Merged mining refers to the concept of mining more than one cryptocurrency without necessitating additional proof-of-work effort. Although merged mining has been adopted by a number of cryptocurrencies already, to this date little is known about the effects and implications. We shed light on this topic area by performing a comprehensive analysis of merged mining in practice. As part of this analysis, we present a block attribution scheme for mining pools to assist in the evaluation of mining centralization. Our findings disclose that mining pools in merge-mined cryptocurrencies have operated at the edge of, and even beyond, the security guarantees offered by the underlying Nakamoto consensus for extended periods. We discuss the implications and security considerations for these cryptocurrencies and the mining ecosystem as a whole, and link our findings to the intended effects of merged mining.

We introduce a new approach to actively secure two-party computation based on so-called oblivious linear function evaluation (OLE), a natural generalisation of oblivious transfer (OT) and a special case of the notion of oblivious polynomial evaluation introduced by Naor and Pinkas at STOC 1999. OLE works over a finite field $\F$. In an OLE the sender inputs two field elements $a \in \F$ and $b \in \F$, and the receiver inputs a field element $x \in \F$ and learns only $f(x) = ax + b$. Our protocol can evaluate an arithmetic circuit over a finite field $\F$ given black-box access to OLE for $\F$. The protocol is unconditionally secure and consumes only a constant number of OLEs per multiplication gate. An OLE over a field $\F$ of size $O(2^\kappa)$ can be implemented with communication $O(\kappa)$. This gives a protocol with communication complexity $O(\vert C \vert \kappa)$ for large enough fields, where $C$ is an arithmetic circuit computing the desired function. This asymptotically matches the best previous protocols, but our protocol at the same time obtains significantly smaller constants hidden by the big-O notation, yielding a highly practical protocol. Conceptually our techniques lift the techniques for basing practical actively secure 2PC of Boolean circuits on OT introduced under the name TinyOT by Nielsen, Nordholt, Orlandi and Burra at Crypto 2012 to the arithmetic setting. In doing so we develop several novel techniques for generating various flavours of OLE and combining these. We believe that the efficiency of our protocols, both in asymptotic and practical terms, establishes OLE and its variants as an important foundation for efficient actively secure 2PC.

The most important pre-quantum threat to AES-128 is the 1994 van Oorschot--Wiener "parallel rho method", a low-communication parallel pre-quantum multi-target preimage-search algorithm. This algorithm uses a mesh of p small processors, each running for approximately 2^128/pt fast steps, to find one of t independent AES keys k_1,...,k_t, given the ciphertexts AES_{k_1}(0),...,AES_{k_t}(0) for a shared plaintext 0. NIST has claimed a high post-quantum security level for AES-128, starting from the following rationale: "Grover's algorithm requires a long-running serial computation, which is difficult to implement in practice. In a realistic attack, one has to run many smaller instances of the algorithm in parallel, which makes the quantum speedup less dramatic." NIST has also stated that resistance to multi-key attacks is desirable; but, in a realistic parallel setting, a straightforward multi-key application of Grover's algorithm costs more than targeting one key at a time. This paper introduces a different quantum algorithm for multi-target preimage search. This algorithm shows, in the same realistic parallel setting, that quantum preimage search benefits asymptotically from having multiple targets. The new algorithm requires a revision of NIST's AES-128, AES-192, and AES-256 security claims.

Although AES is designed to be secure against a wide variety of linear and differential attacks, security ultimately depends on a combination of the engineering implementation and proper application by intended users. In this work, we attack a publicly-available VHDL implementation of AES by exploiting a partial result visible at the top-level public interface of the implementation. The vulnerability renders the security of the implementation equivalent to a one-round version of AES. An algorithm is presented that exploits this vulnerability to recover the secret key in 2^31 operations. The algorithm is coded in an interpreted high-level language and successfully recovers secret keys, with one set of known plaintext, using a general-purpose CPU in an average of 30 minutes.

Opaque predicates are a commonly used technique in program obfuscation, intended to add complexity to control flow and to insert dummy code or watermarks. However, there are many attacks known to detect opaque predicates and remove dummy code. We survey these attacks and argue that many types of programs cannot be securely obfuscated using opaque predicates. In particular we explain that most previous works on control flow obfuscation have introduced predicates that are easily distinguished from naturally occurring predicates in code, and hence easily removed by an attacker. We state two conditions that are necessary for a program to be suitable for control flow obfuscation. We give an integrated approach to control flow obfuscation that simultaneously obfuscates real predicates and introduces opaque predicates. The opaque predicates are indistinguishable from the obfuscated real predicates in the program. If an attacker applies the usual approaches (both static and dynamic) to identify and remove opaque predicates then they are likely to remove critical functionality and introduce errors. We have implemented our obfuscator in LLVM. We provide an analysis of the performance of the resulting obfuscated code.

Cryptographic channels aim to enable authenticated and confidential communication over the Internet. The general understanding seems to be that providing security in the sense of authenticated encryption for every (unidirectional) point-to-point link suffices to achieve this goal. As recently shown (in FSE17/ToSC17), however, the security properties of the unidirectional links do not extend, in general, to the bidirectional channel as a whole. Intuitively, the reason for this is that the increased interaction in bidirectional communication can be exploited by an adversary. The same applies, a fortiori, in a multi-party setting where several users operate concurrently and the communication develops in more directions. In the cryptographic literature, however, the targeted goals for group communication in terms of channel security are still unexplored. Applying the methodology of provable security, we fill this gap by defining exact (game-based) authenticity and confidentiality goals for broadcast communication, and showing how to achieve them. Importantly, our security notions also account for the causal dependencies between exchanged messages, thus naturally extending the bidirectional case where causal relationships are automatically captured by preserving the sending order. On the constructive side we propose a modular and yet efficient protocol that, assuming only point-to-point links between users, leverages (non-cryptographic) broadcast and standard cryptographic primitives to a full-fledged broadcast channel that provably meets the security notions we put forth.

In the simplest setting of proxy reencryption, there are three parties: Alice, Bob, and Polly (the proxy). Alice keeps some encrypted data that she can decrypt with a secret key known only to her. She wants to communicate the data to Bob, but not to Polly (nor anybody else). Using proxy reencryption, Alice can create a reencryption key that will enable Polly to reencrypt the data for Bob's use, but which will not help Polly learn anything about the data. There are two well-studied notions of security for proxy reencryption schemes: security under chosen-plaintext attacks (CPA) and security under chosen-ciphertext attacks (CCA). Both definitions aim to formalize the security that Alice enjoys against both Polly and Bob. In this work, we demonstrate that CPA security guarantees much less security against Bob than was previously understood. In particular, CPA security does not prevent Bob from learning Alice's secret key after receiving a single honestly reencrypted ciphertext. We also show that an existing construction of CPA secure proxy reencryption suffers from this type of weakness. As a result, CPA security provides scant guarantees in common applications. We propose security under honest reencryption attacks (HRA), a strengthening of CPA security that better captures the goals of proxy reencryption. In applications, HRA security provides much more robust security. We identify a property of proxy reencryption schemes that suffices to amplify CPA security to HRA security and show that two existing proxy reencryption schemes are in fact HRA secure.

Secure channels are one of the most pivotal building blocks of cryptography today. Internet connections, secure messaging, protected IoT data, etc., all rely upon the security of the underlying channel. In this work we define channel protocols, as well as security for channels constructed from stateful length-hiding authenticated encryption (stLHAE) schemes. Furthermore, we initiate the concept of secure termination where, upon receipt of a signifying message, a receiver is guaranteed to have received every message that has been sent, and will ever be sent, on the channel. We apply our results to real-world protocols, linking the channel environment to previous analyses of TLS 1.2, and demonstrating that TLS 1.2 achieves secure termination via fatal alerts and close_notify messages, per the specification of the Alert Protocol.

Hardware manipulations pose a serious threat to numerous systems, ranging from a myriad of smart-X devices to military systems. In many attack scenarios an adversary merely has access to the low-level, potentially obfuscated gate-level netlist. In general, the attacker possesses minimal information and faces the costly and time-consuming task of reverse engineering the design to identify security-critical circuitry, followed by the insertion of a meaningful hardware Trojan. These challenges have been considered only in passing by the research community. The contribution of this work is threefold: First, we present HAL, a comprehensive reverse engineering and manipulation framework for gate-level netlists. HAL allows automating defensive design analysis (e.g., including arbitrary Trojan detection algorithms with minimal effort) as well as offensive reverse engineering and targeted logic insertion. Second, we present a novel static analysis Trojan detection technique ANGEL which considerably reduces the false-positive detection rate of the detection technique FANCI. Furthermore, we demonstrate that ANGEL is capable of automatically detecting Trojans obfuscated with DeTrust. Third, we demonstrate how a malicious party can semi-automatically inject hardware Trojans into third-party designs. We present reverse engineering algorithms to disarm and trick cryptographic self-tests, and subtly leak cryptographic keys without any a priori knowledge of the design’s internal workings.

Cloud computing is very popular for its computing and storage capacity at lower cost. More and more data are being moved to the cloud to reduce storage cost. On the other hand, since the cloud is not fully trustable, in order to protect data privacy against third-parties and even the cloud server, they are usually encrypted before uploading. However, many operations, such as searching, are hard to perform on encrypted data. To solve this problem, searchable encryption has emerged. Searchable encryption in multi-user setting is much less efficient than in single-user setting. In order to address this problem, we propose a multi-owner to multi-user searchable encryption scheme based on attribute-based encryption. Our scheme keeps data secure in the cloud even against the cloud server. It allows users with appropriate authorizations to perform search operations on encrypted data. In addition, search tokens are generated by users instead of data owners. We prove that token privacy and index privacy are well protected in our scheme. The cloud server and illegimate users are not able to get any useful information about search tokens and ciphertexts. Ciphertexts of our scheme are constant-size, which reduce the time-complexity and bandwidth overhead of our scheme.

In an accountable anonymous system, a user is guaranteed anonymity and unlinkability unless some well-defined condition is met. A line of research focus on schemes that do not rely on any trusted third party capable of de-anonymising users. Notable examples include $k$-times anonymous authentication ($k$-TAA), blacklistable anonymous credentials (BLAC) and linkable ring signatures (LRS). All instances of these schemes are based on traditional number theoretic assumptions, which are vulnerable to quantum attacks. One common feature of these schemes is the need to limit the number of times a key can be (mis-)used. Traditionally, it is usually achieved through the use of a pseudorandom function (PRF) which maps a user's key to a pseudonym, along with a proof of correctness. However, existing lattice-based PRFs do not interact well with zero-knowledge proofs. To bridge this gap, we propose and develop the following techniques and primitives: We formalize the notion of weak PRF with efficient protocols, which allows a prover to convince a verifier that the function $\mathsf{F}$ is evaluated correctly. Specifically, we provide an efficient construction based on the learning with rounding problem, which uses abstract Stern's Protocol to prove $y = \mathsf{F}_k(x)$ and $y \neq \mathsf{F}_k(x)$ without revealing $x$, $y$ or $k$. We develop a general framework, which we call extended abstract Stern's protocol, to construct zero-knowledge arguments system for statements formed by conjunction and disjunction of sub-statements, who (or whose variants) are provable using abstract Stern's Protocol. Specifically, our system supports arbitrary monotonic propositions and allows a prover to argue polynomial relationships of the witnesses used in these sub-statements. As many existing lattice-based primitives also admit proofs using abstract Stern's protocol, our techniques can easily glue different primitives together for privacy-enhancing applications in a simple and clean way. Indeed, we propose three new schemes, all of which are the first of its kind, in the lattice setting. They also enjoy additional advantages over instances of the number-theoretic counterpart. Our $k$-TAA and BLAC schemes support concurrent enrollment while our LRS features logarithmic signature size without relying on a trusted setup. Our techniques enrich the arsenal of privacy-enhancing techniques and could be useful in the constructions of other schemes such as e-cash, unique group signatures, public key encryption with verifiable decryption, etc.

