International Association
for Cryptologic Research

A white-box cryptographic implementation is to defend against white-box attacks that allow access and modification of memory or internal resources in the computing device. In particular, linear and non-linear transformations applied to this table-based cryptographic implementation are used to prevent key-dependent intermediate values from being seen by white-box attackers. However, it has been shown that there is a correlation before and after the linear and non-linear transformations, so that even a gray-box attacker can reveal secret keys hidden in a white-box cryptographic implementation. In this paper, we focus on the problem of linear transformations including the characteristics of block invertible binary matrices and the distribution of intermediate values. Based on our observation, we find out that a random byte insertion in the intermediate values before linear transformations can eliminate a problematic correlation to the key, and propose our white-box AES implementation using this principle. Our proposed implementations reduce the memory requirement by at most 33 percent compared to the masked implementations and also slightly reduce the number of table lookups. In addition, our method is a non-masking technique and does not require a static or dynamic run-time random source, unlike the existing gray-box (power analysis) countermeasures.

Little theoretical work has been done on $(n,m)$-functions when $\frac {n}{2}<m<n$, even though these functions can be used in Feistel ciphers, and actually play an important role in several block ciphers. Nyberg has shown that the differential uniformity of such functions is bounded below by $2^{n-m}+2$ if $n$ is odd or if $m>\frac {n}{2}$. In this paper, we first characterize the differential uniformity of those $(n,m)$-functions of the form $F(x,z)=\phi(z)I(x)$, where $I(x)$ is the $(m,m)$-Inverse function and $\phi(z)$ is an $(n-m,m)$-function. Using this characterization, we construct an infinite family of differentially $\Delta$-uniform $(2m-1,m)$-functions with $m\geq 3$ achieving Nyberg's bound with equality, which also have high nonlinearity and not too low algebraic degree. We then discuss an infinite family of differentially $4$-uniform $(m+1,m)$-functions in this form, which leads to many differentially $4$-uniform permutations. We also present a method to construct infinite families of $(m+k,m)$-functions with low differential uniformity and construct an infinite family of $(2m-2,m)$-functions with $\Delta\leq2^{m-1}-2^{m-6}+2$ for any $m\geq 8$. The constructed functions in this paper may provide more choices for the design of Feistel ciphers.

We consider the issue of securing dark pools/markets in the financial services sector. These markets currently are executed via trusted third parties, leading to potential fraud being able to be conducted by the market operators. We present a potential solution to this problem by using Multi-Party Computation to enable a trusted third party to be emulated in software. Our experiments show that whilst the standard market clearing mechanism of Continuous Double Auction in lit markets is not currently viable when executed using MPC, a popular mechanism for evaluating dark markets, namely the volume matching methodology, is viable. We present experimental validation of this conclusion by presenting the expected throughputs for such markets in two popular MPC paradigms; namely the two party dishonest majority setting and the honest majority three party setting.

A signature scheme is said to be weakly unforgeable, if it is hard to forge a signature on a message not signed before. A signature scheme is said to be strongly unforgeable, if it is hard to forge a signature on any message. In some applications, the weak unforgeability is not enough and the strong unforgeability is required, e.g., the Canetti, Halevi and Katz transformation.

Leakage-resilience is a property which guarantees that even if secret information such as the secret-key is partially leaked, the security is maintained. Some security models with leakage-resilience have been proposed. The hard-to-invert leakage model, a.k.a. auxiliary (input) leakage model, proposed by Dodis et al. at STOC'09 is especially meaningful one, since the leakage caused by a function which information-theoretically reveals the secret-key, e.g., one-way permutation, is considered.

In this work, we propose a generic construction of digital signature strongly unforgeable and resilient to polynomially hard-to-invert leakage which can be instantiated under standard assumptions such as the decisional linear assumption. We emphasize that our instantiated signature is not only the first one resilient to polynomially hard-to-invert leakage under standard assumptions, but also the first one which is strongly unforgeable and has hard-to-invert leakage-resilience.

Since Cheon et al. introduced a homomorphic encryption scheme for approximate arithmetic (Asiacrypt ’17), it has been recognized as suitable for important real-life usecases of homomorphic encryption, including training of machine learning models over encrypted data. A follow up work by Cheon et al. (Eurocrypt ’18) described an approximate bootstrapping procedure for the scheme. In this work, we improve upon the previous bootstrapping result. We improve the amortized bootstrapping time per plaintext slot by two orders of magnitude, from &#8764; 1 second to &#8764; 0.01 second. To achieve this result, we adopt a smart level-collapsing technique for evaluating DFT-like linear transforms on a ciphertext. Also, we replace the Taylor approximation of the sine function with a more accurate and numerically stable Chebyshev approximation, and design a modified version of the Paterson-Stockmeyer algorithm for fast evaluation of Chebyshev polynomials over encrypted data.

