RFID systems suffer from different location-based attacks such as distance fraud, mafia fraud and terrorist fraud attacks. Among them mafia fraud attack is the most serious since this attack can be mounted without the notice of both the reader and the tag. An adversary performs a kind of man-in-the-middle attack between the reader and the tag. It is very difficult to prevent this attack since the adversary does not change any data between the reader and the tag. Recently distance bounding protocols measuring the round-trip time between the reader and the tag have been researched to prevent this attack. All the existing distance bounding protocols based on binary challenges, without final signature, provide an adversary success probability equal to (3/4)^n where n is the number of rounds in the protocol. In this paper, we introduce a new protocol based on binary mixed challenges that converges toward the expected and optimal (1/2)^n bound. We prove its security in case of both noisy and non-noisy channels.

Fault attacks exploit hardware malfunctions to recover secrets from embedded electronic devices. In the late 90's, Boneh, DeMillo and Lipton introduced fault-based attacks on {\sc crt-rsa}. These attacks factor the signer's modulus when the message padding function is deterministic. However, the attack does not apply when the message is partially unknown, for example when messages contain some randomness which is recovered only when verifying a {\sl correct} signature. In this paper we successfully extends RSA fault attacks to a large class of partially known message configurations. The new attacks rely on Coppersmith's algorithm for finding small roots of multivariate polynomial equations. We illustrate the approach by successfully attacking several randomized versions of the ISO 9796-2 encoding standard. Practical experiments show that a $2048$-bit modulus can be factored in less than a minute given one faulty signature containing $160$ random bits and an unknown $160$-bit message digest.

Certificateless cryptography aims at combining the advantages of identity based and public key cryptography, so as to avoid the key escrow problem inherent in the identity based system and cumbersome certificate management in public key infrastructure. Signcryption achieves confidentiality and authentication simultaneously in an e

We revisit the notion of the anonymous signature, first formalized by Yang, Wong, Deng and Wang, and then further developed by Fischlin, and Zhang and Imai. We present a new formalism of anonymous signature, where instead of the message, a part of the signature is withheld to maintain anonymity. We introduce the notion unpretendability to guarantee infeasibility for someone other than the correct signer to pretend authorship of the message and signature. Our definition retains applicability for all previous applications of the anonymous signature, provides stronger security, and is conceptually simpler. We give a generic construction from any ordinary signature scheme, and also show that the short signature scheme by Boneh and Boyen can be naturally regarded as such a secure anonymous signature scheme according to our formalism.

We study the problem of authenticating the content and creation time of documents generated by an organization and retained in archival storage. Recent regulations (e.g., the Sarbanes-Oxley act and the Securities and Exchange Commission rule) mandate secure retention of important business records for several years. We provide a mechanism to authenticate bulk repositories of archived documents. In our approach, a space efficient local data structure encapsulates a full document repository in a short (e.g., 32-byte) digest. Periodically registered with a trusted party, these commitments enable compact proofs of both document creation time and content integrity. The data structure, an append-only persistent authenticated dictionary, allows for efficient proofs of existence and non-existence, improving on state-of-the-art techniques. We give a rigorous security analysis of our solution and confirm through an experimental evaluation with the Enron email corpus its feasibility in practice.

An $r$-collision for a function is a set of $r$ distinct inputs with identical outputs. Actually finding $r$-collisions for a random map over a finite set of cardinality $N$ requires at least about $N^{(r-1)/r} $ units of time on a sequential machine. For $r$=2, memoryless and well-parallelisable algorithms are known. The current paper describes memory-efficient and parallelisable algorithms for $r \ge 3$. The main results are: (1)~A sequential algorithm for 3-collisions, roughly using memory $N^\alpha$ and time $N^{1-\alpha}$ for $\alpha\le1/3$. I.e., given $N^{1/3}$ units of storage, on can find 3-collisions in time $N^{2/3}$. Note that there is a time-memory tradeoff which allows to reduce the memory consumption. (2)~A parallelisation of this algorithm using $N^{1/3}$ processors running in time $N^{1/3}$. Each single processor only needs a constant amount of memory. (3)~An generalisation of this second approach to $r$-collisions for $r \ge3$: given $N^s$ parallel processors, on can generate $r$-collisions roughly in time $N^{((r-1)/r)-s}$, using memory $N^{((r-2)/r)-s}$ on every processor.

