The importance of efficient MPC in today’s world needs no retelling. An obvious barebones requirement to execute protocols for MPC is the ability of parties to communicate with each other. Traditionally, we solve this problem by assuming that every pair of parties in the network share a dedicated secure link that enables reliable message transmission. This assumption is clearly impractical as the number of nodes in the network grows, as it has today. In their seminal work, Dwork, Peleg, Pippenger and Upfal introduced the notion of almost-everywhere secure primitives in an effort to model the reality of large scale global networks and study the impact of limited connectivity on the properties of fundamental fault-tolerant distributed tasks. In this model, the underlying communication network is sparse and hence some nodes may not even be in a position to participate in the protocol (all their neighbors may be corrupt, for instance). A protocol for almost everywhere reliable message transmission, which would guarantee that a large subset of the network can transmit messages to each other reliably, implies a protocol for almost-everywhere agreement where nodes are required to agree on a value despite malicious or byzantine behavior of some subset of nodes, and an almost-everywhere agreement protocol implies a protocol almost-everywhere secure MPC that is unconditionally or information-theoretically secure. The parameters of interest are the degree d of the network, the number t of corrupted nodes that can be tolerated and the number x of nodes that the protocol may give up. Prior work achieves d=O(1) for t=O(n/logn) and d=O(logqn) for t=O(n) for some fixed constant q>1.
In this work, we first derive message protocols which are efficient with respect to the total number of computations done across the network. We use this result to show an abundance of networks with d=O(1) that are resilient to t=O(n) random corruptions. This randomized result helps us build networks which are resistant to worst-case adversaries. In particular, we improve the state of the art in the almost everywhere reliable message transmission problem in the worst-case adversary model by showing the existence of an abundance of networks that satisfy d=O(logn) for t=O(n), thus making progress on this question after nearly a decade. Finally, we define a new adversarial model of corruptions that is suitable for networks shared amongst a large group of corporations that: (1) do not trust each other, and (2) may collude, and construct optimal networks achieving d=O(1) for t=O(n) in this model.
The right of an individual to request the deletion of their personal data by an entity that might be storing it – referred to as the right to be forgotten – has been explicitly recognized, legislated, and exercised in several jurisdictions across the world, including the European Union, Argentina, and California. However, much of the discussion surrounding this right offers only an intuitive notion of what it means for it to be fulfilled – of what it means for such personal data to be deleted. In this work, we provide a formal definitional framework for the right to be forgotten using tools and paradigms from cryptography. In particular, we provide a precise definition of what could be (or should be) expected from an entity that collects individuals’ data when a request is made of it to delete some of this data. Our framework captures several, though not all, relevant aspects of typical systems involved in data processing. While it cannot be viewed as expressing the statements of current laws (especially since these are rather vague in this respect), our work offers technically precise definitions that represent possibilities for what the law could reasonably expect, and alternatives for what future versions of the law could explicitly require. Finally, with the goal of demonstrating the applicability of our framework and definitions, we consider various natural and simple scenarios where the right to be forgotten comes up. For each of these scenarios, we highlight the pitfalls that arise even in genuine attempts at implementing systems offering deletion guarantees, and also describe technological solutions that provably satisfy our definitions. These solutions bring together techniques built by various communities.
In this work we present a duplex-based authenticated encryption scheme Friet based on a new permutation called Friet-P. We designed Friet-P with a novel approach for cryptographic permutations and block ciphers that takes fault-attack resistance into account and that we introduce in this paper. In this method, we build a permutation fC to be embedded in a larger one, f . First, we define f as a sequence of steps that all abide a chosen error-correcting code C, i.e., that map C-codewords to C-codewords. Then, we embed fC in f by first encoding its input to an element of C, applying f and then decoding back from C. This last step detects a fault when the output of f is not in C. We motivate the design of the permutation we use in Friet and report on performance in soft- and hardware. We evaluate the fault-detection capabilities of the software and simulated hardware implementations with attacks. Finally, we perform a leakage evaluation. Our code is available at https://github.com/thisimon/Friet.git.
