Updated: 5 hours 51 min ago

### Four Round Secure Computation without Setup

Sat, 03/10/2018 - 17:45
We construct a 4-round multi-party computation protocol in the plain model for any functionality, secure against a malicious adversary. Our protocol relies on the sub-exponential hardness of the Learning with Errors (LWE) problem with slightly super-polynomial noise ratio, and on the existence of adaptively secure commitments. Lin, Pass and Soni (FOCS '17) provide an adaptively secure commitment scheme from Time-Lock Puzzles. Our round complexity matches a lower bound of Garg et al. (EUROCRYPT '16), and outperforms the state of the art of 6 rounds based on similar assumptions to ours, and 5 rounds relying on indistinguishability obfuscation and other strong assumptions. To do this, we construct an LWE based multi-key FHE scheme with a very simple one-round distributed setup procedure (vs. the trusted setup required in previous LWE based constructions). This lets us construct the first 3-round semi-malicious MPC protocol without setup from standard LWE using the approach of Mukherjee and Wichs (EUROCRYPT '16). Finally, subexponential hardness and adaptive commitments are used to ''compile'' the protocol into the fully malicious setting.

### MDS Matrices with Lightweight Circuits

Fri, 03/09/2018 - 12:30
MDS matrices are an important element for the design of block ciphers such as the AES. In recent years, there has been a lot of work on the construction of MDS matrices with a low implementation cost, in the context of lightweight cryptography. Most of the previous efforts focused on local optimization, constructing MDS matrices with coefficients that can be efficiently computed. In particular, this led to a matrix with a direct xor count of only 106, while a direct implementation of the MixColumn matrix of the AES requires 152 bitwise xors. More recently, techniques based on global optimization have been introduced, were the implementation can reuse some intermediate variables. In particular, Kranz \emph{et al.} used optimization tools to a find good implementation from the description of an MDS matrix. They have lowered the cost of implementing the MixColumn matrix to 97 bitwise xors, and proposed a new matrix with only 72 bitwise xors, the lowest cost known so far. In this work we propose a different approach to global optimization. Instead of looking for an optimized circuit of a given matrix, we run a search through a space of circuits, to find optimal circuits yielding MDS matrices. This results in MDS matrices with an even lower cost, with only 67 bitwise xors.

### The Death and Rebirth of Privacy-Preserving WiFi Fingerprint Localization with Paillier Encryption

Fri, 03/09/2018 - 12:30
Localization based on premeasured WiFi fingerprints is a popular method for indoor localization where satellite based positioning systems are unavailable. In these systems, privacy of the users' location is lost because the location is computed by the service provider. In INFOCOM'14, Li et al. presented PriWFL, a WiFi fingerprint localization system based on additively homomorphic Paillier encryption, that was claimed to protect both the users' location privacy and the service provider's database privacy. In this paper, we demonstrate a severe weakness in PriWFL that allows an attacker to compromise the service provider's database under a realistic attack model and also identify certain other problems in PriWFL that decrease its localization accuracy. Hence, we show that PriWFL does not solve the privacy problems of WiFi fingerprint localization. We also explore different solutions to implement secure privacy-preserving WiFi fingerprint localization and propose two schemes based on Paillier encryption which do not suffer from the weakness of PriWFL and offer the same localization accuracy as the privacy-violating schemes.

### Fault Analysis of the KTANTAN Family of Block Ciphers: A Revisited Work of Fault Analysis of the KATAN Family of Block Ciphers

Fri, 03/09/2018 - 12:27
This paper investigates the security of the KTANTAN block cipher against differential fault analysis. This attack is considered to be first side channel analysis of KTANTAN in the literature. KTANTAN is a relative to the KATAN block cipher. Therefore, the previous fault analysis on KATAN family of block cipher is revisited. Similar to KATAN, KTANTAN has three variants namely KTANTAN32, KTANTAN48 and KTANTAN64. The inner structure of KTANTAN is similar to KATAN except the key schedule algorithms. KATAN has been practically broken by using fault analysis, employing a transient single-bit fault model, with the assumption is that the attacker is able to inject faults randomly into the internal state of the cipher. The attack is empowerd by extended cube method similarly as applied on KATAN. The complexity of this attack is $2^{74}$ for KTANTAN32 and $2^{76}$ for both KTANTAN48 and KTANTAN64. Furthermore, based on the obtained results, this paper concludes that KTANTAN is more robust against fault analysis compared to KATAN.