Bilinear pairings derived from supersingular elliptic curves of embedding degrees 4 and 6 over finite fields of characteristic two and three, respectively, have been used to implement pairing-based cryptographic protocols. The pairing values lie in certain prime-order subgroups of certain cyclotomic subgroups. It was previously known how to compress the pairing values over characteristic two fields by a factor of 2, and the pairing values over characteristic three fields by a factor of 6. In this paper, we show how the pairing values over characteristic two fields can be compressed by a factor of 4. Moreover, we present and compare several algorithms for performing exponentiation in the prime-order subgroups using the compressed representations. In particular, in the case where the base is fixed, we expect to gain at least a 54% speed up over the fastest previously known exponentiation algorithm that uses factor-6 compressed representations.

We address the problem of designing optimal schemes for the generation of secure cryptographic keys from continuous noisy data. We argue that, contrary to the discrete case, a universal fuzzy extractor does not exist. This implies that in the continuous case, key extraction schemes have to be designed for particular probability distributions. We extend the known definitions of the correctness and security properties of fuzzy extractors. Our definitions apply to continuous as well as discrete variables. We propose a generic construction for fuzzy extractors from noisy continuous sources, using independent partitions. The extra freedom in the choice of discretisation, which does not exist in the discrete case, is advantageously used to give the extracted key a uniform distribution. We analyze the privacy properties of the scheme and the error probabilities in a one-dimensional toy model with simplified noise. Finally, we study the security implications of incomplete knowledge of the source's probability distribution P. We derive a bound on the min-entropy of the extracted key under the worst case assumption, where the attacker knows P exactly.

ESSENCE is a hash function submitted to the NIST Hash Competition that stands out as a hardware-friendly and highly parallelizable design. Previous analysis showed some non-randomness in the compression function which could not be extended to an attack on the hash function and ESSENCE remained unbroken. Preliminary analysis in its documentation argues that it resists standard differential cryptanalysis. This paper disproves this claim, showing that advanced techniques can be used to significantly reduce the cost of such attacks: using a manually found differential characteristic and an advanced search algorithm, we obtain collision attacks on the full ESSENCE-256 and ESSENCE-512, with respective complexities 2^67.4 and 2^134.7. In addition, we show how to use these attacks to forge valid (message, MAC) pairs for HMAC-ESSENCE-256 and HMAC-ESSENCE-512, essentially at the same cost as a collision.

In a compartmented access structure, there are disjoint participants C1, . . . ,Cm. The access structure consists of subsets of participants containing at least ti from Ci for i = 1, . . . ,m, and a total of at least t0 participants. Tassa [2] asked: whether there exists an efficient ideal secret sharing scheme for such an access structure? Tassa and Dyn [5] presented a solution using the idea of bivariate interpolation and the concept of dual program [9, 10]. For the purpose of practical applications, it is advantageous to have a simple scheme solving the problem. In this paper a simple scheme is given for this problem using the similar idea from [5].

We treat the security of group key exchange (GKE) in the universal composability (UC) framework. Analyzing GKE protocols in the UC framework naturally addresses attacks by malicious insiders. We define an ideal functionality for GKE that captures contributiveness in addition to other desired security goals. We show that an efficient two-round protocol securely realizes the proposed functionality in the random oracle model. As a result, we obtain the most efficient UC-secure contributory GKE protocol known.

