Using our new benchmarking environment, we extensively evaluate Pasta in SEAL and HElib and compare its properties to 8 existing ciphers in two use cases. Our evaluations show that Pasta outperforms its competitors for HHE both in terms of homomorphic evaluation time and noise consumption, showing its efficiency for applications in real-world HE use cases. Concretely, Pasta outperforms Agrasta by a factor of up to 82, Masta by a factor of up to 6 and Hera up to a factor of 11 when applied to the two use cases.

Universal hash functions play a prominent role in the design of message authentication codes and the like. Whereas it is known how to build highly efficient sequential universal hash functions, parallel non-algebraic universal hash function designs are always built on top of a PRP. In such case, one employs a relatively strong primitive to obtain a function with a relatively weak security model. In this work, we present EliHash, a construction of a parallel universal hash function from non-compressing universal hash functions, and we back it up with supporting security analysis. We use this construction to design EliMAC, a message authentication code similar to LightMAC. We consider a heuristic instantiation of EliMAC with roundreduced AES, and argue that this instantiation of EliMAC is much more efficient than LightMAC, it is around 21% faster, and additionally allows for precomputation of the keys, albeit with a stronger assumption on the AES primitive than in LightMAC. These observations are backed up with an implementation of our scheme.

Differential fault analysis (DFA) is a very powerful attack vector on implementations of symmetric cryptography. Most countermeasures are applied at the implementation level. At ASIACRYPT 2021, Baksi et al. proposed a design strategy that aims to provide inherent cipher level resistance against DFA by using S-boxes with linear structures. They argue that in their instantiation, the block cipher DEFAULT, a DFA adversary can learn at most 64 of the 128 key bits, so the remaining brute-force complexity of 2^64 is impractical. In this paper, we show that a DFA adversary can combine information across rounds to recover the full key, invalidating their security claim. In particular, we observe that such ciphers exhibit large classes of equivalent keys that can be represented efficiently in normalized form using linear equations. We exploit this in combination with the specifics of DEFAULT's strong key schedule to recover the key using less than 100 faulty computation and negligible time complexity. Moreover, we show that even an idealized version of DEFAULT with independent round keys is vulnerable to our information-combining attacks based on normalized keys.

Recently, a memory safety concept called Cryptographic Capability Computing (C3) has been proposed. C3 is the first memory safety mechanism that works without requiring extra storage for metadata and hence, has the potential to significantly enhance the security of modern IT-systems at a rather low cost. To achieve this, C3 heavily relies on ultra-low-latency cryptographic primitives. However, the most crucial primitive required by C3 demands uncommon dimensions. To partially encrypt 64-bit pointers, a 24-bit tweakable block cipher with a 40-bit tweak is needed. The research on low-latency tweakable block ciphers with such small dimensions is not very mature. Therefore, designing such a cipher provides a great research challenge, which we take on with this paper. As a result, we present BipBip, a 24-bit tweakable block cipher with a 40-bit tweak that allows for ASIC implementations with a latency of 3 cycles at a 4.5 GHz clock frequency on a modern 10 nm CMOS technology.

Authenticated encryption satisfies the basic need for authenticity and confidentiality in our information infrastructure. In this paper, we provide the specification of Ascon -128 and Ascon -128a. Both authenticated encryption algorithms provide efficient authenticated encryption on resource-constrained devices and on high-end CPUs. Furthermore, they have been selected as the “primary choice” for lightweight authenticated encryption in the final portfolio of the CAESAR competition. In addition, we specify the hash function Ascon-Hash , and the extendable output function Ascon-Xof . Moreover, we complement the specification by providing a detailed overview of existing cryptanalysis and implementation results.

Side-channel attacks are a threat to secrets stored on a device, especially if an adversary has physical access to the device. As an effect of this, countermeasures against such attacks for cryptographic algorithms are a well-researched topic. In this work, we deviate from the study of cryptographic algorithms and instead focus on the side-channel protection of a much more basic operation, the comparison of a known attacker-controlled value with a secret one. Comparisons sensitive to side-channel leakage occur in tag comparisons during the verification of message authentication codes (MACs) or authenticated encryption, but are typically omitted in security analyses. Besides, also comparisons performed as part of fault countermeasures might be sensitive to side-channel attacks. In this work, we present a formal analysis on comparing values in a leakage resilient manner by utilizing cryptographic building blocks that are typically part of an implementation anyway. Our results indicate that there is no need to invest additional resources into implementing a protected comparison operation itself if a sufficiently protected implementation of a public cryptographic permutation, or a (tweakable) block cipher, is already available. We complement our contribution by applying our findings to the SuKS message authentication code used by lightweight authenticated encryption scheme ISAP, and to the classical Hash-then-PRF construction.