The intractability of solving the LPN problem serves as the security source of many lightweight/post-quantum cryptographic schemes proposed over the past decade. There are several algorithms available so far to fulfill the solving task. In this paper, we present further algorithmic improvements to the existing work. We describe the first efficient algorithm for the single-list $k$-sum problem which naturally arises from the various BKW reduction settings, propose the hybrid mode of BKW reduction and show how to compute the matrix multiplication in the Gaussian elimination step with flexible and reduced time/memory complexities. The new algorithms yield the best known tradeoffs on the %time/memory/data complexity curve and clearly compromise the $80$-bit security of the LPN instances suggested in cryptographic schemes. Practical experiments on reduced LPN instances verified our results.

In this paper we consider ``OR" based monochrome random grid visual cryptographic schemes (RGVCS) for $t$-$(k,n)^*$ access structure which is a generalization of the threshold $(k,n)$ access structure in the sense that in all the successful attempts to recover the secret image, the $t$ essential participants must always be present. Up to the best of our knowledge, the current proposed work is the first in the literature of RGVCS which provides efficient direct constructions for the $t$-$(k,n)^*$-RGVCS for ``OR" based model. Finding the closed form of light contrast is a challenging work. However, in this paper we come up with the closed form of the light contrast for the ``OR" based model. In literature, there are visual cryptographic schemes where the secret reconstruction is done by binary ``XOR" operation instead of ``OR" operation to increase the relative contrast of the decoded image. In this paper, we also propose an extended grid based $t$-$(k,n)^*$-RGVCS in which we replace the traditional ``OR" operation by ``XOR" operation. Note that the use of XOR operation indicates that the decoding must be performed computationally and not visually. We justified our schemes using both experimental as well as simulation based data.

We present ‘MCMix’, an anonymous messaging system that completely hides communication metadata and can scale in the order of hundreds of thousands of users. Our approach is to isolate two suitable functionalities, called dialing and conversation, that when used in succession realize anonymous messaging. With this as a starting point, we apply secure multiparty computation (``MC'' or MPC) and proceed to realize them. We present an implementation using a prevalent MPC system (Sharemind) that is competitive in terms of latency with previous messaging systems that only offer much weaker privacy guarantees. Our solution can be instantiated in a variety of different ways with different MPC implementations, overall illustrating how MPC is a viable and competitive alternative to mix-nets and DC-nets for anonymous communication.

A public key infrastructure (PKI) binds public keys to the identities of their respective owners. It employs certificate authorities or a web of trust over social links to transitively build cryptographic trust across parties in the form of chains of certificates. In existing PKIs, Alice cannot send a message to Bob confidentially until a complete chain of trust from Alice to Bob exists. We observe that this temporal restriction---which may be severely limiting in some contexts like whistleblowing---can be eliminated by combining webs of trust with concepts from hierarchical identity-based encryption. Specifically, we present a novel protocol that allows Alice to securely send a message to Bob, binding to any chain of social links, with the property that Bob can decrypt the message only after trust has been established on all links in the chain. This trust may be established either before or after Alice has sent the message, and it may be established in any order on the links. We prove the protocol's security relative to an ideal functionality, develop a prototypical implementation and evaluate the implementation's performance for a realistic environment obtained by harvesting data from an existing web of trust. We observe that our protocol is fast enough to be used in practice.

Most Multivariate Quadratic (MQ) signature schemes have a very large public key, which makes them unsuitable for many applications, despite attractive features such as speed and small signature sizes. In this paper we introduce a modification of the Unbalanced Oil and Vinegar (UOV) signature scheme that has public keys which are an order of magnitude smaller than other MQ signature schemes. The main idea is to choose UOV keys over the smallest field F2 in order to achieve small keys, but to lift the keys to a large extension field, where solving the MQ problem is harder. The resulting Lifted UOV signature scheme is very competitive with other post-quantum signature schemes in terms of key sizes, signature sizes and speed.

Assuming the existence of a public-key infrastructure (PKI), digital signatures are a fundamental building block in the design of secure consensus protocols with optimal resilience. More recently, with the advent of blockchain protocols like Bitcoin, consensus has been considered in the ``permissionless'' setting where no authentication or even point-to-point communication is available. Yet, despite some positive preliminary results, there has been no attempt to formalize a building block that is sufficient for designing consensus protocols in this setting. In this work we fill this void by putting forth a formalization of such a primitive, which we call {\em signatures of work} (SoW). Distinctive features of our new notion are a lower bound on the number of steps required to produce a signature; fast verification; {\em moderate unforgeability}---producing a sequence of SoWs, for chosen messages, does not provide an advantage to an adversary in terms of running time; and signing time independence---most relevant in concurrent multi-party applications, as we show. Armed with SoW, we then present a new permissionless consensus protocol which is secure assuming an honest majority of computational power, thus providing a blockchain counterpart to the classical Dolev-Strong consensus protocol. The protocol is built on top of a SoW-based blockchain and standard properties of the underlying hash function, thus improving on the only known provably secure consensus protocol in this setting, which relies on the random-oracle model in a fundamental way.

We present an overview of supersingular isogeny cryptography and how it fits into the broad theme of post-quantum public key crypto. The paper also gives a brief tutorial of elliptic curve isogenies and the computational problems relevant for supersingular isogeny crypto. Supersingular isogeny crypto is attracting attention due to the fact that the best attacks, both classical and quantum, require exponential time. However, the underlying computational problems have not been sufficiently studied by quantum algorithm researchers, especially since there are significant mathematical preliminaries needed to fully understand isogeny crypto. The main goal of the paper is to advertise various related computational problems, and to explain the relationships between them, in a way that is accessible to experts in quantum algorithms. This is a post-peer-review, pre-copyedit version of an article to be published as a "perspective paper" in the journal Quantum Information Processing.

Using the cloud to store data offers many advantages for businesses and individuals alike. The cloud storage provider, however, has to be trusted not to inspect or even modify the data they are entrusted with. Encrypting the data offers a remedy, but current solutions have various drawbacks. Providers which offer encrypted storage themselves cannot necessarily be trusted, since they have no open implementation. Existing encrypted file systems are not designed for usage in the cloud and do not hide metadata like file sizes or directory structure, do not provide integrity, or are prohibitively inefficient. Most have no formal proof of security. Our contribution is twofold. We first introduce a comprehensive formal model for the security and integrity of cloud file systems. Second, we present CryFS, a novel encrypted file system specifically designed for usage in the cloud. Our file system protects confidentiality and integrity (including metadata), even in presence of an actively malicious cloud provider. We give a proof of security for these properties. Our implementation is easy and transparent to use and offers performance comparable to other state-of-the-art file systems.

Oblivious RAMs, introduced by Goldreich and Ostrovsky [JACM'96], compile any RAM program into one that is ``memory oblivious'', i.e., the access pattern to the memory is independent of the input. All previous ORAM schemes, however, completely break the locality of data accesses (for instance, by shuffling the data to pseudorandom positions in memory). In this work, we initiate the study of locality-preserving ORAMs --- ORAMs that preserve locality of the accessed memory regions, while leaking only the lengths of contiguous memory regions accessed. Our main results demonstrate the existence of a locality-preserving ORAM with poly-logarithmic overhead both in terms of bandwidth and locality. We also study the tradeoff between locality, bandwidth and leakage, and show that any scheme that preserves locality and does not leak the lengths of the contiguous memory regions accessed, suffers from prohibitive bandwidth. To the best of our knowledge, before our work, the only works combining locality and obliviousness were for symmetric searchable encryption [e.g., Cash and Tessaro (EUROCRYPT'14), Asharov et al. (STOC'16)]. Symmetric search encryption ensures obliviousness if each keyword is searched only once, whereas ORAM provides obliviousness to any input program. Thus, our work generalizes that line of work to the much more challenging task of preserving locality in ORAMs.

We investigate the post-quantum security of hash functions based on the sponge construction. A crucial property for hash functions in the post-quantum setting is the collapsing property (a strengthening of collision-resistance). We show that the sponge construction is collapsing (and in consequence quantum collision-resistant) under suitable assumptions about the underlying block function. In particular, if the block function is a random function or a (non-invertible) random permutation, the sponge construction is collapsing.

Over the past few years, the increased affordability of genome sequencing and the ensuing availability of genetic data have propelled important progress in precision medicine and enabled a market for personal genomic testing. This yields exciting new opportunities for faster and more accurate diagnosis, personalized treatments, and genetically tailored wellness plans. At the same time, however, it also creates important security and privacy threats. In this paper, we present a new cryptographic protocol, PAPEETE (Private, Authorized, fast PErsonal gEnomic TEsting) suitable for running different types of tests on users' genetic data--specifically, SNPs. The protocol, which builds on additively homomorphic encryption, provides privacy for both users and test facilities, and it guarantees that the test is authorized by an appropriate authority like the FDA. Finally, we present a prototype implementation of PAPEETE, and an experimental evaluation that attests to the real-world practicality of our techniques.

Private set intersection (PSI) allows two parties, who each hold a set of items, to compute the intersection of those sets without revealing anything about other items. Recent advances in PSI have significantly improved its performance for the case of semi-honest security, making semi-honest PSI a practical alternative to insecure methods for computing intersections. However, the semi-honest security model is not always a good fit for real-world problems. In this work, we introduce a new PSI protocol that is secure in the presence of malicious adversaries. Our protocol is based entirely on fast symmetric-key primitives and inherits important techniques from state-of-the-art protocols in the semi-honest setting. Our novel technique to strengthen the protocol for malicious adversaries is inspired by the dual execution technique of Mohassel \& Franklin (PKC 2006). Our protocol is optimized for the random-oracle model, but can also be realized (with a performance penalty) in the standard model. We demonstrate our protocol's practicality with a prototype implementation. To securely compute the intersection of two sets of size $2^{20}$ requires only 13 seconds with our protocol, which is $\sim 12\times$ faster than the previous best malicious-secure protocol (Rindal \& Rosulek, Eurocrypt 2017), and only $3\times$ slower than the best semi-honest protocol (Kolesnikov et al., CCS 2016).

Proxy re-encryption (PRE) is a cryptographic primitive introduced by Blaze, Bleumer and Strauss to provide delegation of decryption rights. PRE allows re-encryption of a ciphertext intended for Alice (delegator) to a ciphertext for Bob (delegatee) via a semi-honest proxy, who should not learn anything about the underlying message. In 2003, Al-Riyami and Patterson introduced the notion of certificateless public key cryptography which offers the advantage of identity-based cryptography without suffering from the key escrow problem. The existing certificateless PRE (CLPRE) schemes rely on costly bilinear pairing operations. In ACM ASIA-CCS SCC 2015, Srinivasan et al. proposed the first construction of a certificateless PRE scheme without resorting to pairing in the random oracle model. However, in this work, we demonstrate a flaw in the CCA-security proof of their scheme. Also, we present the first construction of a CLPRE scheme without pairing which meets CCA security under the computational Diffie-Hellman hardness assumption in the random oracle model.

