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Is there an Oblivious RAM Lower Bound for Online Reads?

Tue, 07/03/2018 - 15:45
Oblivious RAM (ORAM), introduced by Goldreich and Ostrovsky (JACM 1996), can be used to read and write to memory in a way that hides which locations are being accessed. The best known ORAM schemes have an $O(\log n)$ overhead per access, where $n$ is the data size. The work of Goldreich and Ostrovsky gave a lower bound showing that this is optimal for ORAM schemes that operate in a ``balls and bins'' model, where memory blocks can only be shuffled between different locations but not manipulated otherwise. The lower bound even extends to weaker settings such as offline ORAM, where all of the accesses to be performed need to be specified ahead of time, and read-only ORAM, which only allows reads but not writes. But can we get lower bounds for general ORAM, beyond ``balls and bins''? The work of Boyle and Naor (ITCS '16) shows that this is unlikely in the offline setting. In particular, they construct an offline ORAM with $o(\log n)$ overhead assuming the existence of small sorting circuits. Although we do not have instantiations of the latter, ruling them out would require proving new circuit lower bounds. On the other hand, the recent work of Larsen and Nielsen (CRYPTO '18) shows that there indeed is an $\Omega(\log n)$ lower bound for general online ORAM. This still leaves the question open for online read-only ORAM or for read/write ORAM where we want very small overhead for the read operations. In this work, we show that a lower bound in these settings is also unlikely. In particular, our main result is a construction of online ORAM where reads (but not writes) have an $o(\log n)$ overhead, assuming the existence of small sorting circuits as well as very good locally decodable codes (LDCs). Although we do not have instantiations of either of these with the required parameters, ruling them out is beyond current lower bounds.

Offline Assisted Group Key Exchange

Tue, 07/03/2018 - 11:52
We design a group key exchange protocol with forward secrecy where most of the participants remain offline until they wish to compute the key. This is well suited to a cloud storage environment where users are often offline, but have online access to the server which can assist in key exchange. We define and instantiate a new primitive, a blinded KEM, which we show can be used in a natural way as part of our generic protocol construction. Our new protocol has a security proof based on a well-known model for group key exchange. Our protocol is efficient, requiring Diffie-Hellman with a handful of standard public key operations per user in our concrete instantiation.

Self-Guarding Cryptographic Protocols against Algorithm Substitution Attacks

Tue, 07/03/2018 - 09:44
We put forward the notion of self-guarding cryptographic protocols as a countermeasure to algorithm substitution attacks. Such self-guarding protocols can prevent undesirable leakage by subverted algorithms if one has the guarantee that the system has been properly working in an initialization phase. Unlike detection-based solutions they thus proactively thwart attacks, and unlike reverse firewalls they do not assume an online external party. We present constructions of basic primitives for (public-key and private-key) encryption and for signatures. We also argue that the model captures attacks with malicious hardware tokens and show how to self-guard a PUF-based key exchange protocol.

On linear hulls in one round of DES

Tue, 07/03/2018 - 09:09
At Indocrypt 2016, Ashur et al. showed that linear hulls are sometimes formed in a single round of a cipher (exemplifying on Simon ciphers) and showed that the success rate of an attack may be influenced by the quality of the estimation of one-round correlations. This paper improves the understanding regarding one-round linear hulls and trails, being dedicated to the study of one-round linear hulls of the DES cipher, more exactly of its $f$-function. It shows that, in the case of DES, the existence of one-round hulls is related to the number of active Sboxes and its correlation depends on a fixed set of key bits. All the ideas presented in this paper are followed by examples and are verified experimentally.

