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15 April 2021
Anita Aghaie, Amir Moradi
The developments in the areas of strong Physical Unclonable Functions (PUFs) predicate an ongoing struggle between designers and attackers. Such a combat motivated the atmosphere of open research, hence enhancing PUF designs in
the presence of Machine Learning (ML) attacks. As an example of this controversy, at CHES 2019, a novel delay-based PUF (iPUF) has been introduced and claimed to be resistant against various ML and reliability attacks. At CHES 2020, a new
divide-and-conquer modeling attack (splitting iPUF) has been presented showing the vulnerability of even large iPUF variants.
Such attacks and analyses are naturally examined purely in the simulation domain, where some metrics like uniformity are assumed to be ideal. This assumption is motivated by a common belief that implementation defects (such as bias) may ease
the attacks. In this paper, we highlight the critical role of uniformity in the success of ML attacks, and for the first time present a case where the bias originating from implementation defects hardens certain learning problems in complex PUF architectures. We present the result of our investigations conducted on a cluster of 100 Xilinx Artix 7 FPGAs, showing the incapability of the splitting iPUF attack to model even small iPUF instances when facing a slight non-uniformity. In fact, our findings imply that non-ideal conditions due to implementation defects should also be considered when developing an attack vector on complex PUF architectures like iPUF. On the other hand, we observe a relatively low uniqueness even when following the suggestions made by the iPUFs original authors with respect to the FPGA implementations, which indeed questions the promised physical unclonability.
Alexander Heinrich, Matthias Hollick, Thomas Schneider, Milan Stute, Christian Weinert
Apple's offline file-sharing service AirDrop is integrated into more than 1.5 billion end-user devices worldwide. We discovered two design flaws in the underlying protocol that allow attackers to learn the phone numbers and email addresses of both sender and receiver devices. As a remediation, we study the applicability of private set intersection (PSI) to mutual authentication, which is similar to contact discovery in mobile messengers. We propose a novel optimized PSI-based protocol called PrivateDrop that addresses the specific challenges of offline resource-constrained operation and integrates seamlessly into the current AirDrop protocol stack. Using our native PrivateDrop implementation for iOS and macOS, we experimentally demonstrate that PrivateDrop preserves AirDrop's exemplary user experience with an authentication delay well below one second. We responsibly disclosed our findings to Apple and open-sourced our PrivateDrop implementation.
Jakub Klemsa
With the rise of lattice cryptography, (negacyclic) convolution has received increased attention. E.g., the NTRU scheme internally employs cyclic polynomial multiplication, which is equivalent to the standard convolution, on the other hand, many Ring-LWE-based cryptosystems perform negacyclic polynomial multiplication. A method by Crandall implements an efficient negacyclic convolution over a finite field of prime order using an extended Discrete Galois Transform (DGT) a finite field analogy to Discrete Fourier Transform (DFT). Compared to DGT, the classical DFT runs faster by an order of magnitude, however, it suffers from inevitable rounding errors due to finite floating-point number representation. In a recent Fully Homomorphic Encryption (FHE) scheme by Chillotti et al. named TFHE, small errors are acceptable (although not welcome), therefore we decided to investigate the application of DFT for negacyclic convolution.
The primary goal of this paper is to suggest a method for fast negacyclic convolution over integer coefficients using an extended DFT. The key contribution is a thorough analysis of error propagation, as a result of which we derive parameter bounds that can guarantee even error-free results. We also suggest a setup that admits rare errors, which allows to increase the degree of the polynomials and/or their maximum norm at a fixed floating-point precision. Finally, we run benchmarks with parameters derived from a practical TFHE setup. We achieve around 24× better times than the generic NTL library (comparable to Crandalls method) and around 4× better times than a naı̈ve approach with DFT, with no errors.