### On Quantum Indifferentiability

Fri, 03/09/2018 - 12:14
We study the indifferentiability of classical constructions in the quantum setting, such as the Sponge construction or Feistel networks. (But the approach easily generalizes to other constructions, too.) We give evidence that, while those constructions are known to be indifferentiable in the classical setting, they are not indifferentiable in the quantum setting. Our approach is based on an quantum-information-theoreoretical conjecture.

### QC-MDPC: A Timing Attack and a CCA2 KEM

Fri, 03/09/2018 - 12:12
In 2013, Misoczki, Tillich, Sendrier and Barreto proposed a variant of the McEliece cryptosystem based on quasi-cyclic moderate-density parity-check (QC-MDPC) codes. This proposal uses an iterative bit-flipping algorithm in its decryption procedure. Such algorithms fail with a small probability. At Asiacrypt 2016, Guo, Johansson and Stankovski (GJS) exploited these failures to perform a key recovery attack. They introduced the notion of the distance spectrum of a sparse vector and showed that the knowledge of the spectrum is enough to find the vector. By observing many failing plaintexts they recovered the distance spectrum of the QC-MDPC secret key. In this work, we explore the underlying causes of this attack, ways in which it can be improved, and how it can be mitigated. We prove that correlations between the spectrum of the key and the spectrum of the error induce a bias on the distribution of the syndrome weight. Hence, the syndrome weight is the fundamental quantity from which secret information leaks. Assuming a side-channel allows the observation of the syndrome weight, we are able to perform a key-recovery attack, which has the advantage of exploiting all known plaintexts, not only those leading to a decryption failure. Based on this study, we derive a timing attack. It performs well on most decoding algorithms, even on the recent variants where the decryption failure rate is low, a case which is more challenging to the GJS attack. To our knowledge, this is the first timing attack on a QC-MDPC scheme. Finally, we show how to construct a new KEM, called ParQ that can reduce the decryption failure rate to a level negligible in the security parameter, without altering the QC-MDPC parameters. This is done through repeated encryption. We formally prove the IND-CCA2 security of ParQ, in a model that considers decoding failures. This KEM offers smaller key sizes and is suitable for purposes where the public key is used statically.

### Capsule: A Protocol for Secure Collaborative Document Editing

Thu, 03/08/2018 - 07:11
Today's global society strongly relies on collaborative document editing, which plays an increasingly large role in sensitive workflows. While other collaborative venues, such as secure messaging, have seen secure protocols being standardized and widely implemented, the same cannot be said for collaborative document editing. Popular tools such as Google Docs, Microsoft Office365 and Etherpad are used to collaboratively write reports and other documents which are frequently sensitive and confidential, in spite of the server having the ability to read and modify text undetected. Capsule is the first formalized and formally verified protocol standard that addresses secure collaborative document editing. Capsule provides confidentiality and integrity on encrypted document data, while also guaranteeing the ephemeral identity of collaborators and preventing the server from adding new collaborators to the document. Capsule also, to an extent, prevents the server from serving different versions of the document being collaborated on. In this paper, we provide a full protocol description of Capsule. We also provide formal verification results on the Capsule protocol in the symbolic model. Finally, we present a full software implementation of Capsule, which includes a novel formally verified signing primitive implementation.