Physical random number generators (a.k.a. TRNGs) appear to be critical components of many cryptographic systems. Yet, such building blocks are still too seldom provided with a formal assessment of security, in comparison to what is achieved for conventional cryptography. In this work, we present a comprehensive statistical study of TRNGs based on the sampling of an oscillator subject to phase noise (a.k.a. phase jitters). This classical layout, typically instantiated with a ring oscillator, provides a simple and attractive way to implement a TRNG on a chip. Our mathematical study allows one to evaluate and control the main security parameters of such a random source, including its entropy rate and the biases of certain bit patterns, provided that a small number of physical parameters of the oscillator are known. In order to evaluate these parameters in a secure way, we also provide an experimental method for filtering out the global perturbations affecting a chip and possibly visible to an attacker. Finally, from our mathematical model, we deduce specific statistical tests applicable to the bit stream of a TRNG. In particular, in the case of an insecure configuration, we show how to recover the parameters of the underlying oscillator.

Certificateless cryptography introduced by Al-Riyami and Paterson eliminates the key escrow problem inherent in identity based cryptosystems. Even though building practical identity based signcryption schemes without bilinear pairing are considered to be almost impossible, it will be interesting to explore possibilities of constructing such systems in other settings like certificateless cryptography. Often for practical systems, bilinear pairings are considered to induce computational overhead. Signcryption is a powerful primitive that offers both confidentiality and authenticity to noteworthy messages. Though some prior attempts were made for designing certificateless signcryption schemes, almost all the known ones have security weaknesses. Specifically, in this paper we demonstrate the security weakness of the schemes in \cite{BF08}, \cite{DRJR08} and \cite{CZ08}. We also present the first provably secure certificateless signcryption scheme without bilinear pairing and prove it in the random oracle model.

In this paper, we present a new distinguisher for HC-128 which is the best known so far. The distinguisher requires approximately $2^{138}$ keystream words with success probability 0.9772.

In the present paper we study properties of perfectly balanced Boolean functions. Based on the concept of Boolean function barrier, we propose a novel approach to construct large classes of perfectly balanced Boolean functions.

Key-Policy Attribute-Based Encryption (KP-ABE) is a promising cryptographic primitive which enables fine-grained access control over sensitive data. However, key abuse attacks in KP-ABE may impede its wide application especially in copyright-sensitive systems. To defend against this kind of attacks, this paper proposes a novel KP-ABE scheme which is able to disclose any illegal key distributor’s ID when key abuse is detected. In our scheme, each bit of user ID is defined as an attribute and the user secret key is associated with his unique ID. The tracing algorithm fulfills its task by tricking the pirate device into decrypting the ciphertext associated with the corresponding bits of his ID. Our proposed scheme has the salient property of black box tracing, i.e., it traces back to the illegal key distributor’s ID only by observing the pirate device’s outputs on certain inputs. In addition, it does not require the pirate device’s secret keys to be well-formed as compared to some previous work. Our proposed scheme is provably secure under the Decisional Bilinear Diffie-Hellman (DBDH) assumption and the Decisional Linear (DL) assumption.

Most existing anonymous networks focus on providing strong anonymity for the price of having lower bandwidth, higher latency and degraded usability when compared with the conventional use of the Internet. They also often anonymize only a few specific applications. In this paper, we propose a new approach of constructing an anonymous network. The network consists of an overlay network, which provides anonymity to all applications running on top of it, and a routing protocol, which can be considered as an anonymized version of path vector routing. The protocol preserves the high performance characteristics of the path vector routing and also has the added advantage of hiding the overlay network topology. Our simulation results show that the expected latency of our approach is 50% better than that of existing systems. Besides the new anonymous routing protocol, this paper aims to provide the general overview of this new anonymous overlay network which may serve as the input for further research.

Attribute-based encryption (ABE) has been envisioned as a promising cryptographic primitive for realizing secure and flexible access control. However, ABE is being criticized for its high scheme overhead as extensive pairing operations are usually required. In this paper, we focus on improving the efficiency of ABE by leveraging a previously overlooked fact, i.e., the often-found hierarchical relationships among the attributes that are inherent to many access control scenarios. As the first research effort along this direction, we coin the notion of hierarchical ABE (\textsf{HABE}), which can be viewed as the generalization of traditional ABE in the sense that both definitions are equal when all attributes are independent. We further give a concrete \textsf{HABE} construction considering a tree hierarchy among the attributes, which is provably secure. More importantly, our construction exhibits significant improvements over the traditional ABE when attribute hierarchies exist.