Physical attacks are a known threat to secure embedded systems. Notable among these is laser fault injection, which is probably the most powerful fault injection technique. Indeed, powerful injection techniques like laser fault injection provide a high spatial accuracy, which enables an attacker to induce bit level faults. However, experience gained from attacking 8-bit targets might not be relevant on more advanced micro-architectures and these attacks become increasingly challenging on 32-bit microcontrollers. In this article, we show that the flash memory area of a 32-bit microcontroller is sensitive to laser fault injection. These faults occur during the instruction fetch process, hence the stored value remains unaltered. After a thorough characterisation of the induced faults and the associated fault model, we provide detailed examples of bit-level corruptions of instruction and demonstrate practical applications in compromising the security of real-life codes. Based on these experimental results, we formulate a hypothesis about the underlying micro-architectural features that could explain the observed fault model.

Homomorphic Encryption (HE) is a powerful cryptographic primitive to address privacy and security issues in outsourcing computation on sensitive data to an untrusted computation environment. Comparing to secure Multi-Party Computation (MPC), HE has advantages in supporting non-interactive operations and saving on communication costs. However, it has not come up with an optimal solution for modern learning frameworks, partially due to a lack of efficient matrix computation mechanisms.

In this work, we present a practical solution to encrypt a matrix homomorphically and perform arithmetic operations on encrypted matrices. Our solution includes a novel matrix encoding method and an efficient evaluation strategy for basic matrix operations such as addition, multiplication, and transposition. We also explain how to encrypt more than one matrix in a single ciphertext, yielding better amortized performance.

Our solution is generic in the sense that it can be applied to most of the existing HE schemes. It also achieves reasonable performance for practical use; for example, our implementation takes 9.21 seconds to multiply two encrypted square matrices of order 64 and 2.56 seconds to transpose a square matrix of order 64.

Our secure matrix computation mechanism has a wide applicability to our new framework EDM, which stands for encrypted data and encrypted model. To the best of our knowledge, this is the first work that supports secure evaluation of the prediction phase based on both encrypted data and encrypted model, whereas previous work only supported applying a plain model to encrypted data. As a benchmark, we report an experimental result to classify handwritten images using convolutional neural networks (CNN). Our implementation on the MNIST dataset takes 28.59 seconds to compute ten likelihoods of 64 input images simultaneously, yielding an amortized rate of 0.45 seconds per image.

Event date: 16 June to 21 June 2019
Submission deadline: 15 March 2019
Notification: 15 April 2019

The deadline for nominating IACR members for the 2019 IACR Fellows class is earlier than in previous years! As announced at the Eurocrypt and Crypto membership meetings, all nomination materials must be received by November 15.

The IACR Fellows Program recognizes outstanding IACR members for technical and professional contributions to the field of cryptology. Information about nominating a Fellow is available from the IACR Fellows website.

We present practical attacks against OCB2, an ISO-standard authenticated encryption (AE) scheme. OCB2 is a highly-efficient blockcipher mode of operation. It has been extensively studied and widely believed to be secure thanks to the provable security proofs. Our attacks allows the adversary to create forgeries with (almost-known) single encryption query. The source of our attacks is the way OCB2 implements AE using a tweakable blockcipher, called XEX*. We have verified our attacks using a reference code of OCB2. Our attacks do not break the privacy of OCB2, and are not applicable to the others, including OCB1 and OCB3.

Mimblewimble is an electronic cash system proposed by an anonymous author in 2016. It combines several privacy-enhancing techniques initially envisioned for Bitcoin, such as Confidential Transactions (Maxwell, 2015), non-interactive merging of transactions (Saxena, Misra, Dhar, 2014), and cut-through of transaction inputs and outputs (Maxwell, 2013). As a remarkable consequence, coins can be deleted once they have been spent while maintaining public verifiability of the ledger, which is not possible in Bitcoin. This results in tremendous space savings for the ledger and efficiency gains for new users, who must verify their view of the system.

In this paper, we provide a provable-security analysis for Mimblewimble. We give a precise syntax and formal security definitions for an abstraction of Mimblewimble that we call an aggregate cash system. We then formally prove the security of Mimblewimble in this definitional framework. Our results imply in particular that two natural instantiations (with Pedersen commitments and Schnorr or BLS signatures) are provably secure against inflation and coin theft under standard assumptions.

It is well established that the method of choice for implementing a side-channel secure modular inversion, is to use Fermat's little theorem. So $1/x = x^{p-2} \bmod p$. This can be calculated using any multiply-and-square method safe in the knowledge that no branching or indexing with potentially secret data (such as $x$) will be required. However in the case where the modulus $p$ is a pseudo-Mersenne, or Mersenne, prime of the form $p=2^n-c$, where $c$ is small, this process can be optimized to greatly reduce the number of multiplications required. Unfortunately an optimal solution must it appears be tailored specifically depending on $n$ and $c$. What appears to be missing from the literature is a near-optimal heuristic method that works reasonably well in all cases.