AEZ is an authenticated encryption algorithm, submitted to the CAESAR competition. It has been selected for the third round of the competition. While some classical analysis on the algorithm have been published, the cost of these attacks is beyond the security claimed by the designers. In this paper, we show that all the versions of AEZ are completely broken against a quantum adversary. For this, we propose a generalisation of Simon's algorithm for quantum period finding that allows to build efficient attacks.

In 2012 Guneysu, et al. proposed GLP, a practical and efficient post-quantum digital signature scheme based on the computational hardness of the Ring Learning With Errors problem. It has some advantages over more recent efficient post-quantum digital signature proposals such as BLISS and Ring-TESLA, but Ring Learning With Errors hardness is more fully understood now than when GLP was published a half decade ago. Although not broken, GLP as originally proposed is no longer considered to offer strong levels of security. We propose GLYPH, a new instantiation of GLP, parametrised for 128 bits of security under the very conservative assumptions proposed in [2], which gives a strong assurance that it will be secure against forgery even if there are further developments in lattice cryptanalysis. Parameters to obtain this strong security level in an efficient manner were not possible within the original formulation of GLP, as they are not compatible with a signature compression algorithm, and to address this we also propose a new form of the compression algorithm which works efficiently with wider ranges of parameters. We have produced a software implementation of GLYPH, and we place it in the public domain at github.com/quantumsafelattices/glyph.

After the introduction of some stream ciphers with the minimal internal state, the design idea of these ciphers (i.e. the design of stream ciphers by using a secret key, not only in the initialization but also permanently in the keystream generation) has been developed. The idea lets to design lighter stream ciphers that they are suitable for devices with limited resources such as RFID, WSN. We present necessary conditions for designing a secure stream cipher with the minimal internal state. Based on the conditions, we propose Fruit-128 stream cipher for 128-bit security against all types of attacks. Our implementations showed that the area size of Fruit-128 is about 25.2% smaller than that of Grain-128a. The discussions are presented that Fruit-128 is more resistant than Grain-128a to some attacks such as Related key chosen IV attack. Sprout, Fruit-v2 and Plantlet ciphers are vulnerable to time-memory-data trade-off (TMDTO) distinguishing attacks. For the first time, IV bits were permanently used to strengthen Fruit-128 against TMDTO attacks. We will show that if IV bits are not permanently available during the keystream production step, we can eliminate the IV mixing function from it. In this case, security level decreases to 69-bit against TMDTO distinguishing attacks (that based on the application might be tolerable). Dynamic initialization is another contribution of the paper (that it can strengthen initialization of all stream ciphers with low area cost).

Side-channel distinguishers play an important role in differential power analysis, where real world leakage information is compared against hypothetical predictions in order to guess at the underlying secret key. A class of distinguishers which can be described as `cluster-based' have the advantage that they are able to exploit multi-dimensional leakage samples in scenarios where only loose, `semi-profiled' approximations of the true leakage forms are available. This is by contrast with univariate distinguishers exploiting only single points (e.g.\ correlation), and Template Attacks requiring concise fitted models which can be overly sensitive to mismatch between the profiling and attack acquisitions. This paper collects together---to our knowledge, for the first time---the various different proposals for cluster-based DPA (concretely, Differential Cluster Analysis, First Principal Components Analysis, and Linear Discriminant Analysis), and shows how they fit within the robust `semi-profiling' attack procedure proposed by Whitnall et al.\ at CHES 2015. We provide discussion of the theoretical similarities and differences of the separately proposed distinguishers as well as an empirical comparison of their performance in a range of (real and simulated) leakage scenarios and with varying parameters. Our findings have application for practitioners constrained to rely on `semi-profiled' models who wish to make informed choices about the best known procedures to exploit such information.

Gentry’s bootstrapping technique is the most famous method of obtaining fully homomorphic encryption. In previous work I proposed a fully homomorphic encryption without bootstrapping which has the weak point in the enciphering function. In this paper I propose the improved fully homomorphic public-key encryption scheme on non-associative octonion ring over finite field without bootstrapping technique. The plaintext p consists of two sub-plaintext u and v. The proposed fully homomorphic public-key encryption scheme is immune from the “p and -p attack”. The cipher text consists of three sub-cipher texts. As the scheme is based on computational difficulty to solve the multivariate algebraic equations of high degree while the almost all multivariate cryptosystems proposed until now are based on the quadratic equations avoiding the explosion of the coefficients. Because proposed fully homomorphic encryption scheme is based on multivariate algebraic equations with high degree or too many variables, it is against the Gröbner basis attack, the differential attack, rank attack and so on.

Machine learning algorithms, such as neural networks, create better predictive models when having access to larger datasets. In many domains, such as medicine and finance, each institute has only access to limited amounts of data, and creating larger datasets typically requires collaboration. However, there are privacy related constraints on these collaborations for legal, ethical, and competitive reasons. In this work, we present a feasible protocol for learning neural networks in a collaborative way while preserving the privacy of each record. This is achieved by combining Differential Privacy and Secure Multi-Party Computation with Machine Learning.

Logic locking is a technique that's proposed to protect outsourced IC designs from piracy and counterfeiting by untrusted foundries. A locked IC preserves the correct functionality only when a correct key is provided. Recently, the security of logic locking is threatened by a new attack called SAT attack, which can decipher the correct key of most logic locking techniques within a few hours even for a reasonably large key-size. This attack iteratively solves SAT formulas which progressively eliminate the incorrect keys till the circuit is unlocked. In this paper, we present a circuit block (referred to as Anti-SAT block) to enhance the security of existing logic locking techniques against the SAT attack. We show using a mathematical proof that the number of SAT attack iterations to reveal the correct key in a circuit comprising an Anti-SAT block is an exponential function of the key-size thereby making the SAT attack computationally infeasible. Besides, we address the vulnerability of the Anti-SAT block to various removal attacks and investigate obfuscation techniques to prevent these removal attacks. More importantly, we provide a proof showing that these obfuscation techniques for making Anti-SAT un-removable would not weaken the Anti-SAT block's resistance to SAT attack. Through our experiments, we illustrate the effectiveness of our approach to securing modern chips fabricated in untrusted foundries.

In this article, we revisit the design strategy of PRESENT, leveraging all the advances provided by the research community in construction and cryptanalysis since its publication, to push the design up to its limits. We obtain an improved version, named GIFT, that provides a much increased efficiency in all domains (smaller and faster), while correcting the well-known weakness of PRESENT with regards to linear hulls. GIFT is a very simple and clean design that outperforms even SIMON or SKINNY for round-based implementations, making it one of the most energy efficient ciphers as of today. It reaches a point where almost the entire implementation area is taken by the storage and the Sboxes, where any cheaper choice of Sbox would lead to a very weak proposal. In essence, GIFT is composed of only Sbox and bit-wiring, but its natural bitslice data flow ensures excellent performances in all scenarios, from area-optimised hardware implementations to very fast software implementation on high-end platforms. We conducted a thorough analysis of our design with regards to state-of-the-art cryptanalysis, and we provide strong bounds with regards to differential/linear attacks.

For a public value $y$ and a linear function $f$, giving a zero-knowledge proof of knowledge of a secret value $x$ that satisfies $f(x)=y$ is a key ingredient in many cryptographic protocols. Lattice-based constructions, in addition, require proofs of ``shortness'' of $x$. Of particular interest are constructions where $f$ is a function over polynomial rings, since these are the ones that result in efficient schemes with short keys and outputs. All known approaches for such lattice-based zero-knowledge proofs are not very practical because they involve a basic protocol that needs to be repeated many times in order to achieve negligible soundness error. In the amortized setting, where one needs to give zero-knowledge proofs for many equations for the same function $f$, the situation is more promising, though still not yet fully satisfactory. Current techniques either result in proofs of knowledge of $x$'s that are exponentially larger than the $x$'s actually used for the proof (i.e. the \emph{slack} is exponential), or they have polynomial slack but require the number of proofs to be in the several thousands before the amortization advantages ``kick in''. In this work, we give a new approach for constructing amortized zero-knowledge proofs of knowledge of short solutions over polynomial rings. Our proof has small polynomial slack and is practical even when the number of relations is as small as the security parameter.

The number field sieve is the best-known algorithm for factoring integers and solving the discrete logarithm problem in prime fields. In this paper, we present some new improvements to various steps of the number field sieve. We apply these improvements on the current 768-bit discrete logarithm record and show that we are able to perform the overall computing time in about 1260 core$\cdot$years using these improvements instead of 2350 core$\cdot$years using the best known parameters for this problem. Moreover, we show that the pre-computation phase for a 768-bit discrete logarithm problem, that allows for example to build a massive decryption tool of IPsec traffic protected by the Oakley group~1, was feasible in reasonable time using technologies available before the year 2000.

Current widely-used key exchange (KE) mechanisms will be vulnerable to quantum attacks when sufficiently strong quantum computers become available. Therefore, devising quantum-resistant replacements that combine efficiency with solid security guarantees is an important and challenging task. This paper proposes several contributions towards this goal. First, we introduce CAKE, a key encapsulation algorithm based on the QC-MDPC McEliece encryption scheme, with two major improvements: a) the use of ephemeral keys that defeats a recent reaction attack against MDPC decoding of the corresponding encryption scheme and b) a highly efficient key generation procedure for QC-MDPC-based cryptosystems. Then, we present an authenticated key exchange protocol based on CAKE, which is suitable for the Internet Key Exchange (IKE) standard. We prove that CAKE is IND-CPA secure, that the protocol is SK-Secure, and suggest practical parameters. Compared to other post-quantum schemes, we believe that CAKE is a promising candidate for post-quantum key exchange standardization.

Delegating the computation of a polynomial to a server in a verifiable way is challenging. An even more challenging problem is ensuring that this polynomial remains hidden to clients who are able to query such a server. In this paper, we formally define the notion of \emph{Private Polynomial Evaluation} (PPE). Our main contribution is to design a rigorous security model along with relations between the different security properties. We define \emph{polynomial protection} (PP), \emph{proof unforgeability} (UNF), and \emph{indistinguishability against chosen function attack} (INDCFA), which formalizes the resistance of a PPE against attackers trying to guess which polynomial is used among two polynomials of their choice. As a second contribution, we give a cryptanalysis of two PPE schemes of the literature. Finally, we design a PPE scheme called PIPE and we prove that it is PP-, UNF- and INDCFA-secure under the decisional Diffie-Hellman assumption in the random oracle model.

A fuzzy extractor (FE), proposed for deriving cryptographic keys from biometric data, enables reproducible generation of high-quality randomness from noisy inputs having sufficient min-entropy. FEs rely in their operation on a public "helper string" that is guaranteed not to leak too much information about the original input. Unfortunately, this guarantee may not hold when multiple independent helper strings are generated from correlated inputs as would occur if a user registers their biometric data with multiple servers; reusable FEs are needed in that case. Although the notion of reusable FEs was introduced in 2004, it has received relatively little attention since then. We first analyze an FE proposed by Fuller et al. (Asiacrypt 2013) based on the learning-with-errors (LWE) assumption, and show that it is not reusable. We then show how to adapt their construction to obtain a weakly reusable FE. We also show a generic technique for turning any weakly reusable FE to a strongly reusable one, in the random-oracle model. Finally, we give a direct construction of a strongly reusable FE based on the LWE assumption, that does not rely on random oracles.