Indistinguishability Obfuscation from Functional Encryption

Tue, 07/03/2018 - 06:33
Indistinguishability obfuscation (IO) is a tremendous notion, powerful enough to give rise to almost any known cryptographic object. Prior candidate IO constructions were based on specific assumptions on algebraic objects called multi-linear graded encodings. We present a generic construction of indistinguishability obfuscation from public-key functional encryption with succinct encryption circuits and subexponential security. This shows the equivalence of indistinguishability obfuscation and public-key functional en- cryption, a primitive that has previously seemed to be much weaker, lacking the power and the staggering range of applications of indistinguishability obfuscation. Our main construction can be based on functional encryption schemes that support a single functional key, and where the encryption circuit grows sub-linearly in the circuit-size of the function. We further show that sublinear succinctness in circuit-size for single-key schemes can be traded with sublinear succinctness in the number of keys (also known as the collusion-size) for multi-key schemes. We also show that, under the Learning with Errors assumption, our techniques imply that any indistinguishability obfuscator can be converted into one where the size of obfuscated circuits is twice that of the original circuit plus an additive overhead that is polynomial in its depth, input length, and the security parameter.

Public-Key Encryption Indistinguishable Under Plaintext-Checkable Attacks

Mon, 07/02/2018 - 09:44
Indistinguishability under chosen-ciphertext attack (INDCCA) is now considered the de facto security notion for public-key encryption. However, this sometimes offers a stronger security guarantee than what is needed. In this paper, we consider a weaker security notion, termed indistinguishability under plaintext-checking attacks (INDPCA), in which the adversary has only access to an oracle indicating whether or not a given ciphertext encrypts a given message. After formalizing this notion, we design a new public-key encryption scheme satisfying it. The new scheme is a variant of the Cramer-Shoup encryption scheme with shorter ciphertexts. Its security is also based on the plain Decisional Diffie-Hellman (DDH) assumption. Additionally, the algebraic properties of the new scheme allow proving plaintext knowledge using Groth-Sahai non-interactive zero-knowledge proofs or smooth projective hash functions. Finally, as a concrete application, we show that, for many password-based authenticated key exchange (PAKE) schemes in the Bellare-Pointcheval-Rogaway security model, we can safely replace the underlying INDCCA encryption schemes with our new INDPCA one. By doing so, we reduce the overall communication complexity of these protocols and obtain the most efficient PAKE schemes to date based on plain DDH.

Revisiting AES Related-Key Differential Attacks with Constraint Programming

Mon, 07/02/2018 - 05:45
The Advanced Encryption Standard (AES) is one of the most studied symmetric encryption schemes. During the last years, several attacks have been discovered in different adversary models. In this paper, we focus on related-key differential attacks, where the adversary may introduce differences in plaintext pairs and also in keys. We show that Constraint Programming (CP) can be used to model these attacks, and that it allows us to efficiently find all optimal related-key differential characteristics for AES-128, AES-192 and AES-256. In particular, we improve the best related-key differential for the whole AES-256 and give the best related-key differential on 10 rounds of AES-192, which is the differential trail with the longest path. Those results allow us to improve existing related-key distinguishers, basic related-key attacks and $q$-multicollisions on AES-256.

Bulletproofs: Short Proofs for Confidential Transactions and More

Sun, 07/01/2018 - 20:56
We propose Bulletproofs, a new non-interactive zero-knowledge proof protocol with very short proofs and without a trusted setup; the proof size is only logarithmic in the witness size. Bulletproofs are especially well suited for efficient range proofs on committed values: they enable proving that a committed value is in a range using only $2\log_2(n)+9$ group and field elements, where $n$ is the bit length of the range. Proof generation and verification times are linear in $n$. Bulletproofs greatly improve on the linear (in $n$) sized range proofs in existing proposals for confidential transactions in Bitcoin and other cryptocurrencies. Moreover, Bulletproofs supports aggregation of range proofs, so that a party can prove that $m$ commitments lie in a given range by providing only an additive $O(\log(m))$ group elements over the length of a single proof. To aggregate proofs from multiple parties, we enable the parties to generate a single proof without revealing their inputs to each other via a simple multi-party computation (MPC) protocol for constructing Bulletproofs. This MPC protocol uses either a constant number of rounds and linear communication, or a logarithmic number of rounds and logarithmic communication. We show that verification time, while asymptotically linear, is very efficient in practice. Moreover, the verification of multiple Bulletproofs can be batched for further speed-up. Concretely, the marginal time to verify an aggregation of 16 range proofs is about the same as the time to verify 16 ECDSA signatures. Bulletproofs build on the techniques of Bootle et al. (EUROCRYPT 2016). Beyond range proofs, Bulletproofs provide short zero-knowledge proofs for general arithmetic circuits while only relying on the discrete logarithm assumption and without requiring a trusted setup. We discuss many applications that would benefit from Bulletproofs, primarily in the area of cryptocurrencies. The efficiency of Bulletproofs is particularly well suited for the distributed and trustless nature of blockchains.