### Topology-Hiding Computation Beyond Semi-Honest Adversaries

Wed, 03/07/2018 - 14:32
Topology-hiding communication protocols allow a set of parties, connected by an incomplete network with unknown communication graph, where each party only knows its neighbors, to construct a complete communication network such that the network topology remains hidden even from a powerful adversary who can corrupt parties. This communication network can then be used to perform arbitrary tasks, for example secure multi-party computation, in a topology-hiding manner. Previously proposed protocols could only tolerate so-called passive corruption. This paper proposes protocols that can also tolerate so-called fail-corruption (i.e., the adversary can crash any player at any point in time) and so-called semi-malicious corruption (i.e., the adversary can control a corrupted party's randomness), without leaking more than an arbitrarily small fraction of a bit of information about the topology. A small-leakage protocol was recently proposed by Ball et al. [Eurocrypt'18], but only under the unrealistic set-up assumption that each party has a trusted hardware module containing secret correlated pre-set keys, and with the further two restrictions that only passively corrupted parties can be crashed by the adversary, and semi-malicious corruption is not tolerated. Since leaking a small amount of information is unavoidable, as is the need to abort the protocol in case of failures, our protocols seem to achieve the best possible goal in a model with fail-corruption. Further contributions of the paper are applications of the protocol to obtain secure MPC protocols, which requires a way to bound the aggregated leakage when multiple small-leakage protocols are executed in parallel or sequentially. Moreover, while previous protocols are based on the DDH assumption, a new so-called PKCR public-key encryption scheme based on the LWE assumption is proposed, allowing to base topology-hiding computation on LWE. Furthermore, a protocol using fully-homomorphic encryption achieving very low round complexity is proposed.

### Logistic Regression Model Training based on the Approximate Homomorphic Encryption

Wed, 03/07/2018 - 14:32
Security concerns have been raised since big data became a prominent tool in data analysis. For instance, many machine learning algorithms aim to generate prediction models using training data which contain sensitive information about individuals. Cryptography community is considering secure computation as a solution for privacy protection. In particular, practical requirements have triggered research on the efficiency of cryptographic primitives. This paper presents a practical method to train a logistic regression model while preserving the data confidentiality. We apply the homomorphic encryption scheme of Cheon et al. (ASIACRYPT 2017) for an efficient arithmetic over real numbers, and devise a new encoding method to reduce storage of encrypted database. In addition, we adapt Nesterov's accelerated gradient method to reduce the number of iterations as well as the computational cost while maintaining the quality of an output classifier. Our method shows a state-of-the-art performance of homomorphic encryption system in a real-world application. The submission based on this work was selected as the best solution of Track 3 at iDASH privacy and security competition 2017. For example, it took about six minutes to obtain a logistic regression model given the dataset consisting of 1579 samples, each of which has 18 features with a binary outcome variable.

### The Limit of Blockchains: Infeasibility of a Smart Obama-Trump Contract

Wed, 03/07/2018 - 14:29
Blockchains have become a buzzword and many blockchain proponents believe that smart contract is a panacea to redefine the digital economy. The community has a misconception that any kind of contracts could be implemented as a blockchain smart contract. There is no doubt that Turing-complete scripting languages in blockchain techniques such as Ethereum can be used to draft many important smart contracts. However, digital economy is much more than Turing-complete smart contracts. Many protocols/contracts in our daily life could not be implemented using Turing- complete smart contracts. As an example, we formulate an Obama-Trump contract and show that this kind of contract could not be implemented using blockchain smart contract techniques. It is straightforward to observe that many contracts in our daily life could be described in terms of an Obama-Trump contract. As a background discussion, we also give a comprehensive review of historical cryptographic currency techniques, bitcoin smart contract techniques, and Turing-complete smart contract techniques in modern blockchains.