For 1≤m≤n, we consider a natural m-out-of-n multi-instance scenario for a public-key encryption (PKE) scheme. An adversary, given n independent instances of PKE, wins if he breaks at least m out of the n instances. In this work, we are interested in the scaling factor of PKE schemes, SF, which measures how well the difficulty of breaking m out of the n instances scales in m. That is, a scaling factor SF=ℓ indicates that breaking m out of n instances is at least ℓ times more difficult than breaking one single instance. A PKE scheme with small scaling factor hence provides an ideal target for mass surveillance. In fact, the Logjam attack (CCS 2015) implicitly exploited, among other things, an almost constant scaling factor of ElGamal over finite fields (with shared group parameters).
For Hashed ElGamal over elliptic curves, we use the generic group model to argue that the scaling factor depends on the scheme’s granularity. In low granularity, meaning each public key contains its independent group parameter, the scheme has optimal scaling factor SF=m; In medium and high granularity, meaning all public keys share the same group parameter, the scheme still has a reasonable scaling factor SF=m−−√. Our findings underline that instantiating ElGamal over elliptic curves should be preferred to finite fields in a multi-instance scenario.
As our main technical contribution, we derive new generic-group lower bounds of Ω(mp−−−√) on the difficulty of solving both the m-out-of-n Gap Discrete Logarithm and the m-out-of-n Gap Computational Diffie-Hellman problem over groups of prime order p, extending a recent result by Yun (EUROCRYPT 2015). We establish the lower bound by studying the hardness of a related computational problem which we call the search-by-hypersurface problem.
We study the security of schemes related to Schnorr signatures in the algebraic group model (AGM) proposed by Fuchsbauer, Kiltz, and Loss (CRYPTO 2018), where the adversary can only compute new group elements by applying the group operation. Schnorr signatures can be proved secure in the random oracle model (ROM) under the discrete logarithm assumption (DL) by rewinding the adversary; but this security proof is loose. We start with giving a tight security proof under DL in the combination of the AGM and the ROM. Our main focus are blind Schnorr signatures, whose only known security proof is in the combination of the ROM and the generic group model, under the assumption that the so-called ROS problem is hard. We show that in the AGM+ROM the scheme is secure assuming hardness of the one-more discrete logarithm problem and the ROS problem. As the latter can be solved in sub-exponential time using Wagner’s algorithm, this is not entirely satisfying. Hence, we then propose a very simple modification of the scheme (which leaves key generation and signature verification unchanged) and show that, instead of ROS, its security relies on the hardness of a related problem which appears much harder than ROS. Finally, we give a tight reduction of the CCA2 security of Schnorr-signed ElGamal key encapsulation to DL, again in the AGM+ROM.
A non-interactive zero-knowledge (NIZK) protocol enables a prover to convince a verifier of the truth of a statement without leaking any other information by sending a single message. The main focus of this work is on exploring short pairing-based NIZKs for all NP languages based on standard assumptions. In this regime, the seminal work of Groth, Ostrovsky, and Sahai (J.ACM’12) (GOS-NIZK) is still considered to be the state-of-the-art. Although fairly efficient, one drawback of GOS-NIZK is that the proof size is multiplicative in the circuit size computing the NP relation. That is, the proof size grows by O(|C|λ), where C is the circuit for the NP relation and λ is the security parameter. By now, there have been numerous follow-up works focusing on shortening the proof size of pairing-based NIZKs, however, thus far, all works come at the cost of relying either on a non-standard knowledge-type assumption or a non-static q-type assumption. Specifically, improving the proof size of the original GOS-NIZK under the same standard assumption has remained as an open problem.
Our main result is a construction of a pairing-based NIZK for all of NP whose proof size is additive in |C|, that is, the proof size only grows by |C|+\poly(λ), based on the decisional linear (DLIN) assumption. Since the DLIN assumption is the same assumption underlying GOS-NIZK, our NIZK is a strict improvement on their proof size.
As by-products of our main result, we also obtain the following two results: (1) We construct a perfectly zero-knowledge NIZK (NIPZK) for NP relations computable in NC1 with proof size |w|⋅\poly(λ) where |w| is the witness length based on the DLIN assumption. This is the first pairing-based NIPZK for a non-trivial class of NP languages whose proof size is independent of |C| based on a standard assumption. (2)~We construct a universally composable (UC) NIZK for NP relations computable in NC1 in the erasure-free adaptive setting whose proof size is |w|⋅\poly(λ) from the DLIN assumption. This is an improvement over the recent result of Katsumata, Nishimaki, Yamada, and Yamakawa (CRYPTO’19), which gave a similar result based on a non-static q-type assumption.