The primary goal of this paper is to suggest a method for fast negacyclic convolution over integer coefficients using an extended DFT. The key contribution is a thorough analysis of error propagation, as a result of which we derive parameter bounds that can guarantee even error-free results. We also suggest a setup that admits rare errors, which allows to increase the degree of the polynomials and/or their maximum norm at a fixed floating-point precision. Finally, we run benchmarks with parameters derived from a practical TFHE setup. We achieve around 24× better times than the generic NTL library (comparable to Crandalls method) and around 4× better times than a naı̈ve approach with DFT, with no errors.
Tim Fritzmann, Michiel Van Beirendonck, Debapriya Basu Roy, Patrick Karl, Thomas Schamberger, Ingrid Verbauwhede, Georg Sigl
Side-channel attacks can break mathematically secure cryptographic systems leading to a major concern in applied cryptography. While the cryptanalysis and security evaluation of Post-Quantum Cryptography (PQC) have already received an increasing research effort, a cost analysis of efficient side-channel countermeasures is still lacking. In this work, we propose a masked HW/SW codesign of the NIST PQC finalists Kyber and Saber, suitable for their different characteristics. Among others, we present a novel masked ciphertext compression algorithm for non-power-of-two moduli. To accelerate linear performance bottlenecks, we developed a generic Number Theoretic Transform (NTT) multiplier, which, in contrast to previously published accelerators, is also efficient and suitable for schemes not based on NTT. For the critical non-linear operations, masked HW accelerators were developed, allowing a secure execution using RISC-V instruction set extensions. Our experimental results show a cycle count reduction factor of 3.18 for Kyber (K:245k/E:319k/D:339k) and 2.66 for Saber (K:229k/E:308k/D:347k) compared to the latest optimized ARM Cortex-M4 implementations. While Saber performs slightly better for the key generation and encapsulation, Kyber has slight performance advantages for the decapsulation. The masking overhead for the first-order secure decapsulation operation including randomness generation is around 4.14 for Kyber (D:1403k) and 2.63 for Saber (D:915k).
Yaron Gvili, Julie Ha, Sarah Scheffler, Mayank Varia, Ziling Yang, Xinyuan Zhang
In this work, we present a zero knowledge argument for general arithmetic circuits that is public-coin and constant rounds, so it can be made non-interactive and publicly verifiable with the Fiat-Shamir heuristic. The construction is based on the MPC-in-the-head paradigm, in which the prover jointly emulates all MPC protocol participants and can provide advice in the form of Beaver triples whose accuracy must be checked by the verifier. Our construction follows the Beaver triple sacrificing approach used by Baum and Nof [PKC 2020]. Our improvements reduce the communication per multiplication gate from 4 to 2 field elements, matching the performance of the cut-and-choose approach taken by Katz, Kolesnikov, and Wang [CCS 2018] and with lower additive overhead for some parameter settings. We implement our protocol and analyze its cost on Picnic-style post-quantum digital signatures based on the AES family of circuits.
Agathe Cheriere, Lina Mortajine, Tania Richmond, Nadia El Mrabet
ROLLO is a candidate to the second round of NIST Post-Quantum Cryptography standardization process. In the last update in
April 2020, there was a key encapsulation mechanism (ROLLO-I) and a public-key encryption scheme (ROLLO-II). In this paper, we propose an attack to recover the syndrome during the decapsulation process of ROLLO-I. From this syndrome, we explain how to perform a private key-recovery.
We target two constant-time implementations: the C reference implementation and a C implementation available on GitHub. By getting power measurements during the execution of the Gaussian elimination function, we are able to extract on a single trace each element of the syndrome. This attack can also be applied to the decryption process of ROLLO-II.