### VeritasDB: High Throughput Key-Value Store with Integrity

Wed, 03/07/2018 - 14:27
While businesses shift their databases to the cloud, they continue to depend on them to operate correctly. Alarmingly, cloud services constantly face threats from exploits in the privileged computing layers (e.g. OS, Hypervisor) and attacks from rogue datacenter administrators, which tamper with the database's storage and cause it to produce incorrect results. Although integrity verification of outsourced storage and file systems is a well-studied problem, prior techniques impose prohibitive overheads (up to 30x in throughput) and place additional responsibility on clients. We present VeritasDB, a key-value store that guarantees data integrity to the client in the presence of exploits or implementation bugs in the database server. VeritasDB is implemented as a network proxy that mediates communication between the unmodified client(s) and the unmodified database server, which can be any off-the-shelf database engine (e.g., Redis, RocksDB, Apache Cassandra). The proxy transforms each client request before forwarding it to the server and checks the correctness of the server's response before forwarding it to the client. To ensure the proxy is trusted, we use the protections of modern trusted hardware platforms, such as Intel SGX, to host the proxy's code and trusted state, thus completely eliminating trust on the cloud provider. To maintain high performance in VeritasDB while scaling to large databases, we design an authenticated Merkle B+-tree that leverages features of SGX (modest amount of protected RAM, direct access to large unprotected RAM, and CPU parallelism) to implement several novel optimizations based on caching, concurrency, and compression. On standard YCSB and Visa transaction workloads, we observe an average overhead of 2.8x in throughput and 2.5x in latency, compared to the (insecure) system with no integrity checks --- using CPU parallelism, we bring the throughput overhead down to 1.05x.

### Making Public Key Functional Encryption Function Private, Distributively

Wed, 03/07/2018 - 14:25
We put forth a new notion of distributed public key functional encryption. In such a functional encryption scheme, the secret key for a function $f$ will be split into shares $sk_i^f$. Given a ciphertext $ct$ that encrypts a message $x$, a secret key share $sk_i^f$, one can evaluate and obtain a shared value $y_i$. Adding all the shares up can recover the actual value of $f(x)$, while partial shares reveal nothing about the plaintext. More importantly, this new model allows us to establish {\em function privacy} which was not possible in the setting of regular public key functional encryption. We formalize such notion and construct such a scheme from any public key functional encryption scheme together with learning with error assumption. We then consider the problem of hosting services in the untrusted cloud. Boneh, Gupta, Mironov, and Sahai (Eurocrypt 2014) first studied such application and gave a construction based on indistinguishability obfuscation. Their construction had the restriction that the number of corrupted clients has to be bounded and known. They left an open problem how to remove such restriction. We resolve this problem by applying our function private (distributed) public key functional encryption to the setting of hosting service in multiple clouds. Furthermore, our construction provides a much simpler and more flexible paradigm which is of both conceptual and practical interests. Along the way, we strengthen and simplify the security notions of the underlying primitives, including function secret sharing.

### Analysis of Deutsch-Jozsa Quantum Algorithm

Wed, 03/07/2018 - 14:20
Deutsch-Jozsa quantum algorithm is of great importance to quantum computation. It directly inspired Shor's factoring algorithm. In this note, we remark that Deutsch-Jozsa algorithm has confused two unitary transformations: one is performed on a pure state, the other is performed on a superposition. In the past decades, no constructive specification on the essential unitary operator performed on a superposition has been found. Thus, we think the algorithm needs more specifications so as to facilitate the construction of the wanted quantum oracle.

### Stake-Bleeding Attacks on Proof-of-Stake Blockchains

Wed, 03/07/2018 - 14:19
We describe a general attack on proof-of-stake (PoS) blockchains without checkpointing. Our attack leverages transaction fees, the ability to treat transactions "out of context," and the standard longest chain rule to completely dominate a blockchain. The attack grows in power with the number of honest transactions and the stake held by the adversary, and can be launched by an adversary controlling any constant fraction of the stake. With the present statistical profile of blockchain protocols, the attack can be launched given a few years of prior blockchain operation; hence it is within the realm of feasibility for PoS protocols. Most importantly, it demonstrates how closely transaction fees and rewards are coupled with the security properties of PoS protocols. More broadly, our attack must be reflected and countered in any future PoS design that avoids checkpointing, as well as any effort to remove checkpointing from existing protocols. We describe several mechanisms for protecting against the attack that include context-sensitivity of transactions and chain density statistics.