The main building block for all of our NIZKs is a constrained signature scheme with decomposable online-offline efficiency. This is a property which we newly introduce in this paper and construct from the DLIN assumption. We believe this construction is of an independent interest.
Consider the setup where $n$ parties are each given a number $x_i \in \mathbb{F}_q$ and the goal is to compute the sum $\sum_i x_i$ in a secure fashion and with as little communication as possible. We study this problem in the anonymized model of Ishai et al. (FOCS 2006) where each party may broadcast anonymous messages on an insecure channel.
We present a new analysis of the one-round “split and mix” protocol of Ishai et al. In order to achieve the same security parameter, our analysis reduces the required number of messages by a $\Theta(\log n)$ multiplicative factor. We complement our positive result with lower bounds showing that the dependence of the number of messages on the domain size, the number of parties, and the security parameter is essentially tight.
Using a reduction of Balle et al. (2019), our improved analysis of the protocol of Ishai et al. yields, in the same model, an $\left(\varepsilon, \delta\right)$-differentially private protocol for aggregation that, for any constant $\varepsilon > 0$ and any $\delta = \frac{1}{\mathrm{poly}(n)}$, incurs only a constant error and requires only a constant number of messages per party. Previously, such a protocol was known only for $\Omega(\log n)$ messages per party.
Grover’s search algorithm gives a quantum attack against block ciphers by searching for a key that matches a small number of plaintext-ciphertext pairs. This attack uses O(N−−√) calls to the cipher to search a key space of size N. Previous work in the specific case of AES derived the full gate cost by analyzing quantum circuits for the cipher, but focused on minimizing the number of qubits. In contrast, we study the cost of quantum key search attacks under a depth restriction and introduce techniques that reduce the oracle depth, even if it requires more qubits. As cases in point, we design quantum circuits for the block ciphers AES and LowMC. Our circuits give a lower overall attack cost in both the gate count and depth-times-width cost models. In NIST’s post-quantum cryptography standardization process, security categories are defined based on the concrete cost of quantum key search against AES. We present new, lower cost estimates for each category, so our work has immediate implications for the security assessment of post-quantum cryptography. As part of this work, we release Q# implementations of the full Grover oracle for AES-128, -192, -256 and for the three LowMC instantiations used in \picnic, including unit tests and code to reproduce our quantum resource estimates. To the best of our knowledge, these are the first two such full implementations and automatic resource estimations.
We present the first protocols for private information retrieval that allow fast (sublinear-time) database lookups without increasing the server-side storage requirements. To achieve these efficiency goals, our protocols work in an offline/online model. In an offline phase, which takes place before the client has decided which database bit it wants to read, the client fetches a short string from the servers. In a subsequent online phase, the client can privately retrieve its desired bit of the database by making a second query to the servers. By pushing the bulk of the server-side computation into the offline phase (which is independent of the client’s query), our protocols allow the online phase to complete very quickly—in time sublinear in the size of the database. Our protocols can provide statistical security in the two-server setting and computational security in the single-server setting. Finally, we prove that, in this model, our protocols are optimal in terms of the trade-off they achieve between communication and running time.
Current formal approaches have been successfully used to find design flaws in many security protocols. However, it is still challenging to automatically analyze protocols due to their large or infinite state spaces. In this paper, we propose SmartVerif, a novel and general framework that pushes the limit of automation capability of state-of-the-art verification approaches. The primary technical contribution is the dynamic strategy inside SmartVerif, which can be used to smartly search proof paths. Different from the non-trivial and error-prone design of existing static strategies, the design of our dynamic strategy is simple and flexible: it can automatically optimize itself according to the security protocols without any human intervention. With the optimized strategy, SmartVerif can localize and prove supporting lemmata, which leads to higher probability of success in verification. The insight of designing the strategy is that the node representing a supporting lemma is on an incorrect proof path with lower probability, when a random strategy is given. Hence, we implement the strategy around the insight by introducing a reinforcement learning algorithm. We also propose several methods to deal with other technical problems in implementing SmartVerif. Experimental results show that SmartVerif can automatically verify all security protocols studied in this paper. The case studies also validate the efficiency of our dynamic strategy.
