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2
April
2019

TLS 1.3 allows two parties to establish a shared session key from an out-of-band agreed
Pre Shared Key (PSK) is used to mutually authenticate the parties, under the assumption that it is not shared with others. This allows the parties to skip the certificate verification steps, saving bandwidth, communication rounds, and latency.

We identify a security vulnerability in this TLS 1.3 path, by showing a new reflection attack that we call ``Selfie''. The Selfie attack breaks the mutual authentication. It leverages the fact that TLS does not mandate explicit authentication of the server and the client in every message.

The paper explains the root cause of this TLS 1.3 vulnerability, demonstrates the Selfie attack on the TLS implementation of OpenSSL and proposes appropriate mitigation.

The attack is surprising because it breaks some assumptions and uncovers an interesting gap in the existing TLS security proofs. We explain the gap in the model assumptions and subsequently in the security proofs. We also provide an enhanced Multi-Stage Key Exchange (MSKE) model that captures the additional required assumptions of TLS 1.3 in its current state. The resulting security claims in the case of external PSKs are accordingly different.

We identify a security vulnerability in this TLS 1.3 path, by showing a new reflection attack that we call ``Selfie''. The Selfie attack breaks the mutual authentication. It leverages the fact that TLS does not mandate explicit authentication of the server and the client in every message.

The paper explains the root cause of this TLS 1.3 vulnerability, demonstrates the Selfie attack on the TLS implementation of OpenSSL and proposes appropriate mitigation.

The attack is surprising because it breaks some assumptions and uncovers an interesting gap in the existing TLS security proofs. We explain the gap in the model assumptions and subsequently in the security proofs. We also provide an enhanced Multi-Stage Key Exchange (MSKE) model that captures the additional required assumptions of TLS 1.3 in its current state. The resulting security claims in the case of external PSKs are accordingly different.

ePrint Report
Yet Another Side Channel Cryptanalysis on SM3 Hash Algorithm
Christophe Clavier, Leo Reynaud, Antoine Wurcker

SM3, the Chinese standard hash algorithm inspired from SHA2, can be
attacker by similar means than SHA2 up to an adaptation to its differences. But this
kind of attack is based on targeting point of interest of different kinds, some are end
of computation results, that are stored when others are in intermediate computational
data. The leakage effectiveness of the later could be subject to implementation choices,
device type or device type of leakage. In this paper, we propose a new approach that
targets only the first kind of intermediate data that are more susceptible to appear.
As an example, we targeted the HMAC construction using SM3, where our method
allows to recover the first half of the secret information. reducing the security of the
HMAC protocol.

Second-order analyses have shown a great interest to defeat first level of masking protections.
Their practical realization remains tedious in a lot of cases.
This is partly due to the difficulties of achieving a fine alignment of two areas that are combined together afterward.
Classical protections makes therefore use of random jitter or shuffling to make the alignment difficult or even impossible.
This paper extends Scatter attack to high-order analyses.
Processing the jointdistribution of two selection of points, it becomes possible to retrieve the secret key even when traces are not fully aligned.
The results presented in this paper are validated through practical experimentation and compared with existing window-based techniques, such as the FFT.
Scatter shows the best results when misalignment is significant.
This illustrates that Scatter offers an alternative to existing high-order attacks and can target all kinds of cryptography implementations, regardless they are executed in hardware or software.
With the ability to exploit several leakage points, it may be valuable also when applying a second-order attack on aligned traces.

ePrint Report
Cryptanalysis of Curl-P and Other Attacks on the IOTA Cryptocurrency
Ethan Heilman, Neha Narula, Garrett Tanzer, James Lovejoy, Michael Colavita, Madars Virza, Tadge Dryja

We present attacks on the cryptography formerly used in the IOTA blockchain, including under certain conditions the ability to forge signatures. We developed practical attacks on IOTA's cryptographic hash function Curl-P-27, allowing us to quickly generate short colliding messages. These collisions work even for messages of the same length. Exploiting these weaknesses in Curl-P-27, we broke the EU-CMA security of the former IOTA Signature Scheme (ISS). Finally, we show that in a chosen-message setting we could forge signatures and multi-signatures of valid spending transactions (called bundles in IOTA).

ePrint Report
Optimizations of Side-Channel Attack on AES MixColumns Using Chosen Input
Aurelien Vasselle, Antoine Wurcker

Considering AES sub-steps that can be attacked with a small guess space,
the most practicable is to target SubBytes of extremal rounds. For its contrast
between candidates (non-linearity) and that the search space is reduced to 28 -sized
blocks. But when such point of interests are not available, MixColumns may be
considered but involve search spaces of 2^32 -sized blocks. This number of attacks to
run being often considered as unrealistic to reach, published papers propose to attack
using chosen inputs in order to reduce back search space to 2^8 -sized blocks. Several
sets of chosen inputs acquisition will then be required to succeed an attack.
Our contribution consists in an optimization of usage of gained information that
allows to drastically reduce the number of set needed to realize such an attack, even
to only one set in some configurations.