Signal is a famous secure messaging protocol used by billions of people, by virtue of many secure text messaging applications including Signal itself, WhatsApp, Facebook Messenger, Skype, and Google Allo. At its core it uses the concept of "double ratcheting," where every message is encrypted and authenticated using a fresh symmetric key; it has many attractive properties, such as forward security, post-compromise security, and "immediate (no-delay) decryption," which had never been achieved in combination by prior messaging protocols.

While the formal analysis of the Signal protocol, and ratcheting in general, has attracted a lot of recent attention, we argue that none of the existing analyses is fully satisfactory. To address this problem, we give a clean and general definition of secure messaging, which clearly indicates the types of security we expect, including forward security, post-compromise security, and immediate decryption. We are the first to explicitly formalize and model the immediate decryption property, which implies (among other things) that parties seamlessly recover if a given message is permanently lost---a property not achieved by any of the recent "provable alternatives to Signal." We build a modular "generalized Signal protocol" from the following components: (a) continuous key agreement (CKA), a clean primitive we introduce and which can be easily and generically built from public-key encryption (not just Diffie-Hellman as is done in the current Signal protocol) and roughly models "public-key ratchets;" (b) forward-secure authenticated encryption with associated data (FS-AEAD), which roughly captures "symmetric-key ratchets;" and (c) a two-input hash function that is a pseudorandom function (resp. generator with input) in its first (resp. second) input, which we term PRF-PRNG. As a result, in addition to instantiating our framework in a way resulting in the existing, widely-used Diffie-Hellman based Signal protocol, we can easily get post-quantum security and not rely on random oracles in the analysis.

We further show that our design can be elegantly extended to include other forms of "fine-grained state compromise" recently studied at CRYPTO'18, but without sacrificing the immediate decryption property. However, we argue that the additional security offered by these modifications is unlikely to justify the efficiency hit of using much heavier public-key cryptography in place of symmetric-key cryptography.

Whether there exist Almost Perfect Non-linear permutations (APN) operating on an even number of bit is the so-called Big APN Problem. It has been solved in the 6-bit case by Dillon et al. in 2009 but, since then, the general case has remained an open problem.

In 2016, Perrin et al. discovered the butterfly structure which contains Dillon et al.'s permutation over $\mathbb{F}_{2^6}$. Later, Canteaut et al. generalised this structure and proved that no other butterflies with exponent $3$ can be APN. Recently, Yongqiang et al. further generalized the structure with Gold exponent and obtained more differentially 4-uniform permutations with the optimal nonlinearity. However, the existence of more APN permutations in their generalization was left as an open problem.

In this paper, we adapt the proof technique of Canteaut et al. to handle all Gold exponents and prove that a generalised butterfly with Gold exponents over $\mathbb{F}_{2^{2n}}$ can never be APN when $n>3$. More precisely, we prove that such a generalised butterfly being APN implies that the branch size is strictly smaller than 5. Hence, the only APN butterflies operate on 3-bit branches, i.e. on 6 bits in total.

In this paper we focus on Polynomial Learning with Errors (PLWE). This problem is parametrized by a polynomial and we are interested in relating the hardness of the $\text{PLWE}^f$ and $\text{PLWE}^h$ problems for different polynomials $f$ and $h$. More precisely, our main result shows that for a fixed monic polynomial $f$, $\text{PLWE}^{f\circ g}$ is at least as hard as $\text{PLWE}^f$, in both search and decision variants, for any monic polynomial $g$. As a consequence, $\text{PLWE}^{\phi_n}$ is harder than $\text{PLWE}^{f},$ for a minimal polynomial $f$ of an algebraic integer from the cyclotomic field $\mathbb{Q}(\zeta_n)$ with specific properties. Moreover, we prove in decision variant that in the case of power-of-2 polynomials, $\text{PLWE}^{\phi_n}$ is at least as hard as $\text{PLWE}^f,$ for a minimal polynomial $f$ of algebraic integers from the $n$th cyclotomic field with weaker specifications than those from the previous result.

We show how to build distributed key generation and distributed decryption procedures for the LIMA Ring-LWE based post-quantum cryptosystem. Our protocols implement the CCA variants of distributed decryption and are actively secure (with abort) in the case of three parties and honest majority. Our protocols make use of a combination of problem specific MPC protocols, generic garbled circuit based MPC and generic Linear Secret Sharing based MPC. We also, as a by-product, report on the first run-times for the execution of the SHA-3 function in an MPC system.