Side-channel attacks exploiting (EC)DSA nonce leakage easily lead to full key recovery. Although (EC)DSA implementations have already been hardened against side-channel leakage using the constant-time paradigm, the long-standing cat-and-mouse-game of attacks and patches continues. In particular, current code review is prone to miss less obvious side channels hidden deeply in the call stack. To solve this problem, a systematic study of nonce leakage is necessary. We present a systematic analysis of nonce leakage in cryptographic implementations. In particular, we expand DATA, an open-source side-channel analysis framework, to detect nonce leakage. Our analysis identified multiple unknown nonce leakage vulnerabilities across all essential computation steps involving nonces. Among others, we uncover inherent problems in Bignumber implementations that break claimed constant-time guarantees of (EC)DSA implementations if secrets are close to a word boundary. We found that lazy resizing of Bignumbers in OpenSSL and LibreSSL yields a highly accurate and easily exploitable side channel, which has been acknowledged with two CVEs. Surprisingly, we also found a tiny but expressive leakage in the constant-time scalar multiplication of OpenSSL and BoringSSL. Moreover, in the process of reporting and patching, we identified newly introduced leakage with the support of our tool, thus preventing another attack-patch cycle. We open-source our tool, together with an intuitive graphical user interface we developed.
Trusted Platform Module (TPM) serves as a hardware-based root of trust that protects cryptographic keys from privileged system and physical adversaries. In this work, we per-form a black-box timing analysis of TPM 2.0 devices deployed on commodity computers. Our analysis reveals thatsome of these devices feature secret-dependent execution times during signature generation based on elliptic curves. In particular, we discovered timing leakage on an Intel firmware-based TPM as well as a hardware TPM. We show how this information allows an attacker to apply lattice techniques torecover 256-bit private keys for ECDSA and EC Schnorr signatures. On Intel fTPM, our key recovery succeeds after about1,300 observations and in less than two minutes. Similarly, weextract the private ECDSA key from a hardware TPM manufactured by STMicroelectronics, which is certified at Common Criteria (CC) EAL 4+, after fewer than 40,000 observations.We further highlight the impact of these vulnerabilities by demonstrating a remote attack against a StrongSwan IPsecVPN that uses a TPM to generate the digital signatures for authentication. In this attack, the remote client recovers the server’s private authentication key by timing only 45,000authentication handshakes via a network connection.The vulnerabilities we have uncovered emphasize the difficulty of correctly implementing known constant-time techniques, and show the importance of evolutionary testing and transparent evaluation of cryptographic implementations.Even certified devices that claim resistance against attacks require additional scrutiny by the community and industry, as we learn more about these attacks.
We revisit the problem of performing secure computation of graph-parallel algorithms, focusing on the applications of securely outsourcing matrix factorization, and histograms. Leveraging recent results in low-communication secure multi-party computation, and a security relaxation that allows the computation servers to learn some differentially private leakage about user inputs, we construct a new protocol that reduces overall runtime by 320X, reduces the number of AES calls by 750X , and reduces the total communication by 200X . Our system can securely compute histograms over 300 million items in about 4 minutes, and it can perform sparse matrix factorization, which is commonly used in recommendation systems, on 20 million records in about 6 minutes. Furthermore, in contrast to prior work, our system is secure against a malicious adversary that corrupts one of the computing servers.
In Proof-of-Stake (PoS) and permissioned blockchains, a committee of verifiers agrees and sign every new block of transactions. These blocks are validated, propagated, and stored by all users in the network. However, posterior corruptions pose a common threat to these designs, because the adversary can corrupt committee verifiers after they certified a block and use their signing keys to certify a different block. Designing efficient and secure digital signatures for use in PoS blockchains can substantially reduce bandwidth, storage and computing requirements from nodes, thereby enabling more efficient applications.