This paper applies generalized birthday attacks to the FSB compression function, and shows how to adapt the attacks so that they run in far less memory. In particular, this paper presents details of a parallel implementation attacking FSB48 , a scaled-down version of FSB proposed by the FSB submitters. The implementation runs on a cluster of 8 PCs, each with only 8GB of RAM and 700GB of disk. This situation is very interesting for estimating the security of systems against distributed attacks using contributed off-the-shelf PCs.

A key exchange protocol allows a set of parties to agree upon a secret session key over a public network. Two-party key exchange (2PKE) protocols have been rigorously analyzed under various models considering different adversarial actions. However, the analysis of group key exchange (GKE) protocols has not been as extensive as that of 2PKE protocols. Particularly, the security attribute of key compromise impersonation (KCI) resilience has so far been ignored for the case of GKE protocols. We first model the security of GKE protocols addressing KCI attacks by both outsider and insider adversaries. We then show that a few existing protocols are not secure even against outsider KCI attacks. The attacks on these protocols demonstrate the necessity of considering KCI resilience for GKE protocols. Finally, we give a new proof of security for an existing GKE protocol under the revised model assuming random oracles.

An identity based signature scheme allows any pair of users to communicate securely and to verify each others signatures without exchanging public key certificates. An aggregate signature scheme is a digital signature scheme which supports aggregation of signatures. Batch verification is a method to verify multiple signatures at once. Aggregate signature is useful in reducing both communication and computation cost. In this paper, we describe the breaks possible in some of the aggregate signature schemes and batch verification scheme.

In order to save bandwidth and thus battery power, sensor network measurements are sometimes aggregated en-route while being reported back to the querying server. Authentication of the measurements then becomes a challenge if message integrity is important for the application. At ESAS 2007, the End-by-Hop protocol for securing in-network aggregation for sensor nodes was presented. The solution was claimed to be secure and efficient and to provide the possibility of trading off bandwidth against computation time on the server. In this paper, we disprove these claims. We describe several attacks against the proposed solution and point out shortcomings in the original complexity analysis. In particular, we show that the proposed solution is inferior to a naive solution without in-network aggregation both in security and in efficiency.

In this paper, we propose two authenticated key exchange (AKE) protocols, SMEN and SMEN−, which have efficient online computation and tight security proof in the extended Canetti-Krawczyk (eCK) model. SMEN takes 1.25 exponentiations in online computation, close to that (1.17 exponentiations) of the most efficient AKEs MQV and its variants HMQV and CMQV. SMEN has a security reduction as tight as that of NAXOS, which is the first AKE having a tight security reduction in the eCK model. As a comparison, MQV does not have a security proof; both HMQV and CMQV have a highly non-tight security reduction, and HMQV needs a non-standard assumption; NAXOS takes 2.17 exponentiations in online computation; NETS, a NAXOS variant, takes two online exponentiations in online computation. SMEN simultaneously achieves online efficiency and a tight security proof at a cost of 0.17 more exponentiations in offline computation and the restriction that one party is not allowed to establish a key with itself. SMEN− takes 1.29 exponentiations in online computation, but SMEN− does not use the static private key to compute the ephemeral public key (as does in SMEN, NAXOS, CMQV, and NETS), and hence reduces the risk of leaking the static private key.

