System-level Non-interference for Constant-time Cryptography

International audienceCache-based attacks are a class of side-channel attacks that are particularly effective in virtualized or cloud-based en-vironments, where they have been used to recover secret keys from cryptographic implementations. One common ap-proach to thwart cache-based attacks is to use constant-time implementations, i.e. which do not branch on secrets and do not perform memory accesses that depend on secrets. How-ever, there is no rigorous proof that constant-time implemen-tations are protected against concurrent cache-attacks in virtualization platforms with shared cache; moreover, many prominent implementations are not constant-time. An alter-native approach is to rely on system-level mechanisms. One recent such mechanism is stealth memory, which provisions a small amount of private cache for programs to carry po-tentially leaking computations securely. Stealth memory in-duces a weak form of constant-time, called S-constant-time, which encompasses some widely used cryptographic imple-mentations. However, there is no rigorous analysis of stealth memory and S-constant-time, and no tool support for check-ing if applications are S-constant-time. We propose a new information-flow analysis that checks if an x86 application executes in constant-time, or in S-constant-time. Moreover, we prove that constant-time (resp. S-constant-time) programs do not leak confidential infor-mation through the cache to other operating systems exe-cuting concurrently on virtualization platforms (resp. plat-forms supporting stealth memory). The soundness proofs are based on new theorems of independent interest, includ-ing isolation theorems for virtualization platforms (resp. plat-forms supporting stealth memory), and proofs that constant-time implementations (resp. S-constant-time implementa-tions) are non-interfering with respect to a strict information flow policy which disallows that control flow and memory ac-cesses depend on secrets. We formalize our results using the Coq proof assistant and we demonstrate the effectiveness of our analyses on cryptographic implementations, including PolarSSL AES, DES and RC4, SHA256 and Salsa20

Thwarting Fault Attacks using the Internal Redundancy Countermeasure (IRC)

A growing number of connected objects, with their high performance and low-resources constraints, are embedding lightweight ciphers for protecting the confidentiality of the data they manipulate or store. Since those objects are easily accessible, they are prone to a whole range of physical attacks, one of which are fault attacks against for which countermeasures are usually expensive to implement, especially on off-the-shelf devices. For such devices, we propose a new generic software countermeasure, called the Internal Redundancy Countermeasure (IRC), to thwart most fault attacks while preserving the performances of the targeted cipher. We report practical experiments showing that IRC successfully thwarts fault attacks on the block cipher PRIDE and on the stream cipher TRIVIUM for which we protect both the initialization and the keystream generation

A Public Key Encryption Scheme Based on the Polynomial Reconstruction Problem

International audienceThe Polynomial Reconstruction problem (PR) has been introduced in 1999 as a new hard problem. Several cryptographic primitives established on this problem have been constructed, for instance Naor and Pinkas have proposed a protocol for oblivious polynomial evaluation. Then it has been studied from the point of view of robustness, and several important properties have been discovered and proved by Kiayias and Yung. Furthermore the same authors constructed a symmetric cipher based on the PR problem. In the present paper, we use the published security results and construct a new public key encryption scheme based on the hardness of the problem of Polynomial Reconstruction. The scheme presented is the first public key encryption scheme based on this Polynomial Reconstruction problem. We also present some attacks, discuss their performances and state the size of the parameters required to reach the desired security level. In conclusion, this leads to a cryptosystem where the cost of encryption and decryption per bit is low, and where the public key is kept relatively small