SETLA: Signature and Encryption from Lattices

Sun, 07/01/2018 - 17:06
In data security, the main objectives one tries to achieve are privacy, data integrity and authentication. In a public-key setting, privacy is reached through asymmetric encryption and both data integrity and authentication through signature. Meeting all the security objectives for data exchange requires to use a concatenation of those primitives in an encrypt-then-sign or sign-then-encrypt fashion. Signcryption aims at providing all the security requirements in one single primitive at a lower cost than using encryption and signature together. Most existing signcryption schemes are using ElGamal-based or pairing-based techniques and thus rely on the decisional Diffie-Hellman assumption. With the current growth of a quantum threat, we seek for post-quantum counterparts to a vast majority of public-key primitives. In this work, we propose a lattice-based signcryption scheme in the random oracle model inspired from a construction of Malone-Lee. It comes in two flavors, one integrating the usual lattice-based key exchange into the signature and the other merging the scheme with a RLWE encryption. Our instantiation is based on a ring version of the scheme of Bai and Galbraith as was done in ring-TESLA and TESLA$\sharp$. It targets 128 bits of classical security and offers a save in bandwidth over a naive concatenation of state-of-the-art key exchanges and signatures from the literature. Another lightweight instantiation derived from GLP is feasible but raises long-term security concerns since the base scheme is somewhat outdated.

Efficient verifiable delay functions

Sun, 07/01/2018 - 15:41
We construct a verifiable delay function (VDF). A VDF is a function whose evaluation requires to run a given number of sequential steps, yet the result can be efficiently verified. They have applications in decentralised systems, such as the generation of trustworthy public randomness in a trustless environment, or resource-efficient blockchains. To construct a VDF, we actually build a trapdoor VDF. A trapdoor VDF is essentially a VDF which can be evaluated efficiently by parties who know a secret (the trapdoor). By setting up this scheme in a way that the trapdoor is unknown (not even by the party running the setup, so that there is no need for a trusted setup environment), we obtain a simple VDF. Our construction is based on groups of unknown order (such as an RSA group, or the class group of an imaginary quadratic field). The output of our construction is very short (the result and the proof of correctness are both a single element of the group), and the verification of correctness is very efficient.

Simple Verifiable Delay Functions

Sun, 07/01/2018 - 05:59
We construct a verifable delay function (or unique publicly verifiable proof of sequential work) by showing how the Rivest-Shamir-Wagner time-lock puzzle can be made publicly verifiable. Concretely, we give a statistically sound public-coin protocol to prove that a tuple $(N,x,T,y)$ satisfies $y=x^{2^T}\pmod N$ where the prover doesn't know the factorization of $N$ and its running time is dominated by solving the puzzle, that is, compute $x^{2^T}$, which is conjectured to require $T$ sequential squarings. The motivation for this work comes from the Chia blockchain design, which uses a VDF as a key ingredient. For typical parameters, our proofs are of size around $10KB$ and verification cost around three RSA exponentiations.