### Hardware-Supported ORAM in Effect: Practical Oblivious Search and Update on Very Large Dataset

Wed, 03/07/2018 - 14:19
Ability to query and update over encrypted data is an essential feature to enable breach-resilient cyber-infrastructures. Statistical attacks on searchable encryption (SE) have demonstrated the importance of sealing information leakages in access patterns. In response to such attacks, Oblivious Random Access Machine (ORAM) has been proposed. However, the composition of ORAM and SE is extremely costly in client-server model, and this poses a critical barrier towards its practical adaptations. In this paper, we create a new hardware-supported privacy-enhancing platform called as Practical Oblivious Search and Update Platform (POSUP), which enables oblivious keyword search/update operations on very large datasets with a high efficiency. We harness Intel SGX to realize highly optimized oblivious data structures for oblivious search/update purposes. We implemented POSUP and evaluated its performance with Wikipedia dataset containing $\ge 2^{29}$ keyword-file pairs. Our implementation is highly efficient, where it takes 1ms to access a 3 KB block with Circuit-ORAM. Our experiments have shown that POSUP offers up to $70\times$ less end-to-end delay and $100\times$ reduced bandwidth consumption, compared with the traditional ORAM-SE composition without secure hardware. POSUP is also at least $10\times$ faster for up to 99.5% fraction of keywords to be searched, compared with existing Intel SGX-assisted search platforms.

### Universally Verifiable MPC with Applications to IRV Ballot Counting

Wed, 03/07/2018 - 14:18
We present a very simple universally verifiable MPC protocol. The first component is a threshold somewhat homomorphic cryptosystem that permits an arbitrary number of additions (in the source group), followed by a single multiplication, followed by an arbitrary number of additions in the target group. The second component is a black-box construction of universally verifiable distributed encryption switching between any public key encryption schemes supporting shared setup and key generation phases, as long as the schemes satisfy some natural additive-homomorphic properties. This allows us to switch back from the target group to the source group, and hence perform an arbitrary number of multiplications. The key generation algorithm of our prototypical cryptosystem, which is based upon concurrent verifiable secret sharing, permits robust re-construction of powers of a shared secret. We demonstrate the scalability of distribution switching as a viable approach to secure vote tallying by implementing a private verifiable form of Instant Runoff Voting on real Australian election data comprising 40,000 votes.

### Secure Search via Multi-Ring Fully Homomorphic Encryption

Wed, 03/07/2018 - 14:16
Secure search is the problem of securely retrieving from a database table (or any unsorted array) the records matching specified attributes, as in SQL SELECT...WHERE...'' queries, but where the database and the query are encrypted. Secure search has been the leading example for practical applications of Fully Homomorphic Encryption (FHE) since Gentry's seminal work in 2009, attaining the desired properties of a single-round low-communication protocol with semantic security for database and query (even during search). Nevertheless, the wide belief was that the high computational overhead of current FHE candidates is too prohibitive in practice for secure search solutions (except for the restricted case of searching for a uniquely identified record as in SQL UNIQUE constrain and Private Information Retrieval). This is due to the high degree in existing solutions that is proportional at least to the number of database records m. We present the first algorithm for secure search that is realized by a polynomial of logarithmic degree (log m)^c for a small constant c>0. We implemented our algorithm in an open source library based on HElib, and ran experiments on Amazon's EC2 cloud with up to 100 processors. Our experiments show that we can securely search to retrieve database records in a rate of searching in millions of database records in less than an hour on a single machine. We achieve our result by: (1) Designing a novel sketch that returns the first strictly-positive entry in a (not necessarily sparse) array of non-negative real numbers; this sketch may be of independent interest. (2) Suggesting a multi-ring evaluation of FHE -- instead of a single ring as in prior works -- and leveraging this to achieve an exponential reduction in the degree.