Commonly used digital signature schemes have a limited lifetime because their security is based on computational assumptions that will potentially break in the future when more powerful computers are available. In 1993, Bayer et al.\ proposed to renew a digital signature by time-stamping the signature together with the signed document. Based on their idea long-term timestamp schemes have been proposed and standardized that allow to protect data integrity over long periods of time. To minimize the risk of a design failure that affects the security of these schemes, it is important to formally analyze their security. However, many of the proposed schemes have not been subject to a formal security analysis yet. In this paper, we address this issue by formally analyzing the security of a hash-based long-term timestamp scheme that is based on the ideas of Bayer et al. Our analysis shows that the security level of this scheme degrades cubic over time, a security loss that needs to be taken into account when the scheme is used in practice.

Game-based proofs are a well-established paradigm for structuring security arguments and simplifying their understanding. We present a novel framework, CryptHOL, for rigorous game-based proofs that is supported by mechanical theorem proving. CryptHOL is based on a new semantic domain with an associated functional programming language for expressing games. We embed our framework in the Isabelle/HOL theorem prover and, using the theory of relational parametricity, we tailor Isabelle’s existing proof automation to game-based proofs. By basing our framework on a conservative extension of higher-order logic and providing automation support, the resulting proofs are trustworthy and comprehensible, and the framework is extensible and widely applicable. We evaluate our framework by formalizing different game-based proofs from the literature and comparing the results with existing formal-methods tools.

Group Homomorphic Encryption (GHE), formally defined by Armknecht, Katzenbeisser and Peter, is a public-key encryption primitive where the decryption algorithm is a group homomorphism. Hence it supports homomorphic evaluation of a single algebraic operation such as modular addition or modular multiplication. Most classical homomorphic encryption schemes such as as Goldwasser-Micali and Paillier are instances of GHE. In this work, we extend GHE to the attribute-based setting. We introduce and formally define the notion of Attribute-Based GHE (ABGHE) and explore its properties. We then examine the algebraic structure on attributes induced by the group operation in an ABGHE. This algebraic stricture is a bounded semilattice. We consider some possible semilattices and how they can be realized by an ABGHE supporting inner product predicates. We then examine existing schemes from the literature and show that they meet our definition of ABGHE for either an additive or multiplicative homomorphism. Some of these schemes are in fact Identity-Based Group Homomorphic Encryption (IBGHE) schemes i.e. instances of ABGHE whose class of access policies are point functions. We then present a possibility result for IBGHE from indistinguishability obfuscation for any group for which a (public-key) GHE scheme exists.

Keeping track of financial transactions (e.g., in banks and blockchains) means keeping track of an ever-increasing list of exchanges between accounts. In fact, many of these transactions can be safely “forgotten”, in the sense that purging a set of them that compensate each other does not impact the network’s semantic meaning (e.g., the accounts’ balances). We call nilcatenation a collection of transactions having no effect on a network’s semantics. Such exchanges may be archived and removed, yielding a smaller, but equivalent ledger. Motivated by the computational and analytic benefits obtained from more compact representations of numerical data, we formalize the problem of finding nilcatenations, and propose detection methods based on graph and lattice-reduction techniques. Atop interesting applications of this work (e.g., decoupling of centralized and distributed databases), we also discuss the original idea of a “community-serving proof of work”: finding nilcatenations constitutes a proof of useful work, as the periodic removal of nilcatenations reduces the transactional graph’s size.

Verifiable random functions are pseudorandom functions producing publicly verifiable proofs for their outputs, allowing for efficient checks of the correctness of their computation. In this work, we introduce a new computational hypothesis, the n-Eigen-Value assumption, which can be seen as a relaxation of the U_n-MDDH assumption, and prove its equivalence with the n-Rank assumption. Based on the newly introduced computational hypothesis, we build the core of a verifiable random function having an exponentially large input space and reaching adaptive security under a static assumption. The final construction achieves shorter public and secret keys compared to the existing schemes reaching the same properties.

We propose the first linear-space searchable encryption scheme with constant locality and \emph{sublogarithmic} read efficiency, strictly improving the previously best known read efficiency bound (Asharov et al., STOC 2016) from $\Theta(\log N \log \log N)$ to $O(\log ^{\gamma} N)$ where $\gamma=\frac{2}{3}+\delta$ for any fixed $\delta>0$. Our scheme employs four different allocation algorithms for storing the keyword lists, depending on the size of the list considered each time. For our construction we develop (i) new probability bounds for the offline two-choice allocation problem; (ii) and a new I/O-efficient oblivious RAM with $\tilde{O}(n^{1/3})$ bandwidth overhead and zero failure probability, both of which can be of independent interest.

With Fully Homomorphic Encryption (FHE), it is possible to process encrypted data without having an access to the private-key. This has a wide range of applications, most notably the offloading of sensitive data processing. Most research on FHE has focused on the improvement of its efficiency, namely by introducing schemes based on the Ring-Learning With Errors (R-LWE) problem, and techniques such as batching, which allows for the encryption of multiple messages in the same ciphertext. Much of the related research has focused on RLWE relying on power-of-two cyclotomic polynomials. While it is possible to achieve efficient arithmetic with such polynomials, one cannot exploit batching. Herein, the efficiency of ring arithmetic underpinned by non-power-of-two cyclomotic polynomials is analysed and improved. Two methods for polynomial reduction are proposed, one based on the Barrett reduction and the other on a Montgomery representation. Speed-ups up to 2.66 are obtained for the reduction operation using an i7-5960X processor when compared with a straightforward implementation of the Barrett reduction. Moreover, the proposed methods are exploited to enhance homomorphic multiplication of FV and BGV encryption schemes, producing experimental speed-ups up to 1.37.

In this paper, we propose a family of lightweight cryptographic permutations called sLiSCP, with the sole aim to provide a realistic minimal design}that suits a variety of lightweight device applications. More precisely, we argue that for such devices the chip area dedicated for security purposes should, not only be consumed by an encryption or hashing algorithm, but also provide as many cryptographic functionalities as possible. Our main contribution is the design of a lightweight permutation employing a 4-subblock Type-2 Generalized-like Structure (GFS) and round-reduced unkeyed Simeck with either 48 or 64-bit block length as the two round functions, thus resulting in two lightweight instances of the permutation, sLiSCP-192 and sLiSCP-256. We leverage the extensive security analysis on both Simeck (Simon-like functions) and Type-2 GFSs and present bounds against differential and linear cryptanalysis. In particular, we provide an estimation on the maximum differential probability of the round-reduced Simeck and use it for bounding the maximum expected differential/linear characteristic probability for our permutation. Due to the iterated nature of the Simeck round function and the simple XOR and cyclic shift mixing layer of the GFS that fosters the propagation of long trails, the long trail strategy}is adopted to provide tighter bounds on both characteristics. Moreover, we analyze sLiSCP against a wide range of distinguishing attacks, and accordingly, claim that there exists no structural distinguishers for sLiSCP with a complexity below $2^{b/2}$ where $b$ is the state size. We demonstrate how sLiSCP can be used as a unified round function in the duplex sponge construction to build (authenticated) encryption and hashing functionalities. The parallel hardware implementation area of the unified duplex mode of sLiSCP-192 (resp. sLiSCP-256) in CMOS $65\,nm$ ASIC is 2289 (resp. 3039) GEs with a throughput of 29.62 (resp. 44.44) kbps, and their areas in CMOS $130\, nm$ are 2498 (resp. 3319) GEs.

In this paper, we revisit the security of factoring-based signature schemes built via the Fiat-Shamir transform and show that they can admit tighter reductions to certain decisional complexity assumptions such as the quadratic-residuosity, the high-residuosity, and the $\phi$-hiding assumptions. We do so by proving that the underlying identification schemes used in these schemes are a particular case of the lossy identification notion recently introduced by Abdalla et al. at Eurocrypt 2012. Next, we show how to extend these results to the forward-security setting based on ideas from the Itkis-Reyzin forward-secure signature scheme. Unlike the original Itkis-Reyzin scheme, our construction can be instantiated under different decisional complexity assumptions and has a much tighter security reduction. Moreover, we also show that the tighter security reductions provided by our proof methodology can result in concrete efficiency gains in practice, both in the standard and forward-security setting, as long as the use of stronger security assumptions is deemed acceptable. Finally, we investigate the design of forward-secure signature schemes whose security reductions are fully tight.

In their foundational paper on pseudorandom bit generation, Blum and Micali showed that the discrete logarithm problem could be solved efficiently given a ``magic box'' oracle that computes the most significant bit of the discrete logarithm with a slight advantage over guessing. This magic box can be realized on a quantum computer with a new, simplified variant of Shor's algorithm. The resulting combination of Blum and Micali's reduction and this new quantum magic box offers an intriguing hybrid approach to solving the discrete logarithm problem with a quantum computer. Because the only requirement on the quantum portion of the algorithm is that it provide an approximate estimate of a single bit of the discrete logarithm, the new algorithm may be easier to implement, more resilient to errors, and more amenable to optimization than previous approaches. Further analysis is needed to quantify the extent of these benefits in practice. The result applies to the discrete logarithm problem over both finite fields and elliptic curves. (The views expressed are my own and do not necessarily reflect those of my employer.)

We present a certificate access management system to support the USDOT's proposed rule on Vehicle-to-Vehicle (V2V) communications, Federal Motor Vehicle Safety Standard (FMVSS) No.~150. Our proposal, which we call Binary Hash Tree based Certificate Access Management (BCAM) eliminates the need for vehicles to have bidirectional connectivity with the Security Credential Management System (SCMS) for certificate update. BCAM significantly improves the ability of the SCMS to manage large-scale software and/or hardware compromise events. Vehicles are provisioned at the start of their lifetime with all the certificates they will need. However, certificates and corresponding private key reconstruction values are provided to the vehicle encrypted, and the keys to decrypt them are only made available to the vehicles shortly before the start of the validity periods of those certificates. Vehicles that are compromised can be effectively removed from the V2V system by preventing them from decrypting the certificates. We demonstrate that the system is feasible with a broadcast channel for decryption keys and other revocation information, even if that channel has a relatively low capacity.

Bernstein et al. have proposed a new permutation, Gimli, which aims to provide simple and performant implementations on a wide variety of platforms. One of the tricks used to make Gimli performant is that it processes data mostly in 96-bit columns, only occasionally swapping 32-bit words between them. Here we show that this trick is dangerous by presenting a distinguisher for reduced-round Gimli. Our distinguisher takes the form of an attack on a simple and practical PRF that should be nearly 192-bit secure. Gimli has 24 rounds. Against 15.5 of those rounds, our distinguisher uses two known plaintexts, takes about $2^{64}$ time and uses enough memory for a set with $2^{64}$ elements. Against 19$\frac12$ rounds, the same attack uses three non-adaptively chosen plaintexts, and uses twice as much memory and about $2^{128}$ time. Against $22\frac12$ rounds, it requires about $2^{138.5}$ work, $2^{129}$ bits of memory and $2^{10.5}$ non-adaptively chosen plaintexts. The same attack would apply to 23$\frac12$ rounds if Gimli had more rounds. Our attack does not use the structure of the SP-box at all, other than that it is invertible, so there may be room for improvement. On the bright side, our toy PRF puts keys and data in different positions than a typical sponge mode would do, so the attack might not work against sponge constructions.