Triathlon of Lightweight Block Ciphers for the Internet of Things

Sat, 06/30/2018 - 06:28
In this paper we introduce a framework for the benchmarking of lightweight block ciphers on a multitude of embedded platforms. Our framework is able to evaluate the execution time, RAM footprint, as well as binary code size, and allows one to define a custom "figure of merit" according to which all evaluated candidates can be ranked. We used the framework to benchmark implementations of 19 lightweight ciphers, namely AES, Chaskey, Fantomas, HIGHT, LBlock, LEA, LED, Piccolo, PRESENT, PRIDE, PRINCE, RC5, RECTANGLE, RoadRunneR, Robin, Simon, SPARX, Speck, and TWINE, on three microcontroller platforms: 8-bit AVR, 16-bit MSP430, and 32-bit ARM. Our results bring some new insights to the question of how well these lightweight ciphers are suited to secure the Internet of Things (IoT). The benchmarking framework provides cipher designers with an easy-to-use tool to compare new algorithms with the state-of-the-art and allows standardization organizations to conduct a fair and consistent evaluation of a large number of candidates.

Middle-Product Learning With Errors

Fri, 06/29/2018 - 10:44
We introduce a new variant MPLWE of the Learning With Errors problem (LWE) making use of the Middle Product between polynomials modulo an integer q. We exhibit a reduction from the Polynomial-LWE problem (PLWE) parametrized by a polynomial f, to MPLWE which is defined independently of any such f. The reduction only requires f to be monic with constant coefficient coprime with q. It incurs a noise growth proportional to the so-called expansion factor of f. We also describe a public-key encryption scheme with quasi-optimal asymptotic efficiency (the bit-sizes of the keys and the run-times of all involved algorithms are quasi-linear in the security parameter), which is secure against chosen plaintext attacks under the MPLWE hardness assumption. The scheme is hence secure under the assumption that PLWE is hard for at least one polynomial f of degree n among a family of f's which is exponential in n.

Efficient Construction of the Boomerang Connection Table

Thu, 06/28/2018 - 09:23
Recently, the Boomerang Connection Table was introduced by Cid et al. as a tool to better evaluate the probability of a boomerang distinguisher. To compute the BCT of an $n$-bit to $n$-bit S-box, the inventors of the BCT proposed an algorithm that takes $O(2^{3n})$ time. We show that one can construct the same table in only $O(2^{2n})$ time.

Machine Learning Attacks on PolyPUFs, OB-PUFs, RPUFs, LHS-PUFs, and PUF-FSMs

Thu, 06/28/2018 - 05:50
A physically unclonable function (PUF) is a circuit of which the input–output behavior is designed to be sensitive to the random variations of its manufacturing process. This building block hence facilitates the authentication of any given device in a population of identically laid-out silicon chips, similar to the biometric authentication of a human. The focus and novelty of this work is the development of efficient impersonation attacks on the following five PUF-based authentication protocols: (1) the so-called PolyPUF protocol of Konigsmark, Chen, and Wong, as published in the IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems in 2016, (2) the so-called OB-PUF protocol of Gao, Li, Ma, Al-Sarawi, Kavehei, Abbott, and Ranasinghe, as presented at the IEEE conference PerCom 2016, (3) the so-called RPUF protocol of Ye, Hu, and Li, as presented at the IEEE conference AsianHOST 2016, (4) the so-called LHS-PUF protocol of Idriss and Bayoumi, as presented at the IEEE conference RFID-TA 2017, and (5) the so-called PUF–FSM protocol of Gao, Ma, Al-Sarawi, Abbott, and Ranasinghe, as published in the IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems in 2018. The common flaw of all five designs is that the use of lightweight obfuscation logic provides insufficient protection against machine learning attacks.