ePrint Report
LightChain: A DHT-based Blockchain for Resource Constrained Environments
Yahya Hassanzadeh-Nazarabadi, Alptekin Küpçü, Öznur Özkasap

As an append-only distributed database, blockchain is utilized in a vast
variety of applications including the cryptocurrency and Internet-of-Things
(IoT). The existing blockchain solutions have downsides in communication and
storage efficiency, convergence to centralization, and consistency problems. In
this paper, we propose LightChain, which is the first blockchain architecture
that operates over a Distributed Hash Table (DHT) of participating peers.
LightChain is a permissionless blockchain that provides addressable blocks and
transactions within the network, which makes them efficiently accessible by all
the peers. Each block and transaction is replicated within the DHT of peers and
is retrieved in an on-demand manner. Hence, peers in LightChain are not
required to retrieve or keep the entire blockchain. LightChain is fair as all
of the participating peers have a uniform chance of being involved in the
consensus regardless of their influence such as hashing power or stake.
LightChain provides a deterministic fork-resolving strategy as well as a
blacklisting mechanism, and it is secure against colluding adversarial peers
attacking the availability and integrity of the system. We provide mathematical
analysis and experimental results on scenarios involving 10K nodes to
demonstrate the security and fairness of LightChain.

ePrint Report
MixEth: efficient, trustless coin mixing service for Ethereum
István András Seres, Dániel A. Nagy, Chris Buckland, Péter Burcsi

Coin mixing is a prevalent privacy-enhancing technology for cryptocurrency users. In this paper, we present MixEth, which is a trustless coin mixing service for Turing-complete blockchains. MixEth does not rely on a trusted setup and is more efficient than any proposed trustless coin tumbler. It requires only 3 on-chain transactions at most per user and 1 off-chain message. It achieves strong notions of anonymity and is able to resist denial-of-service attacks. Furthermore the underlying protocol can also be used to efficiently shuffle ballots, ciphertexts in a trustless and decentralized manner.

Concerning the side-channel attacks on Advanced Encryp-
tion Standard, it seems that majority of studies focus on the lowest size:
AES-128. Even when adaptable to higher sizes (AES-192 and AES-256),
lots of state-of-the-art attacks see their complexity substantially raised.
Indeed, it often requires to perform two consecutive dependent attacks.
The first is similar to the one applied on AES-128, but a part of the key
remains unknown and must be retrieved through a second attack directly
dependent on the success of the first.
This configuration may substantially raise the complexity for the at-
tacker, especially if new signal acquisitions with specific input, built using
the first key part recovered, must be performed. Any error/uncertainty
in the first attack raise the key recovery complexity.
Our contribution is to show that this complexity can be lowered to two
independent attacks by the mean of attacking separately first and last
round keys. We show that the information is enough to recover the main
key (or a very small list of candidates) in a negligible exploratory effort.

ePrint Report
Lightweight Authenticated Encryption Mode of Operation for Tweakable Block Ciphers
Yusuke Naito, Takeshi Sugawara

Using a small block length is a common strategy in designing lightweight block cipher. So far, many $64$-bit primitives have been proposed. However, if we use such a $64$-bit primitive for an authenticated encryption with birthday-bound security, it has only $32$-bit plaintext complexity which is subject to a practical attack. To take advantage of a short block length without losing security, we propose a lightweight AEAD mode $\mathsf{FBAE}$ that achieves beyond-birthday-bound security. For the purpose, we extend the idea of $\mathsf{iCOFB}$, originally defined with a tweakable random function, with tweakable block cipher. More specifically, we fix the tweak length which was variable in $\mathsf{iCOFB}$, and further generalize the feedback function. Moreover, we improve its security bound. We evaluate the concrete hardware performances of $\mathsf{FBAE}$. $\mathsf{FBAE}$ benefits from the small block length and shows the particularly good performances in threshold implementation.

ePrint Report
Garbled Neural Networks are Practical
Marshall Ball, Brent Carmer, Tal Malkin, Mike Rosulek, Nichole Shimanski

We show that garbled circuits are a practical choice for secure evaluation of neural network classifiers. At the protocol level, we start with the garbling scheme of Ball, Malkin & Rosulek (ACM CCS 2016) for arithmetic circuits and introduce new optimizations for modern neural network activation functions. We develop fancy-garbling, the first implementation of the BMR16 garbling scheme along with our new optimizations, as part of heavily optimized garbled-circuits tool that is driven by a TensorFlow classifier description.

We evaluate our constructions on a wide range of neural networks. We find that our approach is up to 100x more efficient than straight-forward boolean garbling (depending on the neural network). Our approach is also roughly 40% more efficient than DeepSecure (Rouhani et al., DAC 2018), the only previous garbled-circuit-based approach for secure neural network evaluation, which incorporates significant optimization techniques for boolean circuits. Furthermore, our approach is competitive with other non-garbled-circuit approaches for secure neural network evaluation.