We present Pixel, a pairing-based forward-secure multi-signature scheme optimized for use in blockchains, that achieves substantial savings in bandwidth, storage requirements, and verification effort. Pixel signatures consist of two group elements, regardless of the number of signers, can be verified using three pairings and one exponentiation, and support non-interactive aggregation of individual signatures into a multi-signature. Pixel signatures are also forward-secure and let signers evolve their keys over time, such that new keys cannot be used to sign on old blocks, protecting against posterior corruptions attacks on blockchains. We show how to integrate Pixel into any PoS blockchain. Next, we evaluate Pixel in a real-world PoS blockchain implementation, showing that it yields notable savings in storage, bandwidth, and block verification time. In particular, Pixel signatures reduce the size of blocks with 1500 transactions by 35% and reduce block verification time by 38%.
Recent results have shown that some post-quantum cryptographic systems have encryption and decryption performance comparable to fast elliptic-curve cryptography (ECC) or even better. However, this performance metric is considering only CPU time and ignoring bandwidth and storage. High-confidence post-quantum encryption systems have much larger keys than ECC. For example, the code-based cryptosystem recommended by the PQCRYPTO project uses public keys of 1MB.
Fast key erasure (to provide “forward secrecy”) requires new public keys to be constantly transmitted. Either the server needs to constantly generate, store, and transmit large keys, or it needs to receive, store, and use large keys from the clients. This is not necessarily a problem for overall bandwidth, but it is a problem for storage and computation time on tiny network servers. All straightforward approaches allow easy denial-of-service attacks.
This paper describes a protocol, suitable for today’s networks and tiny servers, in which clients transmit their code-based one-time public keys to servers. Servers never store full client public keys but work on parts provided by the clients, without having to maintain any per-client state. Intermediate results are stored on the client side in the form of encrypted cookies and are eventually combined by the server to obtain the ciphertext. Requirements on the server side are very small: storage of one long-term private key, which is much smaller than a public key, and a few small symmetric cookie keys, which are updated regularly and erased after use. The protocol is highly parallel, requiring only a few round trips, and involves total bandwidth not much larger than a single public key. The total number of packets sent by each side is 971, each fitting into one IPv6 packet of less than 1280 bytes.
The protocol makes use of the structure of encryption in code-based cryptography and benefits from small ciphertexts in code-based cryptography.
At the core of Apple’s iMessage is a signcryption scheme that involves symmetric encryption of a message under a key that is derived from the message itself. This motivates us to formalize a primitive we call Encryption under Message-Derived Keys (EMDK). We prove security of the EMDK scheme underlying iMessage. We use this to prove security of the signcryption scheme itself, with respect to definitions of signcryption we give that enhance prior ones to cover issues peculiar to messaging protocols. Our provable-security results are quantitative, and we discuss the practical implications for iMessage.
We present a 2-party private set intersection (PSI) protocol which provides security against malicious participants, yet is almost as fast as the fastest known semi-honest PSI protocol of Kolesnikov et al. (CCS 2016).
Our protocol is based on a new approach for two-party PSI, which can be instantiated to provide security against either malicious or semi-honest adversaries. The protocol is unique in that the only difference between the semi-honest and malicious versions is an instantiation with different parameters for a linear error-correction code. It is also the first PSI protocol which is concretely efficient while having linear communication and security against malicious adversaries, while running in the OT-hybrid model (assuming a non-programmable random oracle).
State of the art semi-honest PSI protocols take advantage of cuckoo hashing, but it has proven a challenge to use cuckoo hashing for malicious security. Our protocol is the first to use cuckoo hashing for malicious-secure PSI. We do so via a new data structure, called a probe-and-XOR of strings (PaXoS), which may be of independent interest. This abstraction captures important properties of previous data structures, most notably garbled Bloom filters. While an encoding by a garbled Bloom filter is larger by a factor of $O(\lambda)$ than the original data, we describe a significantly improved PaXoS based on cuckoo hashing that achieves constant rate while being no worse in other relevant efficiency measures.