Motivated by new applications such as secure Multi-Party Computation (MPC), Fully Homomorphic Encryption (FHE), and Zero-Knowledge proofs (ZK), the need for symmetric encryption schemes that minimize the number of field multiplications in their natural algorithmic description is apparent. This development has brought forward many dedicated symmetric encryption schemes that minimize the number of multiplications in GF(2^n) or GF(p), with p being prime. These novel schemes have lead to new cryptanalytic insights that have broken many of said schemes. Interestingly, to the best of our knowledge, all of the newly proposed schemes that minimize the number of multiplications use those multiplications exclusively in S-boxes based on a power mapping that is typically x^3 or x^{-1}. Furthermore, most of those schemes rely on complex and resource-intensive linear layers to achieve a low multiplication count. In this paper, we present Ciminion, an encryption scheme minimizing the number of field multiplications in large binary or prime fields, while using a very lightweight linear layer. In contrast to other schemes that aim to minimize field multiplications in GF(2^n) or GF(p), Ciminion relies on the Toffoli gate to improve the non-linear diffusion of the overall design. In addition, we have tailored the primitive for the use in a Farfalle-like construction in order to minimize the number of rounds of the used primitive, and hence, the number of field multiplications as far as possible.

We formalize and analyze the general suffix keyed sponge construction, a pseudorandom function built on top of a cryptographic permutation. The construction hashes its data using the (keyless) sponge construction, transforms part of the state using the secret key, and generates the tag from the output of a final permutation call. In its simplest form, if the key and tag size are at most the rate of the sponge, one can see the suffix keyed sponge as a simple sponge function evaluation whose input is the plaintext appended with the key. The suffix keyed sponge is, however, much more general: the key and tag size may exceed the rate without any need to make extra permutation calls. We prove that the suffix keyed sponge construction achieves birthday-bound PRF security in the capacity, even if key and tag size exceed the rate. Furthermore, we prove that if the absorption of the key into the state happens in a leakage resilient manner, the suffix keyed sponge itself is leakage resilient as well. Our findings show that the suffix keyed sponge compares favorably with the hash-then-MAC construction. For instance, to reach a security level of k bits, the side-channel protected component in the suffix keyed sponge just needs to process k bits of input besides the key, whereas schemes following the hash-then-MAC construction need a side-channel protected MAC function that processes 2k bits of input besides the key. Moreover, even if we just consider black-box attacks, the MAC function in a hash-then-MAC scheme needs to be cryptographically strong whereas in the suffix keyed sponge the key may be absorbed by a simple XOR. The security proofs are performed using the H-coefficient technique, and make effective use of the multicollision limit function results of Daemen et al. (ASIACRYPT 2017), both for arguing that state manipulation larger than the rate is tolerated after key processing and for upper bounding the amount of leakage an attacker may gain about the secret key.

Cryptographic competitions, like the ongoing NIST call for lightweight cryptography, always provide a thriving research environment, where new interesting ideas are proposed and new cryptographic insights are made. One proposal for this NIST call that is accepted for the second round is Pyjamask. Pyjamask is an authenticated encryption scheme that builds upon two block ciphers, Pyjamask-96 and Pyjamask-128, that aim to minimize the number of AND operations at the cost of a very strong linear layer. A side-effect of this goal is a slow growth in the algebraic degree. In this paper, we focus on the block cipher Pyjamask-96 and are able to provide a theoretical key-recovery attack reaching 14 (out of 14) rounds as well as a practical attack on 8 rounds. We do this by combining higher-order differentials with an in-depth analysis of the system of equations gotten for 2.5 rounds of Pyjamask-96. The AEAD-scheme Pyjamask itself is not threatened by the work in this paper.

Statistical Ineffective Fault Attacks (SIFA) pose a threat for many practical implementations of symmetric primitives. Countermeasures against both power analysis and fault attacks typically do not prevent straightforward SIFA attacks, which require only very limited knowledge about the concrete implementation. Therefore, the exploration of countermeasures against SIFA that do not rely on protocols or physical protection mechanisms is of great interest. In this paper, we describe different countermeasure strategies against SIFA. First, we introduce an abstraction layer between the algorithmic specification of a cipher and its implementation in hardware or software to study and describe resistance against SIFA. We then show that by basing the masked implementation on permutations as building blocks, we can build circuits that withstand single-fault SIFA and DPA attacks. We show how this approach can be applied to 3-bit, 4-bit, and 5-bit S-boxes and the AES S-box. Additionally, we present a strategy based on fine-grained fault detection suitable for protecting any circuit against SIFA attacks. Although this approach may lead to a higher implementation cost due to the fine-grained detection needed, it can be used to protect arbitrary circuits and can be generalized to cover multi-fault SIFA. For single-fault SIFA protection, our countermeasures only have a small computational overhead compared to a simple combination of masking and duplication.