As an invited speaker of the ACISP 2017 conference, Dongxi Liu recently introduced a new lattice-based encryption scheme (joint work with Li, Kim and Nepal) designed for lightweight IoT applications, and announced plans to submit it to the NIST postquantum competition. The new scheme is based on a variant of standard LWE called Compact-LWE, but is claimed to achieve high security levels in considerably smaller dimensions than usual lattice-based schemes. In fact, the proposed parameters, allegedly suitable for 138-bit security, involve the Compact-LWE assumption in dimension only 13. In this note, we show that this particularly aggressive choice of parameters fails to achieve the stated security level. More precisely, we show that ciphertexts in the new encryption scheme can be decrypted using the public key alone with >99.9% probability in a fraction of a second on a standard PC, which is not quite as fast as legitimate decryption, but not too far off.

Public-key encryption with keyword search (PEKS) allows a sender to generate keyword-searchable ciphertexts using a receiver’s public key and upload them to a server. Upon receiving a keyword-search trapdoor from the receiver, the server finds all matching ciphertexts. Due to the characteristics of public-key encryption, PEKS is inherently suitable for the application of numerous senders. Hence, PEKS is a well-known method to achieve secure keyword search over the encrypted email system. However, we find that without a keyword-search trapdoor, the traditional concept of PEKS still allows the server to have the obvious advantage to distinguish ciphertexts in practice. In other words, the traditional PEKS cannot guarantee the well-recognized semantic security in practice. To solve this problem, this paper defines a new concept called dynamic searchable public-key encryption (DSPE). It can hide the relationships between keyword-searchable ciphertexts and their corresponding encrypted files, and guarantee semantic security in both theory and practice. In addition, it allows the server to delete the intended ciphertexts according to the receiver’s requirement. Then, we construct a DSPE instance with provable semantic security in the random oracle model. In terms of performance, the proposed instance also has the advantage that it only requires sublinear complexity to determine all matching ciphertexts or to delete the intended ciphertexts. Finally, we experimentally demonstrate the practicability of the instance.

In the context of the security evaluation of cryptographic implementations, profiling attacks (aka Template Attacks) play a fundamental role. Nowadays the most popular Template Attack strategy consists in approximating the information leakages by Gaussian distributions. Nevertheless this approach suffers from the difficulty to deal with both the traces misalignment and the high dimensionality of the data. This forces the attacker to perform critical preprocessing phases, such as the selection of the points of interest and the realignment of measurements. Some software and hardware countermeasures have been conceived exactly to create such a misalignment. In this paper we propose an end-to-end profiling attack strategy based on the Convolutional Neural Networks: this strategy greatly facilitates the attack roadmap, since it does not require a previous trace realignment nor a precise selection of points of interest. To significantly increase the performances of the CNN, we moreover propose to equip it with the data augmentation technique that is classical in other applications of Machine Learning. As a validation, we present several experiments against traces misaligned by different kinds of countermeasures, including the augmentation of the clock jitter effect in a secure hardware implementation over a modern chip. The excellent results achieved in these experiments prove that Convolutional Neural Networks approach combined with data augmentation gives a very efficient alternative to the state-of-the-art profiling attacks.

We seek to answer the following question: To what extent can we deduplicate replicated storage? To answer this question, we design ReDup, a secure storage system that provides users with strong integrity, reliability, and transparency guarantees about data that is outsourced at cloud storage providers. Users store multiple replicas of their data at different storage servers, and the data at each storage server is deduplicated across users. Remote data integrity mechanisms are used to check the integrity of replicas. We consider a strong adversarial model, in which collusions are allowed between storage servers and also between storage servers and dishonest users of the system. A cloud storage provider (CSP) could store less replicas than agreed upon by contract, unbeknownst to honest users. ReDup defends against such adversaries by making replica generation to be time consuming so that a dishonest CSP cannot generate replicas on the fly when challenged by the users. In addition, ReDup employs transparent deduplication, which means that users get a proof attesting the deduplication level used for their files at each replica server, and thus are able to benefit from the storage savings provided by deduplication. The proof is obtained by aggregating individual proofs from replica servers, and has a constant size regardless of the number of replica servers. Our solution scales better than state of the art and is provably secure under standard assumptions.

In this work, we consider the Intersection-Sum problem: two parties hold datasets containing user identifiers, and the second party additionally has an integer value associated with each user identifier. The parties want to learn the number of users they have in common, and the sum of the associated integer values, but “nothing more”. We present a novel protocol tackling this problem using Diffie-Hellman style Private Set Intersection techniques together with Paillier homomorphic encryption. We prove security of our protocol in the honest-but-curious model. We also discuss applications for the protocol for attributing aggregate ad conversions. Finally, we present a variant of the protocol, which allows aborting if the intersection is too small, in which case neither party learns the intersection-sum.

Reachability, which answers whether one person is reachable from another through a sequence of contacts within a period of time, is of great importance in many domains such as social behavior analysis. Recently, with the prevalence of various location-based services (LBSs), a great amount of spatiotemporal location check-in data is generated by individual GPS-equipped mobile devices and collected by LBS companies, which stimulates research on reachability queries in these location check-in datasets. Meanwhile, a growing trend is for LBS companies to use scalable and cost-effective clouds to collect, store, and analyze data, which makes it necessary to encrypt location check-in data before outsourcing due to privacy concerns. In this paper, for the first time, we propose a scheme, SecReach, to securely evaluate reachability queries on encrypted location check-in data by using somewhat homomorphic encryption (SWHE). We prove that our scheme is secure against a semihonest cloud server. We also present a proof-of-concept implementation using the state-of-the-art SWHE library (i.e., HElib), which shows the efficiency and feasibility of our scheme.

Intel SGX enclaves provide hardware enforced confidentiality and integrity guarantees for running pure computations (\ie, OS-level side-effect-free code) in the cloud environment. In addition, SGX remote attestation enables enclaves to prove that a claimed enclave is indeed running inside a genuine SGX hardware and not some (adversary controlled) SGX simulator. Since cryptographic protocols do not compose well, especially when run concurrently, SGX remote attestation is only a necessary pre-condition for securely instantiating an enclave. In practice, one needs to analyze all the different interacting enclaves as a \textit{single protocol} and make sure that no sub-computation of the protocol can be simulated outside of the enclave. In this paper we describe protocol design problems under (a) sequential-composition, (b) concurrent-composition, and (c) enclave state malleability that must be taken into account while designing new enclaves. We analyze Intel provided EPID \textsf{Provisioning} and \textsf{Quoting} enclave and report our (largely positive) findings. We also provide details about how SGX uses EPID Group Signatures and report (largely negative) results about claimed anonymity guarantees.

As an important cryptographic primitive in cloud computing and outsourced computation, fully homomorphic encryption (FHE) is an animated area of modern cryptography. However, the efficiency of FHE has been a bottleneck that impeding its application. According to Gentry’s blueprint, bootstrapping, which is used to decrease ciphertext errors, is the most important process in FHE. However, bootstrapping is also the most expensive process that affecting the efficiency of the whole system. Firstly, we notice that, hundreds of serial homomorphic additions take most of the time of bootstrapping. We made use of the properties of Boolean circuit to reduce the number of serial homomorphic additions by two-thirds, and thus constructed an efficient FHE scheme with bootstrapping in 10ms. Secondly, the most expensive parts in our bootstrapping, EHCM and addition operations of multiple matrices, can be accelerated by parallel. This parallel may accelerate the bootstrapping. At last, we found a set of more efficient combination of parameters. As a result, our security parameter level is 128 bits and the correctness is elevated, compared with TFHE scheme in ASIACRYPT 2016. Experiments show that the running time of our bootstrapping is 10ms, which is only 52 percent of TFHE, and is less than CGGI17.

Non-malleable commitments are a central cryptographic primitive that guarantee security against man-in-the-middle adversaries, and their exact round complexity has been a subject of great interest. Pass (TCC 2013, CC 2016) proved that non-malleable commitments with respect to commitment are impossible to construct in less than three rounds, via black-box reductions to polynomial hardness assumptions. Obtaining a matching positive result has remained an open problem so far. While three-round constructions of non-malleable commitments have been achieved, beginning with the work of Goyal, Pandey and Richelson (STOC 2016), current constructions require super-polynomial assumptions. In this work, we settle the question of whether three-round non-malleable commitments can be based on polynomial hardness assumptions. We give constructions based on polynomial hardness of Decisional Diffie-Hellman assumption or Quadratic Residuosity or Nth Residuosity, together with ZAPs. Our protocols also satisfy concurrent non-malleability.

This paper compares the efficiency of various algorithms for implementing public key encryption scheme RLCE on 64-bit CPUs. By optimizing various algorithms for polynomial and matrix operations over finite fields, we obtained several interesting (or even surprising) results. For example, it is well known (e.g., Moenck 1976) that Karatsuba's algorithm outperforms classical polynomial multiplication algorithm from the degree 15 and above (practically, Karatsuba's algorithm only outperforms classical polynomial multiplication algorithm from the degree 35 and above ). Our experiments show that 64-bit optimized Karatsuba's algorithm will only outperform 64-bit optimized classical polynomial multiplication algorithm for polynomials of degree 115 and above over finite field $GF(2^{10})$. The second interesting (surprising) result shows that 64-bit optimized Chien's search algorithm ourperforms all other 64-bit optimized polynomial root finding algorithms such as BTA and FFT for polynomials of all degrees over finite field $GF(2^{10})$. The third interesting (surprising) result shows that 64-bit optimized Strassen matrix multiplication algorithm only outperforms 64-bit optimized classical matrix multiplication algorithm for matrices of dimension 750 and above over finite field $GF(2^{10})$. It should be noted that existing literatures and practices recommend Strassen matrix multiplication algorithm for matrices of dimension 40 and above. All experiments are done on a 64-bit MacBook Pro with i7 CPU with a single thread. The reported results should be appliable to 64 or larger bits CPU. For 32 or smaller bits CPUs, these results may not be applicable. The source code and library for the algorithms covered in this paper will be available at http://quantumca.org/.

Ridge regression is an algorithm that takes as input a large number of data points and finds the best-fit linear curve through these points. It is a building block for many machine-learning operations. This report presents a system for privacy-preserving ridge regression. The system outputs the best-fit curve in the clear, but exposes no other information about the input data. This problem was elegantly addressed by Nikolaenko et al. (S\&P 2013). They suggest an approach that combines homomorphic encryption and Yao garbled circuits. The solution presented in this report only involves homomorphic encryption. This improves the performance as Yao circuits were the main bottleneck in the previous solution.

The Bitcoin whitepaper states that security of the system is guaranteed as long as honest miners control more than half of the current total computational power. The whitepaper assumes a static difficulty, thus it is equally hard to solve a cryptographic proof-of-work puzzle for any given moment of the system history. However, the real Bitcoin network is using an adaptive difficulty adjustment mechanism. In this paper we introduce and analyze a new kind of attack on a mining difficulty retargeting function used in Bitcoin. A malicious miner is increasing his mining profits from the attack, named coin-hopping attack, and, as a side effect, an average delay between blocks is increasing. We propose an alternative difficulty adjustment algorithm in order to reduce an incentive to perform coin-hopping, and also to improve stability of inter-block delays. Finally, we evaluate the presented approach and show that the novel algorithm performs better than the original algorithm of Bitcoin.

The LLL algorithm (from Lenstra, Lenstra and Lovász) and its generalization BKZ (from Schnorr and Euchner) are widely used in cryptanalysis, especially for lattice-based cryptography. Precisely understanding their behavior is crucial for deriving appropriate key-size for cryptographic schemes subject to lattice-reduction attacks. Current models, e.g. the Geometric Series Assumption and Chen-Nguyen's BKZ-simulator, have provided a decent first-order analysis of the behavior of LLL and BKZ. However, they only focused on the average behavior and were not perfectly accurate. In this work, we initiate a second order analysis of this behavior. We confirm and quantify discrepancies between models and experiments ---in particular in the head and tail regions--- and study their consequences. We also provide variations around the mean and correlations statistics, and study their impact. While mostly based on experiments, by pointing at and quantifying unaccounted phenomena, our study sets the ground for a theoretical and predictive understanding of LLL and BKZ performances at the second order.