Zero-knowledge (ZK) proofs (ZKP) have received wide attention, focusing on non-interactivity, short proof size, and fast verification time. We focus on the fastest total proof time, in particular for large Boolean circuits. Under this metric, Garbled Circuit (GC)-based ZKP (Jawurek et al., [JKO], CCS 2013) remained the state-of-the-art technique due to the low-constant linear scaling of computing the garbling. We improve GC-ZKP for proof statements with conditional clauses. Our communication is proportional to the longest branch rather than to the entire proof statement. This is most useful when the number m of branches is large, resulting in up to factor $m\times$ improvement over JKO. In our proof-of-concept illustrative application, prover P demonstrates knowledge of a bug in a codebase consisting of any number of snippets of actual C code. Our computation cost is linear in the size of the codebase and communication is constant in the number of snippets. That is, we require only enough communication for a single largest snippet! Our conceptual contribution is stacked garbling for ZK, a privacy-free circuit garbling scheme that can be used with the JKO GC-ZKP protocol to construct more efficient ZKP. Given a Boolean circuit C and computational security parameter $\kappa$, our garbling is $L \cdot \kappa$ bits long, where $L$ is the length of the longest execution path in C. All prior concretely efficient garbling schemes produce garblings of size $|C| \cdot \kappa$. The computational cost of our scheme is not increased over prior state-of-the-art. We implement our GC-ZKP and demonstrate significantly improved ($m\times$ over JKO) ZK performance for functions with branching factor $m$. Compared with recent ZKP (STARK, Libra, KKW, Ligero, Aurora, Bulletproofs), our scheme offers much better proof times for larger circuits ($35-1000\times$ or more, depending on circuit size and compared scheme). For our illustrative application, we consider four C code snippets, each of about 30-50 LOC; one snippet allows an invalid memory dereference. The entire proof takes 0.15 seconds and communication is 1.5 MB.
We describe in this paper how to perform a padding oracle attack against
the GlobalPlatform SCP02 protocol. SCP02 is implemented in smart cards and
used by transport companies, in the banking world and by mobile network operators
(UICC/SIM cards). The attack allows an adversary to efficiently retrieve plaintext
bytes from an encrypted data field. We provide results of our experiments done
with 10 smart cards from six different card manufacturers, and show that, in our
experimental setting, the attack is fully practical. Given that billions SIM cards are
produced every year, the number of affected cards, although difficult to estimate,
is potentially high. To the best of our knowledge, this is the first successful attack
against SCP02.
The surprising success of cryptocurrencies has led to a surge of interest in deploying large scale, highly robust, Byzantine fault tolerant (BFT) proto- cols for mission-critical applications, such as finan- cial transactions. Although the conventional wisdom is to build atop a (weakly) synchronous protocol such as PBFT (or a variation thereof), such protocols rely critically on network timing assumptions, and only guarantee liveness when the network behaves as ex- pected. We argue these protocols are ill-suited for this deployment scenario.
We present an alternative, HoneyBadgerBFT, the first practical asynchronous BFT protocol, which guarantees liveness without making any timing as- sumptions. We base our solution on a novel atomic broadcast protocol that achieves optimal asymptotic efficiency. We present an implementation and ex- perimental results to show our system can achieve throughput of tens of thousands of transactions per second, and scales to over a hundred nodes on a wide area network. We even conduct BFT experi- ments over Tor, without needing to tune any parame- ters. Unlike the alternatives, HoneyBadgerBFT sim- ply does not care about the underlying network.
Cryptocurrencies, such as Bitcoin and 250 similar alt-coins, embody at their core a blockchain protocol — a mechanism for a distributed network of computational nodes to periodically agree on a set of new transactions. Designing a secure blockchain protocol relies on an open challenge in security, that of designing a highly-scalableagreement protocol open to manipulation by byzantine or arbitrarily malicious nodes. Bitcoin’s blockchain agreement protocol exhibits security, but does not scale: it processes 3–7 transactions per second at present, irrespective of the available computation capacity at hand.
In this paper, we propose a new distributed agreement protocol for permission-less blockchains called ELASTICO. ELASTICO scales transaction rates almost linearly with available computation for mining: the more the computation power in the network, the higher the number of transaction blocks selected per unit time. ELASTICO is efficient in its network messages and tolerates byzantine adversaries of up to one-fourth of the total computational power. Technically, ELASTICO uniformly partitions or parallelizes the mining network (securely) into smaller committees, each of which processes a disjoint set of transactions (or “shards”). While sharding is common in non-byzantine settings, ELASTICO is the first candidate for a secure sharding protocol with presence of byzantine adversaries. Our scalability experiments on Amazon EC2 with up to $1, 600$ nodes confirm ELASTICO’s theoretical scaling properties.