Boomerang Connectivity Table:A New Cryptanalysis Tool

A boomerang attack is a cryptanalysis framework that regards a block cipher $E$ as the composition of two sub-ciphers $E_1\circ E_0$ and builds a particular characteristic for $E$ with probability $p^2q^2$ by combining differential characteristics for $E_0$ and $E_1$ with probability $p$ and $q$, respectively. Crucially the validity of this figure is under the assumption that the characteristics for $E_0$ and $E_1$ can be chosen independently. Indeed, Murphy has shown that independently chosen characteristics may turn out to be incompatible. On the other hand, several researchers observed that the probability can be improved to $p$ or $q$ around the boundary between $E_0$ and $E_1$ by considering a positive dependency of the two characteristics, e.g.~the ladder switch and S-box switch by Biryukov and Khovratovich. This phenomenon was later formalised by Dunkelman et al.~as a sandwich attack that regards $E$ as $E_1\circ E_m \circ E_0$, where $E_m$ satisfies some differential propagation among four texts with probability $r$, and the entire probability is $p^2q^2r$. In this paper, we revisit the issue of dependency of two characteristics in $E_m$, and propose a new tool called Boomerang Connectivity Table (BCT), which evaluates $r$ in a systematic and easy-to-understand way when $E_m$ is composed of a single S-box layer. With the BCT, previous observations on the S-box including the incompatibility, the ladder switch and the S-box switch are represented in a unified manner. Moreover, the BCT can detect a new switching effect, which shows that the probability around the boundary may be even higher than $p$ or $q$. To illustrate the power of the BCT-based analysis, we improve boomerang attacks against Deoxys-BC, and disclose the mechanism behind an unsolved probability amplification for generating a quartet in SKINNY. Lastly, we discuss the issue of searching for S-boxes having good BCT and extending the analysis to modular addition

Boolean functions for homomorphic-friendly stream ciphers

The proliferation of small embedded devices having growing but still limited computing and data storage facilities, and the related development of cloud services with extensive storage and computing means, raise nowadays new privacy issues because of the outsourcing of data processing. This has led to a need for symmetric cryptosystems suited for hybrid symmetric-FHE encryption protocols, ensuring the practicability of the FHE solution. Recent ciphers meant for such use have been introduced, such as LowMC, Kreyvium, FLIP, and Rasta. The introduction of stream ciphers devoted to symmetric-FHE frameworks such as FLIP and its recent modification has in its turn posed new problems on the Boolean functions to be used in them as filter functions. We recall the state of the art in this matter and present further studies (without proof)

Legislative History: An Act Concerning the Operation of Emergency Medical Vehicles (SP482)(LD 1303)

https://digitalmaine.com/legishist114/2302/thumbnail.jp

Anomalies and Vector Space Search: Tools for S-Box Analysis

International audienceS-boxes are functions with an input so small that the simplest way to specify them is their lookup table (LUT). How can we quantify the distance between the behavior of a given S-box and that of an S-box picked uniformly at random? To answer this question, we introduce various "anomalies". These real numbers are such that a property with an anomaly equal to should be found roughly once in a set of $2^a$ random S-boxes. First, we present statistical anomalies based on the distribution of the coefficients in the difference distribution table, linear approximation table, and for the first time, the boomerang connectivity table. We then count the number of S-boxes that have block-cipher like structures to estimate the anomaly associated to those. In order to recover these structures, we show that the most general tool for decomposing S-boxes is an algorithm efficiently listing all the vector spaces of a given dimension contained in a given set, and we present such an algorithm. Combining these approaches, we conclude that all permutations that are actually picked uniformly at random always have essentially the same cryptographic properties and the same lack of structure

Polytopic Cryptanalysis

Standard differential cryptanalysis uses statistical dependencies between the difference of two plaintexts and the difference of the respective two ciphertexts to attack a cipher. Here we introduce polytopic cryptanalysis which considers interdependencies between larger sets of texts as they traverse through the cipher. We prove that the methodology of standard differential cryptanalysis can unambiguously be extended and transferred to the polytopic case including impossible differentials. We show that impossible polytopic transitions have generic advantages over impossible differentials. To demonstrate the practical relevance of the generalization, we present new low-data attacks on round-reduced DES and AES using impossible polytopic transitions that are able to compete with existing attacks, partially outperforming these

Faster Correlation Attack on Bluetooth Keystream Generator E0

Abstract. We study both distinguishing and key-recovery attacks against E0, the keystream generator used in Bluetooth by means of correlation. First, a powerful computation method of correlations is formulated by a recursive expression, which makes it easier to calculate correlations of the finite state machine output sequences up to 26 bits for E0 and allows us to verify the two known correlations to be the largest for the first time. Second, we apply the concept of convolution to the analysis of the distinguisher based on all correlations, and propose an efficient distinguisher due to the linear dependency of the largest correlations. Last, we propose a novel maximum likelihood decoding algorithm based on fast Walsh transform to recover the closest codeword for any linear code of dimension L and length n. It requires time O(n + L · 2 L) and memory min(n, 2 L). This can speed up many attacks such as fast correlation attacks. We apply it to E0, and our best key-recovery attack works in 2 39 time given 2 39 consecutive bits after O(2 37) precomputation. This is the best known attack against E0 so far.