We evaluate our constructions on a wide range of neural networks. We find that our approach is up to 100x more efficient than straight-forward boolean garbling (depending on the neural network). Our approach is also roughly 40% more efficient than DeepSecure (Rouhani et al., DAC 2018), the only previous garbled-circuit-based approach for secure neural network evaluation, which incorporates significant optimization techniques for boolean circuits. Furthermore, our approach is competitive with other non-garbled-circuit approaches for secure neural network evaluation.

ePrint Report
Anonymous Deniable Identification in Ephemeral Setup & Leakage Scenarios
Łukasz Krzywiecki, Mirosław Kutyłowski, Jakub Pezda, Marcin Słowik

In this paper we concern anonymous identification, where the verifier
can check that the user belongs to a given group of users (just like in case of
ring signatures), however a transcript of a session executed between a user and a
verifier is deniable. That is, neither the verifier nor the prover can convice a third
party that a given user has been involved in a session but also he cannot prove
that any user has been interacting with the verifier. Thereby one can achieve high
standards for protecting personal data according to the General Data Protection
Regulation – the fact that an interaction took place might be a sensitive data from
information security perspective.
We show a simple realization of this idea based on Schnorr identification scheme
arranged like for ring signatures. We show that with minor modifications one can
create a version immune to leakage of ephemeral keys.
We extend the above scenario to the case of k out of n, where the prover must
use at least k private keys corresponding to the set of n public keys. With the
most probable setting of k = 2 or 3, we are talking about the practical case of
multifactor authentication that might be necessary for applications with higher
security level.

ePrint Report
DEEP-FRI: Sampling Outside the Box Improves Soundness
Eli Ben-Sasson, Lior Goldberg, Swastik Kopparty, Shubhangi Saraf

Motivated by the quest for scalable and succinct zero knowledge arguments, we revisit worst-case-to-average-case reductions for linear spaces, raised by [Rothblum, Vadhan, Wigderson, STOC 2013]. The previous state of the art by [Ben-Sasson, Kopparty, Saraf, CCC 2018] showed that if some member of an affine space $U$ is $\delta$-far in relative Hamming distance from a linear code $V$ — this is the worst-case assumption — then most elements of $U$ are almost-$\delta$-far from $V$ — this is the average case. However, this result was known to hold only below the “double Johnson” function of the relative distance $\delta_V$ of the code $V$ , i.e., only when $\delta < 1 - \sqrt[4]{1 - \delta_V}$.
First, we increase the soundness-bound to the “one-and-a-half Johnson” function of $\delta_V$ and show that the average distance of $U$ from $V$ is nearly $\delta$ for any worst-case distance $\delta$ smaller than $1 - \sqrt[3]{1 - \delta_V}$. This bound is tight, which is somewhat surprising because the one-and-a-half Johnson function is unfamiliar in the literature on error correcting codes.
To improve soundness further for Reed Solomon codes we sample outside the box. We suggest a new protocol in which the verifier samples a single point $z$ outside the box $D$ on which codewords are evaluated, and asks the prover for the value at $z$ of the interpolating polynomial of a random element of $U$. Intuitively, the answer provided by the prover “forces” it to choose one codeword from a list of “pretenders” that are close to $U$. We call this technique Domain Extending for Eliminating Pretenders (DEEP).
The DEEP method improves the soundness of the worst-case-to-average-case reduction for RS codes up their list decoding radius. This radius is bounded from below by the Johnson bound, implying average distance is approximately $\delta$ for all $\delta < 1 - \sqrt{1 - \delta_V}$. Under a plausible conjecture about the list decoding radius of Reed-Solomon codes, average distance from $V$ is approximately $\delta$ for all $\delta$. The DEEP technique can be generalized to all linear codes, giving improved reductions for capacity-achieving list-decodable codes.
Finally, we use the DEEP technique to devise two new protocols:
• An Interactive Oracle Proof of Proximity (IOPP) for RS codes, called DEEP-FRI. This soundness of the protocol improves upon that of the FRI protocol of [Ben-Sasson et al., ICALP 2018] while retaining linear arithmetic proving complexity and logarithmic verifier arithmetic complexity.
• An Interactive Oracle Proof (IOP) for the Algebraic Linking IOP (ALI) protocol used to construct zero knowledge scalable transparent arguments of knowledge (ZK-STARKs) in [Ben-Sasson et al., eprint 2018]. The new protocol, called DEEP-ALI, improves soundness of this crucial step from a small constant $< 1/8$ to a constant arbitrarily close to $1$.

ePrint Report
Examining the Practical Side Channel Resilience of ARX-boxes
Yan Yan, Elisabeth Oswald