Partially specified channels: The TLS 1.3 record layer without elision

Thu, 06/28/2018 - 01:50
This work advances the study of secure stream-based channels (Fischlin et al., CRYPTO ’15) by considering the multiplexing of many data streams over a single channel. This is an essential feature of real-world protocols such as TLS. Our treatment adopts the definitional perspective of Rogaway and Stegers (CSF ’09), which offers an elegant way to reason about what standardizing documents actually provide: a partial specification of a protocol that admits a collection of compliant, fully realized implementations. We formalize partially specified channels as the component algorithms of two parties communicating over a channel. Each algorithm has an oracle that services specification detail queries; intuitively, the algorithms abstract the things that are explicitly specified, while the oracle abstracts the things that are not. Our security notions, which capture a variety of privacy and integrity goals, allow the adversary to respond to these oracle queries; security relative to our notions implies that the channel withstands attacks in the presence of worst-case (i.e., adversarial) realizations of the specification details. Our formalization is flexible enough to provide the first provable security treatment of the TLS 1.3 record layer that does not elide optional behaviors and unspecified details.

DelegaTEE: Brokered Delegation Using Trusted Execution Environments

Wed, 06/27/2018 - 06:30
We introduce a new concept called brokered delegation. Brokered delegation allows users to flexibly delegate credentials and rights for a range of service providers to other users and third parties. We explore how brokered delegation can be implemented using novel trusted execution environments (TEEs). We introduce a system called DelegaTEE that enables users (Delegatees) to log into different online services using the credentials of other users (Owners). Credentials in DelegaTEE are never revealed to Delegatees and Owners can restrict access to their accounts using a range of rich, contextually dependent delegation policies. DelegaTEE fundamentally shifts existing access control models for centralized online services. It does so by using TEEs to permit access delegation at the user's discretion. DelegaTEE thus effectively reduces mandatory access control (MAC) in this context to discretionary access control (DAC). The system demonstrates the significant potential for TEEs to create new forms of resource sharing around online services without the direct support from those services. We present a full implementation of DelegaTEE using Intel SGX and demonstrate its use in four real-world applications: email access (SMTP/IMAP), restricted website access using a HTTPS proxy, e-banking/credit card, and a third-party payment system (PayPal).

New Methods for Indistinguishability Obfuscation: Bootstrapping and Instantiation