\begin{abstract} Generic attacks against classical (balanced) Feistel schemes, unbalanced Feistel schemes with contracting functions and unbalanced Feistel schemes with expanding functions have been studied in \cite {P01}, \cite{Jut}, \cite{PNB06}, \cite{PNB07}. In this paper we study schemes where we use alternatively contracting random functions and expanding random functions. We name these schemes ``Alternating Unbalanced Feistel Schemes''. They allow constructing pseudo-random permutations from $kn$ bits to $kn$ bits where $k \geq 3$. At each round, we use either a random function from $n$ bits to $(k-1)n$ bits or a random function from $(k-1)n$ bits to $n$ bits. We describe the best generic attacks we have found. We present``known plaintext attacks'' (KPA) and ``non-adaptive chosen plaintext attacks'' (CPA-1). Let $d$ be the number of rounds. We show that if $d \leq k$, there are CPA-1 with 2 messages and KPA with $m$ the number of messages about $2^{\frac {(d-1)n}{4}}$. For $d \geq k+1$ we have to distinguish $k$ even and $k$ odd. For $k$ even, we have $m=2$ in CPA-1 and $m \simeq 2^{\frac {kn}{4}}$ in KPA. When $k$ is odd, we show that there exist CPA-1 for $d \leq 2k-1$ and KPA for $d \leq 2k+3$ with less than $2^{kn}$ messages and computations. Beyond these values, we give KPA against generators of permutations. \end{abstract}

Tamper-proof devices are pretty powerful. They typically make security applications simpler (provided that the tamper-proof assumption is not violated). For application requiring privacy, we observe that some properties may become harder (if possible at all) to achieve when devices are maliciously used. We take the example of deniability, receipt-freeness, and anonymity. We formalize the trusted agent model which assumes tamper-proof hardware in a way which captures the notion of programmable secure hardware. This model defines a functionality relative to which deniability requires provers to use a tamper proof hardware. Otherwise, any asymmetric situation in which the malicious verifiers have more powerful tamper-proof devices than the honest ones makes deniability impossible. We conclude by observing that the ability to put boundaries in computing devices prevents from providing full control on how private information spreads: the concept of sealing a device is in some sense incompatible with some privacy notions.

The potential high efficiency of public-key encryption based on structured lattices was first indicated by the NTRU cryptosystem, which was proposed about 10 years ago. Unfortunately, the security of NTRU is only heuristic. Thus, it remained an important research challenge to construct an efficient encryption scheme based on structured lattices which admits a proof of security relative to a well established cryptographic assumption. We make progress in addressing the above challenge. We show how to construct a CPA-secure public-key encryption scheme with security provably based on the worst case hardness of the approximate Shortest Vector Problem in structured ideal lattices. Under the assumption that the latter is exponentially hard to solve even with a quantum computer, our scheme resists any subexponential attack and offers (quasi-)optimal asymptotic performance: if $n$ is the security parameter, both keys are of bit-length $\softO(n)$ and the amortized costs of both encryption and decryption are $\softO(1)$ per message bit. Our construction adapts the trapdoor one-way function of Gentry, Peikert and Vaikuntanathan (STOC 2008), based on the Learning With Errors problem, to structured lattices. Our main technical tools are an adaptation of Ajtai's trapdoor key generation algorithm (ICALP 1999) to structured ideal lattices, and a re-interpretation of Regev's quantum reduction between the Closest Vector Problem and sampling short lattice vectors. We think these techniques are very likely to find further applications in the future.

As a new public key primitive, attribute-based encryption (ABE) is envisioned to be a promising tool for implementing fine-grained access control. To further address the concern of user access privacy, privacy-aware ABE schemes are being developed to achieve hidden access policy recently. For the purpose of secure access control, there is, however, still one critical functionality missing in the existing ABE schemes, which is user accountability. Currently, no ABE scheme can completely prevent the problem of illegal key sharing among users. In this paper, we tackle this problem by firstly proposing the notion of accountable, anonymous, and ciphertext-policy ABE (CP-A$^3$BE, in short) and then giving out a concrete construction. We start by improving the state-of-the-art of anonymous CP-ABE to obtain shorter public parameters and ciphertext length. In the proposed CP-A$^3$BE construction, user accountability can be achieved in black-box model by embedding additional user-specific information into the attribute private key issued to that user, while still maintaining hidden access policy. The proposed constructions are provably secure.