### Improved Masking for Tweakable Blockciphers with Applications to Authenticated Encryption

Tue, 03/06/2018 - 10:05
A popular approach to tweakable blockcipher design is via masking, where a certain primitive (a blockcipher or a permutation) is preceded and followed by an easy-to-compute tweak-dependent mask. In this work, we revisit the principle of masking. We do so alongside the introduction of the tweakable Even-Mansour construction MEM. Its masking function combines the advantages of word-oriented LFSR- and powering-up-based methods. We show in particular how recent advancements in computing discrete logarithms over finite fields of characteristic 2 can be exploited in a constructive way to realize highly efficient, constant-time masking functions. If the masking satisfies a set of simple conditions, then MEM is a secure tweakable blockcipher up to the birthday bound. The strengths of MEM are exhibited by the design of fully parallelizable authenticated encryption schemes OPP (nonce-respecting) and MRO (misuse-resistant). If instantiated with a reduced-round BLAKE2b permutation, OPP and MRO achieve speeds up to 0.55 and 1.06 cycles per byte on the Intel Haswell microarchitecture, and are able to significantly outperform their closest competitors.

### Building PUF based Authentication and Key Exchange Protocol for IoT without Explicit CRPs in Verifier Database

Tue, 03/06/2018 - 07:33
Physically Unclonable Functions (PUFs) promise to be a critical hardware primitive to provide unique identities to billions of connected devices in Internet of Things (IoTs). In traditional authentication protocols a user presents a set of credentials with an accompanying proof such as password or digital certificate. However, IoTs need more evolved methods as these classical techniques suffer from the pressing problems of password dependency and inability to bind access requests to the “things” from which they originate. Additionally, the protocols need to be lightweight and heterogeneous. Although PUFs seem promising to develop such mechanism, it puts forward an open problem of how to develop such mechanism without needing to store the secret challenge-response pair (CRP) explicitly at the verifier end. In this paper, we develop an authentication and key exchange protocol by combining the ideas of Identity based Encryption (IBE), PUFs and Key-ed Hash Function to show that this combination can help to do away with this requirement. The security of the protocol is proved formally under the Session Key Security and the Universal Composability Framework. A prototype of the protocol has been implemented to realize a secured video surveillance camera using a combination of an Intel Edison board, with a Digilent Nexys-4 FPGA board consisting of an Artix-7 FPGA, together serving as the IoT node. We show, though the stand-alone video camera can be subjected to man-in-the-middle attack via IP-spoofing using standard network penetration tools, the camera augmented with the proposed protocol resists such attacks and it suits aptly in an IoT infrastructure making the protocol deployable for the industry.

### Round-Optimal Secure Multi-Party Computation

Mon, 03/05/2018 - 20:54
Secure multi-party computation (MPC) is a central cryptographic task that allows a set of mutually distrustful parties to jointly compute some function of their private inputs where security should hold in the presence of a malicious adversary that can corrupt any number of parties. Despite extensive research, the precise round complexity of this "standard-bearer'' cryptographic primitive is unknown. Recently, Garg, Mukherjee, Pandey and Polychroniadou, in EUROCRYPT 2016 demonstrated that the round complexity of any MPC protocol relying on black-box proofs of security in the plain model must be at least four. Following this work, independently Ananth, Choudhuri and Jain, CRYPTO 2017 and Brakerski, Halevi, and Polychroniadou, TCC 2017 made progress towards solving this question and constructed four-round protocols based on non-polynomial time assumptions. More recently, Ciampi, Ostrovsky, Siniscalchi and Visconti in TCC 2017 closed the gap for two-party protocols by constructing a four-round protocol from polynomial-time assumptions. In another work, Ciampi, Ostrovsky, Siniscalchi and Visconti TCC 2017 showed how to design a four-round multi-party protocol for the specific case of multi-party coin-tossing. In this work, we resolve this question by designing a four-round actively secure multi-party (two or more parties) protocol for general functionalities under standard polynomial-time hardness assumptions with a black-box proof of security.