Aaqib Bashir Dar , Mashhood Jeelani Lone, Nuzhat Hussain
Block ciphers have been extremely predominant in the area of cryptography and due to the paradigm shift towards devices of resource constrained nature, lightweight block ciphers have totally influenced the field and has been a go-to option ever since. The growth of resource constrained devices have put forth a dire need for the security solutions that are feasible in terms of resources without taking a toll on the security that they offer. As the world is starting to move towards Internet of Things (IoT), data security and privacy in this environment is a major concern. This is due to the reason that a huge number of devices that operate in this environment are resource constrained. Because of their resource-constrained nature, advanced mainstream cryptographic ciphers and techniques do not perform as efficiently on such devices. This has led to the boom in the field of 'lightweight cryptography' which aims at developing cryptographic techniques that perform efficiently in a resource constrained environment. Over the period of past two decades or so, a bulk of lightweight block ciphers have been proposed due to the
growing need and demand in lightweight cryptography. In this paper, we review the state-of-the-art lightweight block ciphers, present a comprehensive design niche, give a detailed taxonomy with multiple classifications and present future research directions.
Shahla Atapoor, Nigel P. Smart, Younes Talibi Alaoui
Many central banks, as well as blockchain systems, are looking into distributed versions of interbank payment systems, in particular the netting procedure. When executed in a distributed manner this presents a number of privacy problems. This paper studies a privacy preserving netting protocol to solve the gridlock resolution problem in such Real Time Gross Settlement systems. Our solution utilizes Multi-party Computation and is implemented in the SCALE MAMBA system, using Shamir secret sharing scheme over three parties in an actively secure manner. Our experiments show that, even for large throughput systems, such a privacy preserving operation is often feasible.
Fukang Liu, Takanori Isobe, Willi Meier
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 to 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.
Ryan Karl, Jonathan Takeshita, Alamin Mohammed, Aaron Striegel, and Taeho Jung
In modern times, data collected from multi-user distributed applications must be analyzed on a massive scale to support critical business objectives. While analytics often requires the use of personal data, it may compromise user privacy expectations if this analysis is conducted over plaintext data. Private Stream Aggregation (PSA) allows for the aggregation of time-series data, while still providing strong privacy guarantees, and is significantly more efficient over a network than related techniques (e.g. homomorphic encryption, secure multiparty computation, etc.) due to its asynchronous and efficient protocols. However, PSA protocols face limitations and can only compute basic functions, such as sum, average, etc.. We present Cryptonomial, a framework for converting any PSA scheme amenable to a complex canonical embedding into a secure computation protocol that can compute any function over time- series data that can be written as a multivariate polynomial, by combining PSA and a Trusted Execution Environment. This design allows us to compute the parallelizable sections of our protocol outside the TEE using advanced hardware, that can take better advantage of parallelism. We show that Cryptonomial inherits the security requirements of PSA, and supports fully malicious security. We implement our scheme, and show that our techniques enable performance that is orders of magnitude faster than similar work supporting polynomial calculations.
Ryan Karl, Jonathan Takeshita, Alamin Mohammed, Aaron Striegel, Taeho Jung
Histograms have a large variety of useful applications in data analysis, e.g., tracking the spread of diseases and analyzing public health issues. However, most data analysis techniques used in practice operate over plaintext data, putting the privacy of users data at risk. We consider the problem of allowing an untrusted aggregator to privately compute a histogram over multiple users private inputs (e.g., number of contacts at a place) without learning anything other than the final histogram. This is a challenging problem to solve when the aggregators and the users may be malicious and collude with each other to infer others private inputs, as existing black box techniques incur high communication and computational overhead that limit scalability. We address these concerns by building a novel, efficient, and scalable protocol that intelligently combines a Trusted Execution Environment (TEE) and the Durstenfeld-Knuth uniformly random shuffling algorithm to update a mapping between buckets and keys by using a deterministic cryptographically secure pseudorandom number generator. In addition to being provably secure, experimental evaluations of our technique indicate that it generally outperforms existing work by several orders of magnitude, and can achieve performance that is within one order of magnitude of protocols operating over plaintexts that do not offer any security.