Implementations of ARX ciphers are hoped to have some intrinsic side channel resilience owing to the specific choice of cipher components: modular addition (A), rotation (R) and exclusive-or (X). Previous work has contributed to this understanding by developing theory regarding the side channel resilience of components (pioneered by the early works of Prouff) as well as some more recent practical investigations by Biryukov et al. that focused on lightweight cipher constructions. We add to this work by specifically studying ARX-boxes both mathematically as well as practically. Our results show that previous works' reliance on the simplistic assumption that intermediates independently leak (their Hamming weight) has led to the incorrect conclusion that the modular addition is necessarily the best target and that ARX constructions are therefore harder to attack in practice: we show that on an ARM M0, the best practical target is the exclusive or and attacks succeed with only tens of traces.

ePrint Report
Practically Efficient Secure Distributed Exponentiation without Bit-Decomposition
Abdelrahaman Aly, Aysajan Abidin, Svetla Nikova

Bit-decomposition is a powerful tool which can be used to design constant round protocols for bit-oriented multiparty computation (MPC) problems, such as comparison and Hamming weight computation. However, protocols that involve bit-decomposition are expensive in terms of performance. In this paper, we introduce a set of protocols for distributed exponentiation without bit-decomposition. We build upon the current state-of-the-art by Ning and Xu [ASIACRYPT 2010 & ASIACRYPT 2011], in terms of round and multiplicative complexity.
We consider different cases where the inputs are either private or public and present privacy-preserving protocols for each case. Our protocols offer perfect security against passive and active adversaries and have constant multiplicative and round complexity, for any fixed number of parties. Furthermore, we showcase how these primitives can be used, for instance, to perform secure distributed decryption for some public key schemes, that are based on modular exponentiation.

There are several new efficient approaches to decrease the trust in the CRS creators in the case of non-interactive zero knowledge (NIZK) in the CRS model. Recently, Groth et al. (CRYPTO 2018) defined the notion of NIZK with updatable CRS (updatable NIZK) and described an updatable SNARK. We consider the same problem in the case of QA-NIZKs.
While doing it, we define an important new property: we require that after updating the CRS, one should be able to update a previously generated argument to a new argument that is valid with the new CRS. We propose a general definitional framework for key-and-argument-updatable QA-NIZKs. After that, we describe a key-and-argument-updatable version of the most efficient known QA-NIZK for linear subspaces by Kiltz and Wee. Importantly, for obtaining soundness it suffices to update a universal public key that just consists of a matrix drawn from a KerMDH-hard distribution and thus can be shared by any pairing-based application that relies on the same hardness assumption. After specializing the universal public key to concrete language parameter, one can use the proposed key-and-argument updating algorithms to continue updating to strengthen the soundness guarantee.

ePrint Report
Efficient Private Comparison Queries over Encrypted Databases using Fully Homomorphic Encryption with Finite Fields
Benjamin Hong Meng Tan, Hyung Tae Lee, Huaxiong Wang, Shu Qin Ren, Khin Mi Mi Aung

To achieve security and privacy for data stored on the cloud, we need the ability to secure data in compute. Equality comparisons, ``$x=y, x\neq y$'', have been widely studied with many proposals but there is much room for improvement for order comparisons, ``$x < y,~x \leq y, x > y$ and $x \geq y$''. Most protocols for order comparisons have some limitation, either leaking some information about the data or requiring several rounds of communication between client and server. In addition, little work has been done on retrieving with compound conditions, mixing several equality and order comparisons. Fully homomorphic encryption (FHE) promises the ability to compute arbitrary functions on encrypted data without sacrificing privacy and without communication, but its potential has yet to be fulfilled. Particularly, private comparisons for database queries using FHE are expensive to compute.

In this work, we design efficient private database query (PDQ) protocols which support order comparisons and compound conditions. To this end, we first present a private comparison algorithm on encrypted integers using FHE, which scales efficiently for the length of input integers, by applying techniques from finite field theory. Then, we consider two scenarios for PDQ protocols, the first for retrieving data based on one order comparison and the second based on a conjunction of one order and four equality conditions. The proposed algorithm and protocols are implemented and tested to determine their performance in practice. The proposed comparison algorithm takes about 20.155 seconds to compare 697 pairs of 64-bit integers using Brakerski-Gentry-Vaikuntanathan's leveled FHE scheme with single instruction multiple data (SIMD) techniques at more than 110 bits of security. This yields an amortized rate of just 29 milliseconds per comparison. On top of that, we show that our techniques achieve an efficient PDQ protocol for one order and four equality comparisons, achieving an amortized time and communication cost of 36 milliseconds and 154 bytes per database element.