With the trend to connect more and more devices to the Internet, authenticated encryption has become a major backbone in securing the communication, not only between these devices and servers, but also the direct communication among these devices. Most authenticated encryption algorithms used in practice are developed to perform well on modern high-end devices, but are not necessarily suited for usage on resource-constrained devices. We present a lightweight authenticated encryption scheme, called Elephant. Elephant retains the advantages of GCM such as parallelism, but is tailored to the needs of resource-constrained devices. The two smallest instances of Elephant, Dumbo and Jumbo, are based on the 160-bit and 176-bit Spongent permutation, respectively, and are particularly suited for hardware; the largest instance of Elephant, Delirium, is based on 200-bit Keccak and is developed towards software use. All three instances are parallelizable, have a small state size while achieving a high level of security, and are constant time by design.

We specify Isap v2.0, a lightweight permutation-based authenticated encryption algorithm that is designed to ease protection against side-channel and fault attacks. This design is an improved version of the previously published Isap v1.0, and offers increased protection against implementation attacks as well as more efficient implementations. Isap v2.0 is a candidate in NIST’s LightWeight Cryptography (LWC) project, which aims to identify and standardize authenticated ciphers that are well-suited for applications in constrained environments. We provide a self-contained specification of the new Isap v2.0 mode and discuss its design rationale. We formally prove the security of the Isap v2.0 mode in the leakage-resilient setting. Finally, in an extensive implementation overview, we show that Isap v2.0 can be implemented securely with very low area requirements. https://isap.iaik.tugraz.at

Generic attacks are a vital ingredient in the evaluation of the tightness of security proofs. In this paper, we evaluate the tightness of the suffix keyed sponge (SuKS) bound. As its name suggests, SuKS is a sponge-based construction that absorbs the key after absorbing the data, but before producing an output. This absorption of the key can be done via an easy to invert operation, like an XOR, or a hard to invert operation, like a PRF. Using SuKS with a hard to invert absorption provides benefits with respect to its resistance against side-channel attacks, and such a construction is used as part of the authenticated encryption scheme Isap. We derive two key recovery attacks against SuKS with easy to invert key absorption, and a forgery in case of hard to invert key absorption. The attacks closely match the terms in the PRF security bound of SuKS by Dobraunig and Mennink, ToSC 2019(4), and therewith show that these terms are justified, even if the function used to absorb the key is a PRF, and regardless of whether SuKS is used as a PRF or a MAC.

The design and analysis of dedicated tweakable block ciphers is a quite recent and very active research field that provides an ongoing stream of new insights. For instance, results of Kranz, Leander, and Wiemer from FSE 2017 show that the addition of a tweak using a linear tweak schedule does not introduce new linear characteristics. In this paper, we consider – to the best of our knowledge – for the first time the effect of the tweak on zero-correlation linear cryptanalysis for ciphers that have a linear tweak schedule. It turns out that the tweak can often be used to get zero-correlation linear hulls covering more rounds compared to just searching zero-correlation linear hulls on the data-path of a cipher. Moreover, this also implies the existence of integral distinguishers on the same number of rounds. We have applied our technique on round reduced versions of Qarma, Mantis, and Skinny. As a result, we can present – to the best of our knowledge – the best attack (with respect to number of rounds) on a round-reduced variant of Qarma.

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.

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.

Side-channel attacks, especially differential power analysis (DPA), pose a serious threat to cryptographic implementations deployed in a malicious environment. One way to counter side-channel attacks is to design cryptographic schemes to withstand them, an area that is covered amongst others by leakage resilient cryptography. So far, however, leakage resilient cryptography has predominantly focused on block cipher based designs, and insights in permutation based leakage resilient cryptography are scarce. In this work, we consider leakage resilience of the keyed duplex construction: we present a model for leakage resilient duplexing, derive a fine-grained bound on the security of the keyed duplex in said model, and map it to ideas of Taha and Schaumont (HOST 2014) and Dobraunig et al. (ToSC 2017) in order to use the duplex in a leakage resilient manner.