This work considers statistical analysis of attacks on block ciphers using several linear approximations. A general and unified approach is adopted. To this end, the general key randomisation hypotheses for multidimensional and multiple linear cryptanalysis are introduced. Expressions for the success probability in terms of the data complexity and the advantage are obtained using the general key randomisation hypotheses for both multidimensional and multiple linear cryptanalysis and under the settings where the plaintexts are sampled with or without replacement. Particularising to standard/adjusted key randomisation hypotheses gives rise to success probabilities in 16 different cases out of which in only five cases expressions for success probabilities have been previously reported. Even in these five cases, the expressions for success probabilities that we obtain are more general than what was previously obtained. A crucial step in the analysis is the derivation of the distributions of the underlying test statistics. While we carry out the analysis formally to the extent possible, there are certain inherently heuristic assumptions that need to be made. In contrast to previous works which have implicitly made such assumptions, we carefully highlight these and discuss why they are unavoidable. Finally, we provide a complete characterisation of the dependence of the success probability on the data complexity.

In distributed storage systems, documents are shared among multiple Cloud providers and stored within their respective storage servers. In social secret sharing-based distributed storage systems, shares of the documents are allocated according to the trustworthiness of the storage servers. This paper proposes a trust mechanism using machine learning techniques to compute evidence-based trust values. Our mechanism mitigates the effect of colluding storage servers. More precisely, it becomes possible to detect unreliable evidence and establish countermeasures in order to discourage the collusion of storage servers. Furthermore, this trust mechanism is applied to the social secret sharing protocol AS$^3$, showing that this new evidence-based trust mechanism enhances the protection of the stored documents.

The Number Theoretic Transform (NTT) is the time critical function required by cryptographic protocols based on the Ring Learning With Errors problem (RLWE),a popular choice for post-quantum cryptography. Here we apply a simple methodology to convert the NTT and its inverse from a mathematically correct (but side-channel vulnerable) description, to an efficient constant-time side-channel resistant version.

The polynomials defining the LFSRs of the linux Kernel PRNG are irreducible but not primitive. As a result, the space of numbers generated by these LFSRs does not fill all the space. We propose in this paper more optimal polynomials which increase by a factor of 3 the space of the random numbers generated by these LFSRs. The polynomials used in the current implementation of the PRNG and the point presented here, do not conclude a practical attack on the PRNG.

Distributed storage allows to outsource a document to the cloud such that multiple users can easily access the file. The protection of the document stored relies on secret sharing, which generates and distributes shares of the document to the storage servers. However, the users have to trust that a certain amount of storage servers behaves honestly and do not lose (retrievability) or reveal (confidentiality) the document. To address this so called social secret sharing schemes were developed that allow to adjust the distribution of shares according to the experience made with the involved storage servers. In this work, we provide a framework called AS$^3$ that allows to build social secret sharing schemes based on dynamic secret sharing. The resulting protocol has more freedom in adjusting the parameters of the shares distribution and therefore leads to more efficient and accurate solutions as well as an optimal storage consumption. Furthermore, we provide measures to detect and to prevent that the document is lost or accidentally revealed to individual storage servers. We also demonstrate how to compute trust values for storage servers, how to initialize trust values for newcomers, and provide a proof of concept implementation.

In this work we provide a framework for dynamic secret sharing and present the first dynamic and verifiable hierarchical secret sharing scheme based on Birkhoff interpolation. Since the scheme is dynamic it allows, without reconstructing the message distributed, to add and remove shareholders, to renew shares, and to modify the conditions for accessing the message. Furthermore, each shareholder can verify its share received during these algorithms protecting itself against malicious dealers and shareholders. While these algorithms were already available for classical Lagrange interpolation based secret sharing, corresponding techniques for Birkhoff interpolation based schemes were missing. Note that Birkhoff interpolation is currently the only technique available that allows to construct hierarchical secret sharing schemes that are efficient and allow to provide shares of equal size for all shareholder in the hierarchy. Thus, our scheme is an important contribution to hierarchical secret sharing.

In Attribute-Based Signatures (ABS; first defined by Maji, Prabhakaran and Rosulek, CT-RSA 2011) an authority can generate multiple signing keys, where each key is associated with a constraint $f$. A key respective to $f$ can sign a message $x$ only if $f(x) = 0$. The security requirements are unforgeability and key privacy (signatures should not expose the specific signing key used). In Homomorphic Signatures (HS; first defined by Boneh and Freeman, PKC 2011), given a signature for a data-set $x$, one can evaluate a signature for the pair $(f(x),f)$, for functions $f$. In context-hiding HS, evaluated signatures do not reveal information about the pre-evaluated signature. In this work we start by showing that these two notions are in fact equivalent. The first implication of this equivalence is a new lattice-based ABS scheme for polynomial-depth circuits, based on the HS construction of Gorbunov, Vaikuntanathan and Wichs (GVW; STOC 2015). We then construct a new ABS candidate from a worst case lattice assumption (SIS), with different parameters. Using our equivalence again, now in the opposite direction, our new ABS implies a new lattice-based HS scheme with different parameter trade-off, compared to the aforementioned GVW.

In PETS 2015, Kiayias, Leonardos, Lipmaa, Pavlyk, and Tang proposed the first $(n, 1)$-CPIR protocol with rate $1 - o (1)$. They use advanced techniques from multivariable calculus (like the Newton-Puiseux algorithm) to establish optimal rate among a large family of different CPIR protocols. It is only natural to ask whether one can achieve similar rate but with a much simpler analysis. We propose parameters to the earlier $(n, 1)$-CPIR protocol of Lipmaa (ISC 2005), obtaining a CPIR protocol that is asymptotically almost as communication-efficient as the protocol of Kiayias et al. However, for many relevant parameter choices, it is slightly more communication-efficient, due to the cumulative rounding errors present in the protocol of Kiayias et al. Moreover, the new CPIR protocol is simpler to understand, implement, and analyze. The new CPIR protocol can be used to implement (computationally inefficient) FHE with rate $1 - o (1)$.

The Universal 2nd Factor (U2F) protocol is an open authentication standard to strengthen the two-factor authentication process. It augments the existing password based infrastructure by using a specialized USB, termed as the U2F authenticator, as the 2nd factor. The U2F authenticator is assigned two fixed keys at the time of manufacture, namely the device secret key and the attestation private key. These secret keys are later used by the U2F authenticator during the Registration phase to encrypt and digitally sign data that will help in proper validation of the user and the web server. However, the use of fixed keys for the above processing leaks information through side channel about both the secrets. In this work we show why the U2F protocol is not secure against side channel attacks (SCA). We then present a countermeasure for the SCA based on re-keying technique to prevent the repeated use of the device secret key for encryption and signing. We also recommend a modification in the existing U2F protocol to minimise the effect of signing with the fixed attestation private key. Incorporating our proposed countermeasure and recommended modification, we then present a new variant of the U2F protocol that has improved security guarantees. We also briefly explain how the side channel attacks on the U2F protocol and the corresponding proposed countermeasures are similarly applicable to Universal Authentication Framework (UAF) protocol.

We propose some new baby-step giant-step algorithms for computing "low-weight" discrete logarithms; that is, for computing discrete logarithms in which the radix-b representation of the exponent is known to have only a small number of nonzero digits. Prior to this work, such algorithms had been proposed for the case where the exponent is known to have low Hamming weight (i.e., the radix-2 case). Our new algorithms (i) improve the best-known deterministic complexity for the radix-2 case, and then (ii) generalize from radix-2 to arbitrary radixes b>1. We also discuss how our new algorithms can be used to attack several recent Verifier-based Password Authenticated Key Exchange (VPAKE) protocols from the cryptographic literature with the conclusion that the new algorithms render those constructions completely insecure in practice.

The secure storage of long-lived sensitive data is constantly growing in its relevance due to the ever increasing digitization of documents. One very important challenge of this research field is to provide confidentiality for the stored data even in the long term. The only known approach to achieve this, as required, for instance, for medical records, is to use proactive secret sharing. However, all currently known schemes suffer from being inefficient. They require information-theoretic secure communication channels between any two shareholders and between the client and each shareholder and come with a high communication complexity. Thus, this work addresses the scenario where only a subset of servers holding shares is connected via private channels. Furthermore, it is sufficient if there is only one private channel between the client and one shareholder. In addition to improving practicability the presented proactive secret sharing solution, called EPSS, performs data aggregation to provide an efficient solution with respect to the communication complexity. Nevertheless, it still provides unconditional confidentiality for the data at rest and towards external attackers eavesdropping the communication channels.

Typically, when secrecy multiplication is performed in multiparty computation using Shamir’s (k,n) threshold secret sharing scheme, the result is a polynomial with degree of 2k-2 instead of k-1. This causes a problem where, in order to reconstruct a multiplication result, the number of polynomials needed will increase from k to 2k-1. Shingu et al. proposed a method to solve the problem that the degree of polynomial increases when secrecy multiplication is performed by using the (scalar value×polynomial) approach instead of the typical (polynomial×polynomial). However, this method is not secure when a combination operation, such as a product-sum operation, is performed. In this paper, we propose a multiparty computation that uses a secret sharing scheme that is secure against a product-sum operation but does not increase the degree of polynomial of the output. We prove that all combinations of the basic operations (addition, subtraction, multiplication, and division) can be performed securely using this scheme. We also propose three preconditions and finally show that our proposed method is information-theoretic secure against a passive adversary.

The XOR-Encrypt-XOR (XEX) block cipher mode was introduced by Rogaway in 2004. XEX mode uses nonce-based secret masks $(L)$ that are distinct for each message. The existence of secret masks in XEX mode prevents the application of conventional fault attack techniques, such as differential fault analysis. This work investigates other types of fault attacks against XEX mode that either eliminate the effect of the secret masks or retrieve their values. Either of these outcomes enables existing fault attack techniques to then be applied to recover the secret key. To estimate the success rate and feasibility, we ran simulations for ciphertext-only fault attacks against 128-bit AES in XEX mode. The paper discusses also the relevance of the proposed fault attacks to certain authenticated encryption modes based on XEX, such as OCB2, OTR, COPA, SHELL and ElmD. Finally, we suggest effective countermeasures to provide resistance to these fault attacks.

In this paper, we construct an anonymous and decentralized cryptocash protocol which is secure in the quantum computation model. In order to achieve that, a linkable ring signature based on the ideal lattice is proposed. The size of a signature in our scheme is O(log N ), where N is the number of participants in the ring. The framework of our cryptocash system follows that of CryptoNote with some modifications. By adopting the logarithmic size quantum resistant linkable ring signature scheme, our protocol is efficient and anonymous. We also introduce how to generate the verifying and signing key pairs of the linkable ring signature temporarily. With these techniques, both the sender and the receiver's privacy in transactions are protected even though they are published in the public ledger.

We build a privacy-preserving deep learning system in which many learning participants perform neural network-based deep learning over a combined dataset of all, without actually revealing the participants' local data. To that end, we revisit the previous work by Shokri and Shmatikov (ACM CCS 2015) and point out that local data information may be actually leaked to an honest-but-curious server. We then move on to fix that problem via building an enhanced system with following properties: (1) no information is leaked to the server; and (2) accuracy is kept intact, compared to that of the ordinary deep learning system also over the combined dataset. Our system is a bridge between deep learning and cryptography: we utilise stochastic gradient descent (SGD) applied to neural networks, in combination with additively homomorphic encryption. We show that our usage of encryption adds tolerable overhead to the ordinary deep learning system.