In this work, we design efficient private database query (PDQ) protocols which support order comparisons and compound conditions. To this end, we first present a private comparison algorithm on encrypted integers using FHE, which scales efficiently for the length of input integers, by applying techniques from finite field theory. Then, we consider two scenarios for PDQ protocols, the first for retrieving data based on one order comparison and the second based on a conjunction of one order and four equality conditions. The proposed algorithm and protocols are implemented and tested to determine their performance in practice. The proposed comparison algorithm takes about 20.155 seconds to compare 697 pairs of 64-bit integers using Brakerski-Gentry-Vaikuntanathan's leveled FHE scheme with single instruction multiple data (SIMD) techniques at more than 110 bits of security. This yields an amortized rate of just 29 milliseconds per comparison. On top of that, we show that our techniques achieve an efficient PDQ protocol for one order and four equality comparisons, achieving an amortized time and communication cost of 36 milliseconds and 154 bytes per database element.

ePrint Report
Optimized Supersingular Isogeny Key Encapsulation on ARMv8 Processors
Amir Jalali, Reza Azarderakhsh, Mehran Mozaffari Kermani, Matthew Campagna, David Jao

In this work, we present highly-optimized constant-time software libraries for Supersingular Isogeny Key Encapsulation (SIKE) protocol on ARMv8 processors. Our optimized hand-crafted assembly libraries provide the most efficient timing results on 64-bit ARM-powered devices. Moreover, the presented libraries can be integrated into any other cryptography primitives targeting the same finite field size. We design a new mixed implementation of field arithmetic on 64-bit ARM processors by exploiting the A64 and Advanced SIMD processing units working in parallel. Using these techniques, we are able to improve the performance of the entire protocol by the factor of 5 times compared to optimized C implementations on 64-bit ARM high-performance cores, providing 83-, 124-, and 159-bit quantum-security levels. Furthermore, we compare the performance of our proposed library with the previous highly-optimized ARMv8 assembly library available in the literature. The implementation results illustrate the overall 10% performance improvement in comparison with previous work, highlighting the benefit of using mixed implementation over relatively-large finite field size.

ePrint Report
Practical Supersingular Isogeny Group Key Agreement
Reza Azarderakhsh, Amir Jalali, David Jao, Vladimir Soukharev

We present the first quantum-resistant $n$-party key agreement scheme based on supersingular elliptic curve isogenies. We show that the scheme is secure against quantum adversaries, by providing a security reduction to an intractable isogeny problem. We describe the communication and computational steps required for $n$ parties to establish a common shared secret key. Our scheme is the first non-generic quantum-resistant group key agreement protocol, and is more efficient than generic protocols, with near-optimal communication overhead. In addition, our scheme is contributory, which in some settings is a desirable security property: each party applies a function of their own private key to every further transmission. We implement the proposed protocol in portable C for the special case where three parties establish a shared secret. Moreover, we benchmark our software on two generations of Intel processors, highlighting the feasibility and efficiency of using the proposed scheme in practical settings. The proposed software computes the entire group key agreement in 994 and 1,374 millions of clock cycles on Intel Core i7-6500 Skylake and Core i7-2609 Sandy Bridge processors, respectively.

30
March
2019

ePrint Report
Publicly Verifiable Proofs of Sequential Work
Mohammad Mahmoody, Tal Moran, Salil Vadhan

We construct a publicly verifiable protocol for proving computational work based on collision-resistant hash functions and a new plausible complexity assumption regarding the existence of "inherently sequential" hash functions. Our protocol is based on a novel construction of time-lock puzzles. Given a sampled "puzzle" $P \gets D_n$, where $n$ is the security parameter and $D_n$ is the distribution of the puzzles, a corresponding "solution" can be generated using $N$ evaluations of the sequential hash function, where $N>n$ is another parameter, while any feasible adversarial strategy for generating valid solutions must take at least as much time as $\Omega(N)$ *sequential* evaluations of the hash function after receiving $P$. Thus, valid solutions constitute a "proof" that $\Omega(N)$ parallel time elapsed since $P$ was received. Solutions can be publicly and efficiently verified in time $\poly(n) \cdot \polylog(N)$. Applications of these "time-lock puzzles" include noninteractive timestamping of documents (when the distribution over the possible documents corresponds to the puzzle distribution $D_n$) and universally verifiable CPU benchmarks.

Our construction is secure in the standard model under complexity assumptions (collision-resistant hash functions and inherently sequential hash functions), and makes black-box use of the underlying primitives. Consequently, the corresponding construction in the random oracle model is secure unconditionally. Moreover, as it is a public-coin protocol, it can be made non-interactive in the random oracle model using the Fiat-Shamir Heuristic.

Our construction makes a novel use of ``depth-robust'' directed acyclic graphs---ones whose depth remains large even after removing a constant fraction of vertices---which were previously studied for the purpose of complexity lower bounds. The construction bypasses a recent negative result of Mahmoody, Moran, and Vadhan (CRYPTO `11) for time-lock puzzles in the random oracle model, which showed that it is impossible to have time-lock puzzles like ours in the random oracle model if the puzzle generator also computes a solution together with the puzzle.

Our construction is secure in the standard model under complexity assumptions (collision-resistant hash functions and inherently sequential hash functions), and makes black-box use of the underlying primitives. Consequently, the corresponding construction in the random oracle model is secure unconditionally. Moreover, as it is a public-coin protocol, it can be made non-interactive in the random oracle model using the Fiat-Shamir Heuristic.