Recent developments in multi party computation (MPC) and fully homomorphic encryption (FHE) promoted the design and analysis of symmetric cryptographic schemes that minimize multiplications in one way or another. In this paper, we propose with Rastaa design strategy for symmetric encryption that has ANDdepth d and at the same time only needs d ANDs per encrypted bit. Even for very low values of d between 2 and 6 we can give strong evidence that attacks may not exist. This contributes to a better understanding of the limits of what concrete symmetric-key constructions can theoretically achieve with respect to AND-related metrics, and is to the best of our knowledge the first attempt that minimizes both metrics simultaneously. Furthermore, we can give evidence that for choices of d between 4 and 6 the resulting implementation properties may well be competitive by testing our construction in the use-case of removing the large ciphertext-expansion when using the BGV scheme.

Since the seminal work of Boneh et al., the threat of fault attacks has been widely known and techniques for fault attacks and countermeasures have been studied extensively. The vast majority of the literature on fault attacks focuses on the ability of fault attacks to change an intermediate value to a faulty one, such as differential fault analysis (DFA), collision fault analysis, statistical fault attack (SFA), fault sensitivity analysis, or differential fault intensity analysis (DFIA). The other aspect of faults—that faults can be induced and do not change a value—has been researched far less. In case of symmetric ciphers, ineffective fault attacks (IFA) exploit this aspect. However, IFA relies on the ability of an attacker to reliably induce reproducible deterministic faults like stuck-at faults on parts of small values (e.g., one bit or byte), which is often considered to be impracticable.As a consequence, most countermeasures against fault attacks do not focus on such attacks, but on attacks exploiting changes of intermediate values and usually try to detect such a change (detection-based), or to destroy the exploitable information if a fault happens (infective countermeasures). Such countermeasures implicitly assume that the release of “fault-free” ciphertexts in the presence of a fault-inducing attacker does not reveal any exploitable information. In this work, we show that this assumption is not valid and we present novel fault attacks that work in the presence of detection-based and infective countermeasures. The attacks exploit the fact that intermediate values leading to “fault-free” ciphertexts show a non-uniform distribution, while they should be distributed uniformly. The presented attacks are entirely practical and are demonstrated to work for software implementations of AES and for a hardware co-processor. These practical attacks rely on fault induction by means of clock glitches and hence, are achieved using only low-cost equipment. This is feasible because our attack is very robust under noisy fault induction attempts and does not require the attacker to model or profile the exact fault effect. We target two types of countermeasures as examples: simple time redundancy with comparison and several infective countermeasures. However, our attacks can be applied to a wider range of countermeasures and are not restricted to these two countermeasures.

Implementation attacks like side-channel and fault attacks are a threat to deployed devices especially if an attacker has physical access. As a consequence, devices like smart cards and IoT devices usually provide countermeasures against implementation attacks, such as masking against side-channel attacks and detection-based countermeasures like temporal or spacial redundancy against fault attacks. In this paper, we show how to attack implementations protected with both masking and detection-based fault countermeasures by using statistical ineffective fault attacks using a single fault induction per execution. Our attacks are largely unaffected by the deployed protection order of masking and the level of redundancy of the detection-based countermeasure. These observations show that the combination of masking plus error detection alone may not provide sufficient protection against implementation attacks.

Side-channel attacks and in particular differential power analysis (DPA) attacks pose a serious threat to cryptographic implementations. One approach to counteract such attacks are cryptographic schemes based on fresh re-keying. In settings of pre-shared secret keys, such schemes render DPA attacks infeasible by deriving session keys and by ensuring that the attacker cannot collect side-channel leakage on the session key during cryptographic operations with different inputs. While these schemes can be applied to secure standard communication settings, current re-keying approaches are unable to provide protection in settings where the same input needs to be processed multiple times. In this work, we therefore adapt the re-keying approach and present a symmetric authenticated encryption scheme that is secure against DPA attacks and that does not have such a usage restriction. This means that our scheme fully complies with the requirements given in the CAESAR call and hence, can be used like other noncebased authenticated encryption schemes without loss of side-channel protection. Its resistance against side-channel analysis is highly relevant for several applications in practice, like bulk storage settings in general and the protection of FPGA bitfiles and firmware images in particular.

MANTIS is a lightweight tweakable block cipher published at CRYPTO 2016. In addition to the full 14-round version, MANTIS7, the designers also propose an aggressive 10-round version, MANTIS5. The security claim for MANTIS5 is resistance against “practical attacks”, defined as related-tweak attacks with data complexity 2d less than 230 chosen plaintexts (or 240 known plaintexts), and computational complexity at most 2126−d. We present a key-recovery attack against MANTIS5 with 228 chosen plaintexts and a computational complexity of about 238 block cipher calls, which violates this claim. Our attack is based on a family of differential characteristics and exploits several properties of the lightweight round function and tweakey schedule. To verify the validity of the attack, we also provide a practical implementation which recovers the full key in about 1 core hour using 230 chosen plaintexts.