Tue, 06/26/2018 - 13:05
Constructing indistinguishability obfuscation (iO) [BGI+01] is a central open question in cryptography. We provide new methods to make progress towards this goal. Our contributions may be summarized as follows: 1. Bootstrapping. In a recent work, Lin and Tessaro [LT17] (LT) show that iO may be constructed using i) Functional Encryption (FE) for polynomials of degree $L$ , ii) Pseudorandom Generators (PRG) with blockwise locality $L$ and polynomial expansion, and iii) Learning With Errors (LWE). Since there exist constructions of FE for quadratic polynomials from standard assumptions on bilinear maps [Lin17, BCFG17], the ideal scenario would be to set $L=2$, yielding iO from widely believed assumptions. Unfortunately, it was shown soon after [LV17,BBKK17 ] that PRG with block locality $2$ and the expansion factor required by the LT construction, concretely $\Omega(n \cdot 2^{b(3+\epsilon)})$, where $n$ is the input length and $b$ is the block length, do not exist. While [LV17, BBKK17] provided strong negative evidence for constructing iO based on bilinear maps, they could not rule out the possibility completely; a tantalizing gap has remained. Given the current state of lower bounds, the existence of $2$ block local PRG with expansion factor $\Omega(n \cdot 2^{b(1+\epsilon)})$ remains open, although this stretch does not suffice for the LT bootstrapping, and is hence unclear to be relevant for iO. In this work, we improve the state of affairs as follows. (a) Weakening requirements on PRGs: In this work, we show that the narrow window of expansion factors left open by lower bounds do suffice for iO . We show a new method to construct FE for $NC_1$ from i) FE for degree $L$ polynomials, ii) PRGs of block locality $L$ and expansion factor $\Omega(n \cdot 2^{b(1+\epsilon)})$, and iii) LWE (or RLWE ). Our method of bootstrapping is completely different from all known methods. This re-opens the possibility of realizing iO from $2$ block local PRG, SXDH on Bilinear maps and LWE. (b)Broadening class of sufficient PRGs: Our bootstrapping theorem may be instantiated with a broader class of pseudorandom generators than hitherto considered for iO, and may circumvent lower bounds known for the arithmetic degree of iO -sufficient PRGs [LV17 , BBKK17]; in particular, these may admit instantiations with arithmetic degree $2$, yielding iO with the additional assumptions of SXDH on Bilinear maps and LWE. In more detail, we may use the following two classes of PRG: i) Non-Boolean PRGs: We may use pseudorandom generators whose inputs and outputs need not be Boolean but may be integers restricted to a small (polynomial) range. Additionally, the outputs are not required to be pseudorandom but must only satisfy a milder indistinguishability property. We tentatively propose initializing these PRGs using the multivariate quadratic assumption (MQ) which has been widely studied in the literature [MI88 , Wol05 , DY09] and against the general case of which, no efficient attacks are known. ii) Correlated Noise Generators: We introduce an even weaker class of pseudorandom generators, which we call correlated noise generators (CNG) which may not only be non-Boolean but are required to satisfy an even milder (seeming) indistinguishability property. (c) Assumptions and Efficiency. Our bootstrapping theorems can be based on the hardness of the Learning With Errors problem (LWE) or its ring variant (RLWE) and can compile FE for degree $L$ polynomials directly to FE for $NC_1$. Our method for bootstrapping to $NC_1$ does not go via randomized encodings as in previous works, which makes it simpler and more efficient than in previous works. 2. Instantiating Primitives. In this work, we provide the first direct candidate of FE for constant degree polynomials from new assumptions on lattices. Our construction is new and does not go via multilinear maps or graded encoding schemes as all previous constructions. Our construction is based on the ring learning with errors assumption (RLWE) as well as new untested assumptions on NTRU rings. We provide a detailed security analysis and discuss why previously known attacks in the context of multilinear maps, especially zeroizing attacks and annihilation attacks, do not appear to apply to our setting. We caution that the assumptions underlying our construction must be subject to rigorous cryptanalysis before any confidence can be gained in their security. However, their significant departure from known multilinear map based constructions make them, we feel, a potentially fruitful new direction to explore. Additionally, being based entirely on lattices, we believe that security against classical attacks will likely imply security against quantum attacks.

CHARIOT: Cloud-Assisted Access Control for the Internet of Things

Tue, 06/26/2018 - 12:02
The Internet of Things (IoT) technology has expanded widely across the world, promising new data management opportunities for industries, companies and individuals in different sectors, such as health services or transport logistics. This trend relies on connecting devices/things to collect, exchange and store data. The exponentially increasing number of IoT devices, their origin diversity, their limited capabilities in terms of resources, as well as the ever-increasing amount of data, raise new challenges for security and privacy protection, precluding traditional access control solutions to be integrated to this new environment. In this paper, we propose a reliable server-aided policy-based access control mechanism, named CHARIOT, that enables an IoT platform to verify credentials of different devices requesting access (read/write) to the data stored within it. CHARIOT permits IoT devices to authenticate themselves to the platform without compromising their privacy by using attribute-based signatures. Our solution also allows secure delegation of costly computational operations to a cloud server, hence relieving the workload at IoT devices' side.

Characterizing overstretched NTRU attacks

Tue, 06/26/2018 - 12:01
Overstretched NTRU, an NTRU variant with a large modulus, has been used as a building block for several cryptographic schemes in recent years. Recently, two lattice subfield attacks and a subring attack were proposed that broke some suggested parameters for overstretched NTRU. These attacks work by decreasing the dimension of the lattice to be reduced, which improves the performance of the lattice basis reduction algorithm. However, there are a number of conflicting claims in the literature over which of these attacks has the best performance. These claims are typically based on experiments more than analysis. Furthermore, the metric for comparison has been unclear in some prior work. In this paper, we argue that the correct metric should be the lattice dimension. We show both analytically and experimentally that the subring attack succeeds on a smaller dimension lattice than the subfield attack for the same problem parameters, and also succeeds with a smaller modulus when the lattice dimension is fixed.

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