Our construction makes a novel use of ``depth-robust'' directed acyclic graphs---ones whose depth remains large even after removing a constant fraction of vertices---which were previously studied for the purpose of complexity lower bounds. The construction bypasses a recent negative result of Mahmoody, Moran, and Vadhan (CRYPTO `11) for time-lock puzzles in the random oracle model, which showed that it is impossible to have time-lock puzzles like ours in the random oracle model if the puzzle generator also computes a solution together with the puzzle.

We show that every construction of one-time signature schemes from a random oracle achieves black-box security at most
$2^{(1+o(1))q}$, where $q$ is the total number of oracle queries asked by the key generation, signing, and verification
algorithms. That is, any such scheme can be broken with probability close to $1$ by a (computationally unbounded)
adversary making $2^{(1+o(1))q}$ queries to the oracle. This is tight up to a constant factor in the number of queries, since a
simple modification of Lamport's one-time signatures (Lamport'79) achieves $2^{(0.812-o(1))q}$ black-box security using $q$ queries to the oracle.

Our result extends (with a loss of a constant factor in the number of queries) also to the random permutation and ideal-cipher oracles. Since the symmetric primitives (e.g. block ciphers, hash functions, and message authentication codes) can be constructed by a constant number of queries to the mentioned oracles, as corollary we get lower bounds on the efficiency of signature schemes from symmetric primitives when the construction is black-box. This can be taken as evidence of an inherent efficiency gap between signature schemes and symmetric primitives.

Our result extends (with a loss of a constant factor in the number of queries) also to the random permutation and ideal-cipher oracles. Since the symmetric primitives (e.g. block ciphers, hash functions, and message authentication codes) can be constructed by a constant number of queries to the mentioned oracles, as corollary we get lower bounds on the efficiency of signature schemes from symmetric primitives when the construction is black-box. This can be taken as evidence of an inherent efficiency gap between signature schemes and symmetric primitives.

29
March
2019

Job Posting
Research Associate (Postdoc) in Applied Cryptography
University of Luxembourg, Cryptolux (SnT)

Research area

The successful candidate will join the CryptoLUX group led by Prof. Alex Biryukov. Candidates with proven research track record in one or more of the following areas are particularly encouraged to apply:

- Design/analysis of lightweight crypto (with the goal to evaluate NIST competition submissions)
- Financial cryptography (cryptocurrencies, blockchain tech)
- Privacy enhancing technologies

Your Profile

A Ph.D. degree in Computer Science, Applied Mathematics or a related field; Competitive research record in applied cryptography or information security (at least one paper in top 10 IT security/crypto conferences; or several papers at ToSC(FSE), CHES, PETs or TCC); Strong mathematical and algorithmic CS background; Good skills in programming and scripting languages; Commitment, team working and a critical mind; Fluent written and verbal communication skills in English are mandatory.

The University offers a one years employment contract (with extension possibility). The University offers highly competitive salaries and is an equal opportunity employer. You will work in an exciting international environment of a large IT security-focused research center (SnT/CSC), developing technologies which will have direct impact on the future.

Applications, written in English, should be submitted by e-mail and should include:

- A brief cover letter explaining the candidate\'s motivation and research interests
- Curriculum Vitae (including photo, education/research/ work, publications, interests)
- Information on participation in competitions, Olympiads, CTFs is a plus
- Contact information of 2-3 references

Applications will be considered on receipt therefore applying before the deadline is encouraged.

**Closing date for applications:** 1 May 2019

**Contact:** Prof. Alex Biryukov

**More information:** https://www.cryptolux.org/index.php/Vacancies

Job Posting
Two Project Associate Positions in Hardware Security (Physically Unclonable Functions)
Society for Electronic Transactions and Security [SETS], Chennai, India

Applications are invited from the Indian nationals for the position of Project Associate to work in the field of Hardware Security in the project entitled “Physical Unclonable Function (PUF) based Application Specific IC (ASIC) by Technology-Circuit-System Co-Development for Strategic Applications” funded by the office of Principal Scientific Advisor (PSA) to the Government of India.

Post: Project Associate (Number of Posts: 2)

Project Duration: 3 years (the positions as proposed are purely temporary and would be filled on Contract basis with consolidated salary under the project). The appointment will be made initially for a period of two years and extendable upto further one more year or the closing date of the project whichever is earlier.

Salary: Rs 40,000 to Rs 50,000 per month commensurate to relevant experience

Essential Qualifications: M.E/M.Tech in Electronics and Communication Engineering/ Computer Science and Engineering/ Applied Electronics/ Embedded Systems/ VLSI Design from a recognized university in First Class with 60% or above marks or equivalent.