We introduce two new cryptographic notions in the realm of public and symmetric key encryption. Encryption with invisible edits is an encryption scheme with two tiers of users: "privileged" and "unprivileged". Privileged users know a key pair $(pk, sk)$ and "unprivileged" users know a key pair $(pk_e, sk_e)$ which is associated with an underlying edit $e$ to be applied to messages encrypted. Each key pair on its own works exactly as in standard public-key encryption, but when an unprivileged user attempts to decrypt a ciphertext generated by a privileged user of an underlying plaintext $m$, it will be decrypted to an edited $m' = Edit(m,e)$. Here, $Edit$ is some supported edit function and $e$ is a description of the particular edit to be applied. For example, we might want the edit to overwrite several sensitive blocks of data, replace all occurrences of one word with a different word, airbrush an encrypted image, etc. A user shouldn't be able to tell whether he's an unprivileged or a privileged user. An encryption with deniable edits is an encryption scheme which allows a user who owns a ciphertext $c$ encrypting a large corpus of data $m$ under a secret key $sk$, to generate an alternative but legitimate looking secret key $sk_{e,c}$ that decrypts $c$ to an "edited" version of the data $m'=Edit(m,e)$. This generalizes classical receiver deniable encryption, which can be viewed as a special case of deniable edits where the edit function performs a complete replacement of the original data. The new flexibility allows us to design solutions with much smaller key sizes than required in classical receiver deniable encryption, and in particular allows the key size to only scale with the description size of the edit $e$ which can be much smaller than the size of the plaintext data $m$. We construct encryption schemes with deniable and invisible edits for any polynomial-time computable edit function under minimal assumptions: in the public-key setting we only require the existence of standard public-key encryption and in the symmetric-key setting we only require the existence of one-way functions. The solutions to both problems use common ideas, however there is a significant conceptual difference between deniable edits and invisible edits. Whereas encryption with deniable edits enables a user to modify the meaning of a single ciphertext in hindsight, the goal of encryption with invisible edits is to enable ongoing modifications of multiple ciphertexts.

Secure instant messaging is utilized in two variants: one-to-one communication and group communication. While the first variant has received much attention lately (Frosch et al., EuroS&P16; Cohn-Gordon et al., EuroS&P17; Kobeissi et al., EuroS&P17), little is known about the cryptographic mechanisms and security guarantees of secure group communication in instant messaging. To approach an investigation of group instant messaging protocols, we first provide a comprehensive and realistic security model. This model combines security and reliability goals from various related literature to capture relevant properties for communication in dynamic groups. Thereby the definitions consider their satisfiability with respect to the instant delivery of messages. To show its applicability, we analyze three widely used real-world protocols: Signal, WhatsApp, and Threema. Since these protocols and their implementations are mostly undocumented for the public and two out of three applications among them are closed source, we describe the group protocols employed in Signal, WhatsApp, and Threema. By applying our model, we reveal several shortcomings with respect to the security definition. Therefore we propose generic countermeasures to enhance the protocols regarding the required security and reliability goals. Our systematic analysis reveals that (1) the communications' integrity – represented by the integrity of all exchanged messages – and(2) the groups' closeness – represented by the members' ability of managing the group – are not end-to-end protected. We additionally show that strong security properties, such as Future Secrecy which is a core part of the one-to-one communication in the Signal protocol, do not hold for its group communication.

Families of stable cyclic groups of nonlinear polynomial transformations of affine spaces $K^n$ over general commutative ring $K$ of increasing with $n$ order can be used in the key exchange protocols and related to them El Gamal multivariate cryptosystems. We suggest to use high degree of noncommutativity of affine Cremona group and modify multivariate El Gamal algorithm via the usage of conjugations for two polynomials of kind $g^k$ and $g^{-1}$ given by key holder (Alice) or giving them as elements of different transformation groups. We present key exchange protocols based on twisted discrete logarithms problem which uses noncommutativity of semigroup. Recent results on the existence of families of stable transformations of prescribed degree and density and exponential order over finite fields can be used for the implementation of schemes as above with feasible computational complexity. We introduce an example of a new implemented quadratic multivariate cryptosystem based on the above mentioned ideas.

Composability and robustness against physical defaults (e.g., glitches) are two highly desirable properties for secure implementations of masking schemes. While tools exist to guarantee them separately, no current formalism enables their joint investigation. In this paper, we solve this issue by introducing a new model, the robust probing model, that is naturally suited to capture the combination of these properties. We first motivate this formalism by analyzing the excellent robustness and low randomness requirements of first-order threshold implementations, and highlighting the difficulty to extend them to higher orders. Next, and most importantly, we use our theory to design higher-order secure, robust and composable multiplication gadgets. While admittedly inspired by existing approaches to masking (e.g., Ishai-Sahai-Wagner-like, threshold, domain-oriented), these gadgets exhibit subtle implementation differences with these state-of-the-art solutions (none of which being provably composable and robust). Hence, our results illustrate how sound theoretical models can guide practically-relevant implementations.

A group of $n$ users want to run a distributed protocol $\pi$ over a network where communication occurs via private point-to-point channels. Unfortunately, an adversary, who knows $\pi$, is able to maliciously flip bits on the channels. Can we efficiently simulate $\pi$ in the presence of such an adversary? We show that this is possible, even when $L$, the number of bits sent in $\pi$, and $T$, the number of bits flipped by the adversary are not known in advance. In particular, we show how to create a robust version of $\pi$ that 1) fails with probability at most $\delta$, for any $\delta > 0$; and 2) sends $\tilde{O}(L+T)$ bits, where the $\tilde{O}$ notation hides a $\log(nL/\delta)$ term multiplying $L$. Additionally, we show how to improve this result when the average message size $\alpha$ is not constant. In particular, we give an algorithm that sends $O(L(1 + (1/\alpha) \log(nL/\delta) + T )$ bits. This algorithm is adaptive in that it does not require a priori knowledge of $\alpha$. We note that if $\alpha$ is $\Omega (log(nL/\delta))$, then this improved algorithm sends only $O(L + T)$ bits, and is therefore within a constant factor of optimal.

The advent of large-scale quantum computers has resulted in significant interest in quantum-safe cryptographic primitives. Lattice-based cryptography is one of the most attractive post-quantum cryptographic families due to its well-understood security, efficient operation and versatility. However, LWE-based schemes are still relatively bulky and slow. In this work, we present spKEX, a forward-secret, post-quantum, unauthenticated lattice-based key-exchange scheme that combines four techniques to optimize performance. spKEX relies on Learning with Rounding (LWR) to reduce bandwidth; it uses sparse and ternary secrets to speed up computations and reduce failure probability; it applies an improved key reconciliation scheme to reduce bandwidth and failure probability; and computes the public matrix A by means of a permutation to improve performance while allowing for a fresh A in each key exchange. For a quantum security level of 128 bits, our scheme requires 30% lesser bandwidth than the LWE-based key-exchange proposal Frodo [9] and allows for a fast implementation of the key exchange.

We make a number of remarks about the AES-GCM-SIV nonce-misuse resistant authenticated encryption scheme currently considered for standardization by the Crypto Forum Research Group (CFRG). First, we point out that the security analysis proposed in the ePrint report 2017/168 is incorrect, leading to overly optimistic security claims. We correct the bound and re-assess the security guarantees offered by the scheme for various parameters. Second, we suggest a simple modification to the key derivation function which would improve the security of the scheme with virtually no efficiency penalty.

Linear regression is an important statistical tool that models the relationship between some explanatory values and an outcome value using a linear function. In many current applications (e.g. predictive modelling in personalized healthcare), these values represent sensitive data owned by several different parties that are unwilling to share them. In this setting, training a linear regression model becomes challenging and needs specific cryptographic solutions. In this work, we propose a new system that can train a linear regression model with 2-norm regularization (i.e. ridge regression) on a dataset obtained by merging a finite number of private datasets. Our system is composed of two phases: The first one is based on a simple homomorphic encryption scheme and takes care of securely merging the private datasets. The second phase is a new ad-hoc two-party protocol that computes a ridge regression model solving a linear system where all coefficients are encrypted. The efficiency of our system is evaluated both on synthetically generated and real-world datasets.

Side-channel techniques have well progressed over the last few years, leading to the creation of a variety of statistical tools for extracting the secrets used in cryptographic algorithms. Such techniques are taking advantage of the side-channel traces collected during the executions of secret computations in the product. Noticeably, the vast majority of side-channel attacks requires the traces have been aligned together beforehand. This prerequisite turns out to be more and more challenging in the practical realisation of attacks as many devices include hardware or software countermeasures to increase this difficulty. This is typically achieved applying random jittering or random executions with fake operations. In this paper, we introduce \textsl{scatter}, a new attack which has the potential to tackle most of the alignment issues. scatter brings a new dimension to improve the efficiency of existing attacks and opens the door to a large set of potential new attack techniques. The effectiveness of scatter has been proven on both simulated traces and real word secure products. As a result, scatter is a new side-channel technique particularly powerful when the trace alignment represents an issue, or even when considering low-cost attacks, as the requirements in terms of equipment and expertise are significantly reduced.

Several access control systems are based on the users’ physical location/proximity to the access point. Distance- Bounding (DB) protocols constitute a classical solution to calculate the distance between a trusted verifier (e.g., an access point) and an untrusted prover (e.g., a pervasive device). The main limitation of DB is that the prover and the verifier need to lie in each other’s communication range. In this paper, we introduce the concept of Multi-Hop Distance-Estimation (MHDE) protocols, which enable a verifier to authenticate a possibly far-away prover and estimate its distance to this prover, when they are not in the communication range of each other, using an ad-hoc network of pervasive devices. More precisely, our contributions are three-fold, since we provide: (1) a formal definition for MHDE; (2) a threat model for MHDE that considers a powerful and distributed adversary; and (3) implementation of MHDE protocols with different settings. Additionally, we demonstrate our protocol to be secure in the considered threat model, and we provide a performance analysis regarding the accuracy of the distance estimation and the tolerance of limited mobility of the nodes. The results are promising in order to adopt MHDE in a distributed setting.

Since first introduced by Satoshi Nakamoto in 2008, Bitcoin has become the biggest and most well-known decentralized digital currency. Its anonymity allows users all over the world to make transactions with each other and keep their identities hidden. However, protecting private key becomes a very important issue because it is the only access to a unique account and can hardly be recovered if missing. Storing an encrypted backup of private key and its corresponding advanced key is a traditional but effective way, and many other techniques help to make the backup harder to obtain by attackers. While in this paper, we introduce a new backup scheme that can provide protection when an attacker manages to obtain the backup. It is based on Bitcoin system and ECDSA signature scheme. The biggest difference is the generation and recovery of the backup processes are both related with some specific transactions on blockchain, thus it creates a gap for legal users and attackers who manages to obtain backup to recover key. The gap is decided by the number of accounts and transactions on the blockchain which increases rapidly with the growth of Bitcoin's popularity and therefore strengthens the security of our scheme at the same time. What's more, our technique can also be combined with former ones to achieve better security.