13 April 2021
Copenhagen, Denmark, 25 August - 26 August 2021
Event date: 25 August to 26 August 2021
Submission deadline: 1 May 2021
Notification: 15 June 2021
Submission deadline: 1 May 2021
Notification: 15 June 2021
Virtual event, Anywhere on Earth, 17 August 2021
Event date: 17 August 2021
Submission deadline: 30 April 2021
Notification: 31 May 2021
Submission deadline: 30 April 2021
Notification: 31 May 2021
Kamakura, Japan, 12 June - 24 June 2021
Event date: 12 June to 24 June 2021
Submission deadline: 12 April 2021
Notification: 1 May 2021
Submission deadline: 12 April 2021
Notification: 1 May 2021
Darmstadt, Germany, 8 October 2021
Event date: 8 October 2021
Submission deadline: 20 July 2021
Notification: 30 August 2021
Submission deadline: 20 July 2021
Notification: 30 August 2021
12 April 2021
Cesar Pereida García, Sampo Sovio
Ed25519 has significant performance benefits compared to ECDSA using Weierstrass curves such as NIST P-256, therefore it is considered a good digital signature algorithm, specially for low performance IoT devices. However, such devices often have very limited resources and thus, implementations for these devices need to be as small and as performant as possible while being secure. In this paper we describe a scenario in which an obvious strategy to aggressively optimize an Ed25519 implementation for code size leads to a small memory footprint that is functionally correct but vulnerable to side-channel attacks. This strategy serves as an example of aggressive optimizations that might be considered by cryptography engineers, developers, and practitioners unfamiliar with the power of Side-Channel Analysis (SCA). As a solution to the flawed implementation example, we use a computer-aided cryptography tool generating formally verified finite field arithmetic to generate two secure Ed25519 implementations fulfilling different size requirements. After benchmarking and comparing these implementations to other widely used implementations our results show that computer-aided cryptography is capable of generating competitive code in terms of security, speed, and size.
Benny Applebaum, Oded Nir
A secret-sharing scheme allows to distribute a secret $s$ among $n$ parties such that only some predefined ``authorized'' sets of parties can reconstruct the secret, and all other ``unauthorized'' sets learn nothing about $s$.
The collection of authorized/unauthorized sets can be captured by a monotone function $f:\{0,1\}^n\rightarrow \{0,1\}$.
In this paper, we focus on monotone functions that all their min-terms are sets of size $a$, and on their duals -- monotone functions whose max-terms are of size $b$. We refer to these classes as $(a,n)$-upslices and $(b,n)$-downslices, and note that these natural families correspond to monotone $a$-regular DNFs and monotone $(n-b)$-regular CNFs. We derive the following results.
1. (General downslices) Every downslice can be realized with total share size of $1.5^{n+o(n)}<2^{0.585 n}$. Since every monotone function can be cheaply decomposed into $n$ downslices, we obtain a similar result for general access structures improving the previously known $2^{0.637n+o(n)}$ complexity of Applebaum, Beimel, Nir and Peter (STOC 2020). We also achieve a minor improvement in the exponent of linear secrets sharing schemes.
2. (Random mixture of upslices) Following Beimel and Farras (TCC 2020) who studied the complexity of random DNFs with constant-size terms, we consider the following general distribution $F$ over monotone DNFs: For each width value $a\in [n]$, uniformly sample $k_a$ monotone terms of size $a$, where $k=(k_1,\ldots,k_n)$ is an arbitrary vector of non-negative integers. We show that, except with exponentially small probability, $F$ can be realized with share size of $2^{0.5 n+o(n)}$ and can be linearly realized with an exponent strictly smaller than $2/3$. Our proof also provides a candidate distribution for ``exponentially-hard'' access structure. We use our results to explore connections between several seemingly unrelated questions about the complexity of secret-sharing schemes such as worst-case vs. average-case, linear vs. non-linear and primal vs. dual access structures. We prove that, in at least one of these settings, there is a significant gap in secret-sharing complexity.
1. (General downslices) Every downslice can be realized with total share size of $1.5^{n+o(n)}<2^{0.585 n}$. Since every monotone function can be cheaply decomposed into $n$ downslices, we obtain a similar result for general access structures improving the previously known $2^{0.637n+o(n)}$ complexity of Applebaum, Beimel, Nir and Peter (STOC 2020). We also achieve a minor improvement in the exponent of linear secrets sharing schemes.