Areas of Skill sets/ Knowledge required:

a) Knowledge in cryptology and strong background in digital system design, including project development experience in C, MATLAB, VHDL/Verilog programming. b) In-depth knowledge of front-end digital design process and related design flows (Xilinx FPGA /ASIC digital IC design). c) Candidates with prior industrial/research experience in the field of Hardware Security including Physical Unclonable Function (PUF) and Side Channel Attacks (SCA) are preferred.

Selection Procedure: Written Test and/or Interview

How to Apply?

1) Applications received via email will ONLY be considered. Candidate should write “Application for Project Associate Position” in the subject line of his/her E-mail. 2) The candidate is required to attach the Personal Particulars Form in pdf format duly filled and signed. 3) The email should be sent to *hrpuf2019 (at) setsindia.net*

**Closing date for applications:** 10 April 2019

**Contact:** N. Nalla Anandakumar, Scientist, SETS (Co-Principal Investigator)

**More information:** https://setsindia.in/careers

ePrint Report
Doubly half-injective PRGs for incompressible white-box cryptography
Estuardo Alpirez Bock, Alessandro Amadori, Joppe W. Bos, Chris Brzuska, Wil Michiels

White-box cryptography was originally introduced in the setting of digital rights management with the goal of preventing a user from illegally re-distributing their software decryption program. In recent years, mobile payment has become a popular new application for white-box cryptography. Here, white-box cryptography is used to increase the robustness against external adversaries (i.e., not the user) who aim to misuse/attack the cryptographic functionalities of the payment application. A necessary requirement for secure white-box cryptography is that an adversary cannot extract the embedded secret key from the implementation. However, a white-box implementation needs to fulfill further security properties in order to provide useful protection of an application. In this paper we focus on the popular property incompressibility that is a mitigation technique against code-lifting attacks. We provide an incompressible white-box encryption scheme based on the standard-assumption of one-way permutations whereas previous works used either public-key type assumptions or non-standard symmetric-type assumptions.

ePrint Report
On the Difficulty of Hiding the Balance of Lightning Network Channels
Jordi Herrera-Joancomartí, Guillermo Navarro-Arribas, Alejandro Ranchal-Pedrosa, Joaquín Garcia-Alfaro, Cristina Pérez-Solà

The Lightning Network is a second layer technology running on top of
Bitcoin and other Blockchains. It is composed of a peer-to-peer
network, used to transfer raw information data. Some of the links in
the peer-to-peer network are identified as payment channels, used to
conduct payments between two Lightning Network clients (i.e., the two
nodes of the channel). Payment channels are created with a fixed
credit amount, the channel capacity. The channel capacity, together
with the IP address of the nodes, is published to allow a routing
algorithm to find an existing path between two nodes that do not have
a direct payment channel. However, to preserve users' privacy, the
precise balance of the pair of nodes of a given channel (i.e. the
bandwidth of the channel in each direction), is kept secret. Since
balances are not announced, second-layer nodes probe routes
iteratively, until they find a successful route to the destination for
the amount required, if any. This feature makes the routing discovery
protocol less efficient but preserves the privacy of channel balances.
In this paper, we present an attack to disclose the balance of a
channel in the Lightning Network. Our attack is based on performing
multiple payments ensuring that none of them is finalized, minimizing
the economical cost of the attack. We present experimental results
that validate our claims, and countermeasures to handle the attack.

ePrint Report
Quantum Distinguishing Attacks against Type-1 Generalized Feistel Ciphers
Gembu Ito, Tetsu Iwata

A generalized Feistel cipher is one of the methods to construct block ciphers, and it has several variants. Dong, Li, and Wang showed quantum distinguishing attacks against the $(2d-1)$-round Type-1 generalized Feistel cipher with quantum chosen-plaintext attacks, where $d\ge 3$, and they also showed key recovery attacks [Dong, Li, Wang. Sci China Inf Sci, 2019, 62(2): 022501].

In this paper, we show a polynomial time quantum distinguishing attack against the $(3d-3)$-round version, i.e., we improve the number of rounds by $(d-2)$. We also show a quantum distinguishing attack against the $(d^2-d+1)$-round version in the quantum chosen-ciphertext setting. We apply these quantum distinguishing attacks to obtain key recovery attacks against Type-1 generalized Feistel ciphers.

In this paper, we show a polynomial time quantum distinguishing attack against the $(3d-3)$-round version, i.e., we improve the number of rounds by $(d-2)$. We also show a quantum distinguishing attack against the $(d^2-d+1)$-round version in the quantum chosen-ciphertext setting. We apply these quantum distinguishing attacks to obtain key recovery attacks against Type-1 generalized Feistel ciphers.

ePrint Report
Sublinear Pairing-based Arguments with Updatable CRS and Weaker Assumptions
Alonso González, Carla Ràfols

The need for structured and trusted common parameters is often cited as one of the major drawbacks of pairing-based SNARKs. Although multiparty computation techniques can be used to address this, the resulting parameters are circuit dependent and this costly process must be repeated for every circuit. Recent proposals to switch to a weaker updatable model for parameter generation are not yet sufficiently efficient. We propose a new model for updatability which generates the common reference string in two phases, each of them updatable: in the first phase, parameters are generated for a set of universal quadratic constraints and in the second phase specific circuit dependent parameters which impose some affine constraints can be derived from them non-interactively. We propose a concrete construction based on (but more efficient than) Pinocchio.