Proof of Work (PoW) powered blockchains currently account for more than 90% of the total market capitalization of existing digital currencies. Although the security provisions of Bitcoin have been thoroughly analysed, the security guarantees of variant (forked) PoW blockchains (which were instantiated with different parameters) have not received much attention in the literature.
In this paper, we introduce a novel quantitative framework to analyse the security and performance implications of various consensus and network parameters of PoW blockchains. Based on our framework, we devise optimal adversarial strategies for double-spending and selfish mining while taking into account real world constraints such as network propagation, different block sizes, block generation intervals, information propagation mechanism, and the impact of eclipse attacks. Our framework therefore allows us to capture existing PoW-based deployments as well as PoW blockchain variants that are instantiated with different parameters, and to objectively compare the tradeoffs between their performance and security provisions.
Motivated by the problem of data breaches, we formalize a notion of security for dynamic structured encryption (STE) schemes that guarantees security against a snapshot adversary; that is, an adversary that receives a copy of the encrypted structure at various times but does not see the transcripts related to any queries. In particular, we focus on the construction of dynamic encrypted multi-maps which are used to build efficient searchable symmetric encryption schemes, graph encryption schemes and encrypted relational databases. Interestingly, we show that a form of snapshot security we refer to as breach resistance implies previously-studied notions such as a (weaker version) of history independence and write-only obliviousness. Moreover, we initiate the study of dual-secure dynamic STE constructions: schemes that are forward-private against a persistent adversary and breach-resistant against a snapshot adversary. The notion of forward privacy guarantees that updates to the encrypted structure do not reveal their association to any query made in the past. As a concrete instantiation, we propose a new dual-secure dynamic multi-map encryption scheme that outperforms all existing constructions; including schemes that are not dual-secure. Our construction has query complexity that grows with the selectivity of the query and the number of deletes since the client executed a linear-time rebuild protocol which can be de-amortized. We implemented our scheme (with the de-amortized rebuild protocol) and evaluated its concrete efficiency empirically. Our experiments show that it is highly efficient with queries taking less than 1 microsecond per label/value pair.
Encrypted search algorithms (ESA) are cryptographic algorithms that support search over encrypted data. ESAs can be designed with various primitives including searchable/structured symmetric encryption (SSE/STE) and oblivious RAM (ORAM). Leakage abuse attacks attempt to recover client queries using knowledge of the client’s data. An important parameter for any leakage-abuse attack is its known-data rate; that is, the fraction of client data that must be known to the adversary.
In this work, we revisit leakage abuse attacks in several ways. We first highlight some practical limitations and assumptions underlying the well-known IKK (Islam et al. NDSS ’12) and Count (Cash et al., CCS ’15) attacks. We then design four new leakage-abuse attacks that rely on much weaker assumptions. Three of these attacks are volumetric in the sense that they only exploit leakage related to document sizes. In particular, this means that they work not only on SSE/STE-based ESAs but also against ORAM-based solutions. We also introduce two volumetric injection attack which use adversarial file additions to recover queries even from ORAM-based solutions. As far as we know, these are the first attacks of their kind.
We evaluated all our attacks empirically and considered many experimental settings including different data collections, query selectivities, known-data rates, query space size and composition. From our experiments, we observed that the only setting that resulted in reasonable recovery rates under practical assumptions was the case of high-selectivity queries with a leakage profile that includes the response identity pattern (i.e., the identifiers of the matching documents) and the volume pattern (i.e., the size of the matching documents). All other attack scenarios either failed or relied on unrealistic assumptions (e.g., very high known-data rates). For this specific setting, we propose several suggestions and countermeasures including the use of schemes like PBS (Kamara et al, CRYPTO ’18), VLH/AVLH (Kamara and Moataz, Eurocrypt ’19 ), or the use of padding techniques like the ones recently proposed by Bost and Fouque (Bost and Fouque, IACR ePrint 2017/1060).