2. (Random mixture of upslices) Following Beimel and Farras (TCC 2020) who studied the complexity of random DNFs with constant-size terms, we consider the following general distribution $F$ over monotone DNFs: For each width value $a\in [n]$, uniformly sample $k_a$ monotone terms of size $a$, where $k=(k_1,\ldots,k_n)$ is an arbitrary vector of non-negative integers. We show that, except with exponentially small probability, $F$ can be realized with share size of $2^{0.5 n+o(n)}$ and can be linearly realized with an exponent strictly smaller than $2/3$. Our proof also provides a candidate distribution for ``exponentially-hard'' access structure. We use our results to explore connections between several seemingly unrelated questions about the complexity of secret-sharing schemes such as worst-case vs. average-case, linear vs. non-linear and primal vs. dual access structures. We prove that, in at least one of these settings, there is a significant gap in secret-sharing complexity.
Danilo Gligoroski
The algebraic structures that are non-commutative and non-associative known as entropic groupoids that satisfy the "Palintropic" property i.e., $x^{\mathbf{A} \mathbf{B}} = (x^{\mathbf{A}})^{\mathbf{B}} = (x^{\mathbf{B}})^{\mathbf{A}} = x^{\mathbf{B} \mathbf{A}}$ were proposed by Etherington in '40s from the 20th century. Those relations are exactly the Diffie-Hellman key exchange protocol relations used with groups. The arithmetic for non-associative power indices known as Logarithmetic was also proposed by Etherington and later developed by others in the 50s-70s. However, as far as we know, no one has ever proposed a succinct notation for exponentially large non-associative power indices that will have the property of fast exponentiation similarly as the fast exponentiation is achieved with ordinary arithmetic via the consecutive rising to the powers of two.
In this paper, we define ringoid algebraic structures $(G, \boxplus, *)$ where $(G, \boxplus) $ is an Abelian group and $(G, *)$ is a non-commutative and non-associative groupoid with an entropic and palintropic subgroupoid which is a quasigroup, and we name those structures as Entropoids. We further define succinct notation for non-associative bracketing patterns and propose algorithms for fast exponentiation with those patterns.
Next, by an analogy with the developed cryptographic theory of discrete logarithm problems, we define several hard problems in Entropoid based cryptography, such as Discrete Entropoid Logarithm Problem (DELP), Computational Entropoid Diffie-Hellman problem (CEDHP), and Decisional Entropoid Diffie-Hellman Problem (DEDHP). We post a conjecture that DEDHP is hard in Sylow $q$-subquasigroups. Next, we instantiate an entropoid Diffie-Hellman key exchange protocol. Due to the non-commutativity and non-associativity, the entropoid based cryptographic primitives are supposed to be resistant to quantum algorithms. At the same time, due to the proposed succinct notation for the power indices, the communication overhead in the entropoid based Diffie-Hellman key exchange is very low: for 128 bits of security, 64 bytes in total are communicated in both directions, and for 256 bits of security, 128 bytes in total are communicated in both directions.
Our final contribution is in proposing two entropoid based digital signature schemes. The schemes are constructed with the Fiat-Shamir transformation of an identification scheme which security relies on a new hardness assumption: computing roots in finite entropoids is hard. If this assumption withstands the time's test, the first proposed signature scheme has excellent properties: for the classical security levels between 128 and 256 bits, the public and private key sizes are between 32 and 64, and the signature sizes are between 64 and 128 bytes. The second signature scheme reduces the finding of the roots in finite entropoids to computing discrete entropoid logarithms. In our opinion, this is a safer but more conservative design, and it pays the price in doubling the key sizes and the signature sizes.
We give a proof-of-concept implementation in SageMath 9.2 for all proposed algorithms and schemes in an appendix.