Given a well-chosen additively homomorphic cryptosystem and a $\Sigma$ protocol with a linear answer, Damgård, Fazio, and Nicolosi proposed a non-interactive designated-verifier zero knowledge argument in the registered public key model that is sound under non-standard complexity-leveraging assumptions. In 2015, Chaidos and Groth showed how to achieve the weaker yet reasonable culpable soundness notion under standard assumptions but only if the plaintext space order is prime. It makes use of $\Sigma$ protocols that satisfy what we call the \emph{optimal culpable soundness}. Unfortunately, most of the known additively homomorphic cryptosystems (like the Paillier Elgamal cryptosystem that is secure under the standard Decisional Composite Residuosity Assumption) have composite-order plaintext space. We construct optimally culpable sound $\Sigma$ protocols and thus culpably sound non-interactive designated-verifier zero knowledge protocols for NP under standard assumptions given that the least prime divisor of the plaintext space order is large.

Block cipher modes of operation provide a way to securely encrypt using a block cipher. The main factors in analyzing modes of operation are the level of security achieved (chosen-plaintext security, authenticated encryption, nonce-misuse resistance, and so on) and performance. When measuring the security level of a mode of operation, it does not suffice to consider asymptotics, and a concrete analysis is necessary. This is especially the case today, when encryption rates can be very high, and so birthday bounds may be approached or even reached. In this paper, we show that key-derivation at every encryption significantly improves the security bounds in many cases. We present a new key-derivation method that utilizes a truncated block cipher, and show that this is far better than standard block-cipher based key derivation. We prove that by using our key derivation method, we obtain greatly improved bounds for many modes of operation, with a result that the lifetime of a key can be significantly extended. We demonstrate this for AES-CTR (CPA-security), AES-GCM (authenticated encryption) and AES-GCM-SIV (nonce-misuse resistance). Finally, we demonstrate that when using modern hardware with AES instructions (AES-NI), the performance penalty of deriving keys at each encryption is insignificant for most uses.

We analyse the security of database encryption schemes supporting range queries against persistent adversaries. The bulk of our work applies to a generic setting, where the adversary's view is limited to the set of records matched by each query (known as access pattern leakage). We also consider a more specific setting where certain rank information is also leaked. The latter is inherent to multiple recent encryption schemes supporting range queries, including Kerschbaum's FH-OPE scheme (CCS 2015), Lewi and Wu's order-revealing encryption scheme (CCS 2016), and the recently proposed Arx scheme of Poddar et al. (IACR eprint 2016/568, 2016/591). We provide three attacks. First, we consider full reconstruction, which aims to recover the value of every record, fully negating encryption. We show that for dense datasets, full reconstruction is possible within an expected number of queries $N \log N + O(N)$, where $N$ is the number of distinct plaintext values. This directly improves on a $O(N^2 \log N)$ bound in the same setting by Kellaris et al. (CCS 2016). We also provide very efficient, data-optimal algorithms that succeed with the minimum possible number of queries (in a strong, information theoretical sense), and prove a matching data lower bound for the number of queries required. Second, we present an approximate reconstruction attack recovering all plaintext values in a dense dataset within a constant ratio of error (such as a 5% error), requiring the access pattern leakage of only $O(N)$ queries. We also prove a matching lower bound. Third, we devise an attack in the common setting where the adversary has access to an auxiliary distribution for the target dataset. This third attack proves highly effective on age data from real-world medical data sets. In our experiments, observing only 25 queries was sufficient to reconstruct a majority of records to within 5 years. In combination, our attacks show that current approaches to enabling range queries offer little security when the threat model goes beyond snapshot attacks to include a persistent server-side adversary.

In this work the first linearly homomorphic authenticated encryption scheme with public verifiability and provable correctness, called LEPCoV, is presented. It improves the initial proposal by avoiding false negatives during the verification algorithm. This work provides a detailed description of LEPCoV, a comparison with the original scheme, a security and correctness proof, and a performance analysis showing that all algorithms run in reasonable time for parameters that are currently considered secure. The scheme presented here allows a user to outsource computations on encrypted data to the cloud, such that any third party can verify the correctness of the computations without having access to the original data. This makes this work an important contribution to cloud computing and applications where operations on sensitive data have to be performed, such as statistics on medical records and tallying of electronically cast votes.

We present a generic framework for runtime code polymorphism, applicable to a broad range of computing platforms including embedded systems with low computing resources (e.g. microcontrollers with few kilo-bytes of memory). Code polymorphism is defined as the ability to change the observable behaviour of a software component without changing its functional properties. In this paper we present the implementation of code polymorphism with runtime code generation, which offers many code transformation possibilities: we describe the use of random register allocation, random instruction selection, instruction shuffling and insertion of noise instructions. We evaluate the effectiveness of our framework against correlation power analysis: as compared to an unprotected implementation of AES where the secret key could be recovered in less than 50 traces in average, in our protected implementation, we increased the number of traces necessary to achieve the same attack by more than 20000x. With regards to the state of the art, our implementation shows a moderate impact in terms of performance overhead.

In the Ring-LWE literature, there are several works that use a statistical framework based on delta-subgaussian random variables. These were introduced by Miccancio and Peikert (Eurocrypt 2012) as a relaxation of subgaussian random variables. In this paper, we completely characterise delta-subgaussian random variables. In particular, we show that this relaxation from a subgaussian random variable corresponds only to the shifting of the mean. Next, we give an alternative noncentral formulation for a delta-subgaussian random variable, which we argue is more statistically natural. This formulation enables us to extend prior results on sums of delta-subgaussian random variables, and on their discretisation.

In this paper we introduce a classification of existing re-keying-based approaches to increase the security of block cipher operation modes. We introduce the concepts of external and internal re-keying putting the focus on the second one. Whereas the external re-keying approach is widely used and provides the mechanism of key usage control on a message stream processing level, the internal re-keying approach is the first known mechanism providing such a control on a single message processing level. These approaches can be applied completely independently. The internal re-keying approach was already applied to the CTR encryption mode and yielded the CTR-ACPKM mode. This mode is currently being standardized in ISO and in IETF/IRTF (CFRG). In the current paper we apply the internal re-keying approach to the well-known GCM authenticated encryption mode. The main results of this paper are a new internally re-keyed GCM-ACPKM mode and its security bounds. The proposed mode is also passing through the last formal standardization stages in IETF (CFRG). We estimate the security of the GCM-ACPKM mode respecting standard security notions. We compare both security and performance of the GCM-ACPKM and GCM modes. The results show that changing GCM mode by integrating the ACPKM internal re-keying procedure increases security, significantly extending the lifetime of a key with a negligible loss in performance. Also we show how the re-keying approaches could increase the security of TLS 1.3 cipher suites.

Logic encryption is an important hardware security technique that introduces keys to modify a given combinational circuit in order to lock the functionality from unauthorized uses. Traditional methods are all ad hoc approaches based on inserting lock gates with keys on randomly selected signals in the original circuit. Thus, it was not a surprise that a SAT-based attack developed by Subramanyan et al. successfully defeated almost all of them. New approaches such as SARLock and Anti-SAT were then developed to defend against SAT-based attack. However, they are still vulnerable with extremely low error rates. In this paper, we present a theory on logic encryption that provides a complete understanding on the design space and the trade-o between error rate and attack complexity. An e cient general design scheme is derived from the theory and some speci c designs are also suggested. We also suggest a method to insert one-way function to burden the SAT engine, and a method to obfuscate the whole design. In addition, as an application of the theory, we also develop a scienti c encryption benchmark for approximate attacks. We test our encryption designs and obfuscation techniques by the SAT-based attack, and the results have veri ed the robustness of our approach.

Tokenization is the process of consistently replacing sensitive elements, such as credit cards numbers, with non-sensitive surrogate values. As tokenization is mandated for any organization storing credit card data, many practical solutions have been introduced and are in commercial operation today. However, all existing solutions are static yet, i.e., they do not allow for efficient updates of the cryptographic keys while maintaining the consistency of the tokens. This lack of updatability is a burden for most practical deployments, as cryptographic keys must also be re-keyed periodically for ensuring continued security. This paper introduces a model for updatable tokenization with key evolution, in which a key exposure does not disclose relations among tokenized data in the past, and where the updates to the tokenized data set can be made by an untrusted entity and preserve the consistency of the data. We formally define the desired security properties guaranteeing unlinkability of tokens among different time epochs and one-wayness of the tokenization process. Moreover, we construct two highly efficient updatable tokenization schemes and prove them to achieve our security notions.

We present atomic trade protocols for Bitcoin and Ethereum that can bind two parties to swap coins in the event that two blockchains emerge from a single “pre-fork” blockchain. This work is motivated by a bet between two members of the Bitcoin community, Loaded and Roger Ver, to trade 60,000 bitcoins in the event that Bitcoin Unlimited’s planned hardfork occurs and the blockchain splits into two distinct forks. Additionally we study several ways to provide replay protection in the event of hardfork alongside a novel mechanism called migration inputs. We provide a detailed survey and history of previous softforks and hardforks in Ethereum and Bitcoin.

In this article, we provide the first independent security analysis of Deoxys, a third-round authenticated encryption candidate of the CAESAR competition, and its internal tweakable block ciphers Deoxys-BC-256 and Deoxys-BC-384. We show that the related-tweakey differential bounds provided by the designers can be greatly improved thanks to a MILP-based search tool. In particular, we develop a new method to incorporate linear incompatibility in the MILP model. We use this tool to generate valid differential paths for reduced-round versions of Deoxys-BC-256 and Deoxys-BC-384, later combining them into broader boomerang or rectangle attacks. Here, we also develop a new MILP model which optimises the two paths by taking into account the effect of the ladder switch technique. Interestingly, with the tweak in Deoxys-BC providing extra input as opposed to a classical block cipher, we can even consider beyond full-codebook attacks, and we analyse how our results can be improved in this setting. As these primitives are based on the TWEAKEY framework, we further study how the security of the cipher is impacted when playing with the tweak/key sizes. All in all, we are able to attack 10 rounds of Deoxys-BC-256 (out of 14) and 14 rounds of Deoxys-BC-384 (out of 16). The extra rounds specified in Deoxys-BC to balance the tweak input (when compared to AES) seem to provide about the same security margin as AES-128. Finally we analyse why the authenticated encryption modes of Deoxys mostly prevent our attacks on Deoxys-BC to apply to the authenticated encryption primitive.

A basic question of cryptographic complexity is to combinatorially characterize all randomized functions which have information-theoretic semi-honest secure 2-party computation protocols. The corresponding question for deterministic functions was answered almost three decades back, by Kushilevitz (FOCS 1989). In this work, we make progress towards understanding securely computable `randomized' functions. We bring tools developed in the study of completeness to bear on this problem. In particular, our characterizations are obtained by considering only symmetric functions with a combinatorial property called `simplicity' (Maji et al. Indocrypt 2012). Our main result is a complete combinatorial characterization of randomized functions with `ternary output' kernels, that have information-theoretic semi-honest secure 2-party computation protocols. In particular, we show that there exist simple randomized functions with ternary output that do not have secure computation protocols. (For deterministic functions, the smallest output alphabet size of such a function is 5, due to an example given by Beaver, DIMACS Workshop on Distributed Computing and Cryptography 1989.) Also, we give a complete combinatorial characterization of randomized functions that have `2-round' information-theoretic semi-honest secure 2-party computation protocols. We also give a counter-example to a natural conjecture for the full characterization, namely, that all securely computable simple functions have secure protocols with a unique transcript for each output value. This conjecture is in fact true for deterministic functions, and -- as our results above show -- for ternary functions and for functions with 2-round secure protocols.