We propose two one-round authenticated group-key exchange protocols from newly employed cryptographic invariant maps (CIMs): one is secure under the quantum random oracle model and the other resists against maximum exposure where a non-trivial combination of secret keys is revealed. The security of the former (resp. latter) is proved under the n-way decisional Diffie-Hellman (resp. n-way gap Diffie-Hellman) assumption on the CIMs in the quantum random (resp. random) oracle model. We instantiate the proposed protocols on the hard homogeneous spaces with limitation where the number of the user group is two. In particular, the protocols instantiated by using the CSIDH, commutative supersingular isogeny Diffie-Hellman, key exchange are currently more realistic than the general n-party CIM-based ones due to its implementability. Our two-party one-round protocols are secure against quantum adversaries.

Homomorphic encryption has the purpose to allow computations on encrypted data, without the need for decryption other than that of the final result. This could provide an elegant solution to the problem of privacy preservation in data-based applications, such as those provided and/or facilitated by machine learning techniques, but several limitations and open issues hamper the fulfillment of this plan. In this work we assess the possibility for homomorphic encryption to fully implement its program without the need to rely on other techniques, such as multiparty computation, which may be impossible in many actual use cases (for instance due to the high level of communication required). We proceed in two steps: i) on the basis of the well-known structured program theorem [Bohm and Jacopini] we identify the relevant minimal set of operations homomorphic encryption must be able to perform to implement any algorithm; and ii) we analyse the possibility to solve -and propose an implementation for- the most fundamentally relevant issue as it emerges from our analysis, that is, the implementation of conditionals (which in turn require comparison and selection/jump operations) in full homomorphic encryption. We show how this issue has a serious impact and clashes with the fundamental requirements of homomorphic encryption. This could represent a drawback for its use as a complete solution in data analysis applications, in particular machine learning. It will thus possibly require a deep re-thinking of the homomorphic encryption program for privacy preservation.

We note that our approach to comparisons is novel, and for the first time completely embedded in homomorphic encryption, differently from what proposed in previous studies (and beyond that, we supplement it with the necessary selection/jump operation). A number of studies have indeed dealt with comparisons, but have not managed to perform them in pure homomorphic encryption. Typically their comparison protocols do not utilise homomorphic encryption for the comparison itself, but rely on other cryptographic techniques, such as secure multiparty computation, which a) require a high level of communication between parties (each single comparison in a machine learning training and prediction process must be performed by exchanging several messages), which may not be possible in various use cases, and b) required the data owner to decrypt intermediate results, extract significant bits for the comparison, re-encrypt and send the result back to the other party for the accomplishment of the algorithm. Such ``decryption'' in the middle foils the purpose of homomorphic encryption. Beside requiring only homomorphic encryption, and not any intermediate decryption, our protocol is also provably safe (as it shares the same safety as the homomorphic encryption schemes), differently from other techniques such as OPE/ORE and variations, which have been proved not secure.

The efficient verification of the security of masked hardware implementations is an important issue that hinders the development and deployment of randomness-efficient masking techniques. At EUROCRYPT 2018, Bloem et al. [6] introduced the first practical formal tool to prove the side-channel resilience of masked circuits in the probing model with glitches. Most recently Barthe et al.[2] introduced a more efficient formal tool that builds upon the findings of Bloem et al. for modeling the effects of glitches. While Barthe et al.'s approach greatly improves the first-order verification performance, it shows that higher-order verification in the probing model with glitches is still enormously time-consuming for larger circuits like a second-order AES S-box, for instance. Furthermore, the results of Barthe et al. underline the discrepancy between state-of-the-art formal security notions that allow for faster verification of circuits. Namely the strong non-interference (SNI) notion, and existing masked hardware implementations that are secure in the probing model with glitches. In this work, we extend and improve the formal approaches of Bloem et al. and Barthe et al. on manifold levels. We first introduce a so-called sharing independence notion which helps to reason about the independence of shared variables. We then show how to use this notion to test for the independence of input and output sharings of a module which allows speeding up the formal verification of circuits that do not fulfill the SNI notion. With this extension, we are for the time able to verify the security of a second-order masked DOM AES S-box which takes about 3 seconds, and up to a fifth-order AES S-box which requires about 47 days for verification. Furthermore, we discuss in which case the independence of input and output sharings lead to composability.

In Sweden, a phd studentship is a paid full-time job, with a salary, annual leave and all other social and medical benefits.

We are hiring one phd student to work on (acoustic) side channels, automotive security, or cybercrime at Linköping University.

Candidates with a solid MSc in security or applied crypto are welcome to apply.

PI profile: https://scholar.google.com/citations?hl=en&user=rYhiAEUAAAAJ&view_op=list_works&sortby=pubdate

Closing date for applications: 30 November 2018

Contact: Prof Jeff Yan (jeff.yan (at) liu.se)