In this paper, we define ringoid algebraic structures $(G, \boxplus, *)$ where $(G, \boxplus) $ is an Abelian group and $(G, *)$ is a non-commutative and non-associative groupoid with an entropic and palintropic subgroupoid which is a quasigroup, and we name those structures as Entropoids. We further define succinct notation for non-associative bracketing patterns and propose algorithms for fast exponentiation with those patterns.
Next, by an analogy with the developed cryptographic theory of discrete logarithm problems, we define several hard problems in Entropoid based cryptography, such as Discrete Entropoid Logarithm Problem (DELP), Computational Entropoid Diffie-Hellman problem (CEDHP), and Decisional Entropoid Diffie-Hellman Problem (DEDHP). We post a conjecture that DEDHP is hard in Sylow $q$-subquasigroups. Next, we instantiate an entropoid Diffie-Hellman key exchange protocol. Due to the non-commutativity and non-associativity, the entropoid based cryptographic primitives are supposed to be resistant to quantum algorithms. At the same time, due to the proposed succinct notation for the power indices, the communication overhead in the entropoid based Diffie-Hellman key exchange is very low: for 128 bits of security, 64 bytes in total are communicated in both directions, and for 256 bits of security, 128 bytes in total are communicated in both directions.
Our final contribution is in proposing two entropoid based digital signature schemes. The schemes are constructed with the Fiat-Shamir transformation of an identification scheme which security relies on a new hardness assumption: computing roots in finite entropoids is hard. If this assumption withstands the time's test, the first proposed signature scheme has excellent properties: for the classical security levels between 128 and 256 bits, the public and private key sizes are between 32 and 64, and the signature sizes are between 64 and 128 bytes. The second signature scheme reduces the finding of the roots in finite entropoids to computing discrete entropoid logarithms. In our opinion, this is a safer but more conservative design, and it pays the price in doubling the key sizes and the signature sizes.
We give a proof-of-concept implementation in SageMath 9.2 for all proposed algorithms and schemes in an appendix.
Coşku Acay, Rolph Recto, Joshua Gancher, Andrew C. Myers, Elaine Shi
Modern distributed systems involve interactions between principals with limited trust, so cryptographic mechanisms are needed to protect confidentiality and integrity. At the same time, most developers lack the training to securely employ cryptography. We present Viaduct, a compiler that transforms high-level programs into secure, efficient distributed realizations. Viaduct's source language allows developers to declaratively specify security policies by annotating their programs with information flow labels. The compiler uses these labels to synthesize distributed programs that use cryptography efficiently while still defending the source-level security policy. The Viaduct approach is general, and can be easily extended with new security mechanisms.
Our implementation of the Viaduct compiler comes with an extensible runtime system that includes plug-in support for multiparty computation, commitments, and zero-knowledge proofs. We have evaluated the system on a set of benchmarks, and the results indicate that our approach is feasible and can use cryptography in efficient, nontrivial ways.
Our implementation of the Viaduct compiler comes with an extensible runtime system that includes plug-in support for multiparty computation, commitments, and zero-knowledge proofs. We have evaluated the system on a set of benchmarks, and the results indicate that our approach is feasible and can use cryptography in efficient, nontrivial ways.
Chris Brzuska, Antoine Delignat-Lavaud, Christoph Egger, Cédric Fournet, Konrad Kohbrok, Markulf Kohlweiss
We analyze the security of the TLS 1.3 key establishment protocol, as specified at the end of its rigorous standardization process. We define a core key-schedule and reduce its security to concrete assumptions against an adversary that controls client and server configurations and adaptively chooses some of their keys. Our model supports all key derivations featured in the standard, including its negotiated modes and algorithms that combine an optional Diffie-Hellman exchange for forward
secrecy with optional pre-shared keys supplied by the application or recursively established in prior sessions. We show that the output keys are secure as soon as any of their input key materials are. Our compositional, code-based proof makes use of state separation to yield concrete reductions despite the complexity of the key schedule. We also discuss (late) changes to the standard that would improve its robustness and simplify its analysis.