An additional contribution of the paper is to obtain a very efficient argument for verifiable computation using the same design principles which is based on weaker assumptions. The communication is approximately 4d group elements and verifying a proof requires computing around 4d pairings and O(n+d) exponentiations, where n is the input size and d the circuit depth. While the argument for the quadratic constraints is based on standard falsifiable assumptions, the argument for the linear constraints is based on a very ad-hoc assumption about certain properties of arguments of membership in linear spaces.

An additional contribution of the paper is to obtain a very efficient argument for verifiable computation using the same design principles which is based on weaker assumptions. The communication is approximately 4d group elements and verifying a proof requires computing around 4d pairings and O(n+d) exponentiations, where n is the input size and d the circuit depth. While the argument for the quadratic constraints is based on standard falsifiable assumptions, the argument for the linear constraints is based on a very ad-hoc assumption about certain properties of arguments of membership in linear spaces.

ePrint Report
An Efficient Private Evaluation of a Decision Graph
Hiroki Sudo, Koji Nuida, Kana Shimizu

A decision graph is a well-studied classifier and has been used to solve many real-world problems. We assumed a typical scenario between two parties in this study, in which one holds a decision graph and the other wants to know the class label of his/her query without disclosing the graph and query to the other. We propose a novel protocol for this scenario that can obliviously evaluate a graph that is designed by an efficient data structure called the graph level order unary degree sequence (GLOUDS). The time and communication complexities of this protocol are linear to the number of nodes in the graph and do not include any exponential factors. The experiment results revealed that the actual runtime and communication size were well concordant with theoretical complexities. Our method can process a graph with approximately 500 nodes in only 11 s on a standard laptop computer. We also compared the runtime of our method with that of previous methods and confirmed that it was one order of magnitude faster than the previous methods.

Traceable ring signatures are a variant of ring signatures which allows the identity of a user to be revealed, when it signs two different messages with respect to the same group of users. It has applications in e-voting and in cryptocurrencies, such as the well-known Monero. We propose the first traceable ring signature scheme whose security is based on the hardness of the Syndrome Decoding problem, a problem in coding theory which is conjectured to be unsolvable by both classical and quantum algorithms. To construct the scheme, we use a variant of Stern's protocol and, by applying the Fiat-Shamir transform to it in an ingenious way, we obtain a ring signature that allows traceability. We prove that the resulting protocol has the standard security properties for traceable ring signatures in the random oracle model: tag-linkability, anonymity and exculpability. As far as we know, this is the first proposal for a traceable ring signature scheme in the post-quantum setting.

ePrint Report
Theory and application of computationally independent one-way functions: Interactive proof of ability - Revisited
Sabyasachi Dutta, Kouichi Sakurai

We introduce the concept of computationally independent pair of one-way functions (CI-OWF). We also provide two rich classes of examples of such functions based on standard assumptions. We revisit two-party interactive protocols for proving possession of computational power and existing two-flow challenge-response protocols. We analyze existing protocols for proof of computation power and propose a new two-flow protocol using CI-OWF based on square Diffie-Hellman problem.

ePrint Report
A High-Speed Constant-Time Hardware Implementation of NTRUEncrypt SVES
Farnoud Farahmand, Malik Umar Sharif, Kevin Briggs, Kris Gaj

In this paper, we present a high-speed constant time hardware implementation of NTRUEncrypt Short Vector
Encryption Scheme (SVES), fully compliant with the IEEE 1363.1 Standard Specification for Public Key Cryptographic Techniques Based on Hard Problems over Lattices. Our implementation follows an earlier proposed Post-Quantum Cryptography (PQC) Hardware Application Programming Interface (API), which facilitates its fair comparison with implementations of other PQC schemes. The paper contains the detailed flow and block diagrams, timing analysis, as well as results in terms of latency (in clock cycles), maximum clock frequency, and resource utilization in modern high-performance Field Programmable Gate Arrays (FPGAs). Our design takes full advantage of the ability to parallelize
the major operation of NTRU, polynomial multiplication, in hardware. As a result, the execution time bottleneck shifts to the hash function, SHA-256, which is sequential in nature and as a result cannot be easily sped up in hardware. The obtained FPGA results for NTRU Encrypt SVES are compared with the equivalent results for Classic McEliece, a competing, well-established Post-Quantum Cryptography encryption scheme, with a long history of unsuccessful attempts at breaking. Our code for NTRUEncrypt SVES is being made open-source to speed-up further design-space exploration and benchmarking on multiple hardware platforms.