We present the first signature scheme which is ''short'', stateless and secure under the RSA assumption in the standard model. Prior short, standard model signatures in the RSA setting required either a strong complexity assumption such as Strong RSA or (recently) that the signer maintain state. A signature in our scheme is comprised of one element in ZN* and one integer. The public key is also short, requiring only the modulus N, one element of ZN*, one integer, one PRF seed and some short chameleon hash parameters. To design our signature, we employ the known generic construction of fully-secure signatures from weakly-secure signatures and a chameleon hash. We then introduce a new proof technique for reasoning about weakly-secure signatures. This technique enables the simulator to predict a prefix of the message on which the adversary will forge and to use knowledge of this prefix to embed the challenge. This technique has wider applications beyond RSA. We also use it to provide an entirely new analysis of the security of the Waters signatures: the only short, stateless signatures known to be secure under the Computational Diffie-Hellman assumption in the standard model.

The strongest standard security notion for digital signature schemes is unforgeability under chosen message attacks. In practice, however, this notion can be insufficient due to ``side-channel attacks'' which exploit leakage of information about the secret internal state of the scheme's hardware implementation. In this work we put forward the notion of ``leakage-resilient signatures,'' which strengthens the standard security notion by giving the adversary the additional power to learn a bounded amount of arbitrary information about the secret state that was accessed during every signature generation. This notion naturally implies security against all possible side-channel attacks as long as the amount of information leaked on each invocation is bounded and ``only computation leaks information.'' The main result of this paper is a construction which gives a (tree based, stateful) leakage-resilient signature scheme based on any 3-time signature scheme. The amount of information that our scheme can safely leak per signature generation is $1/3$ of the information the underlying 3-time signature scheme can leak in total. Based on recent works by Alwen, Dodis, Wichs and by Katz we give several efficient instantiations of 3-time signature schemes with the required security properties, hence yielding the first constructions of provably secure leakage-resilient signature schemes.

Cloud Computing has been envisioned as the next-generation architecture of IT Enterprise. It moves the application software and databases to the centralized large data centers, where the management of the data and services may not be fully trustworthy. This unique paradigm brings about many new security challenges, which have not been well understood. This work studies the problem of ensuring the integrity of data storage in Cloud Computing. In particular, we consider the task of allowing a third party auditor (TPA), on behalf of the cloud client, to verify the integrity of the dynamic data stored in the cloud. The introduction of TPA eliminates the involvement of client through the auditing of whether his data stored in the cloud is indeed intact, which can be important in achieving economies of scale for Cloud Computing. The support for data dynamics via the most general forms of data operation, such as block modification, insertion and deletion, is also a significant step toward practicality, since services in Cloud Computing are not limited to archive or backup data only. While prior works on ensuring remote data integrity often lacks the support of either public verifiability or dynamic data operations, this paper achieves both. We first identify the difficulties and potential security problems of direct extensions with fully dynamic data updates from prior works and then show how to construct an elegant verification scheme for seamless integration of these two salient features in our protocol design. In particular, to achieve efficient data dynamics, we improve the Proof of Retrievability model \cite{Shacham:ASIACRYPT:2008} by manipulating the classic Merkle Hash Tree (MHT) construction for block tag authentication. Extensive security and performance analysis show that the proposed scheme is highly efficient and provably secure.

We introduce the first universally anonymous, thus key-private, IBE whose security is based on the standard quadratic residuosity assumption. Our scheme is a variant of Cocks IBE (which is not anonymous) and is efficient and highly parallelizable.

In 2002, algebraic attacks using overdefined systems of equations have been proposed as a potentially very powerful cryptanalysis technique against block ciphers. However, although a number of convincing experiments have been performed against certain reduced algorithms, it is not clear wether these attacks can be successfully applied in general and to a large class of ciphers. In this paper, we show that algebraic techniques can be combined with side-channel attacks in a very effective and natural fashion. As an illustration, we apply them to the block cipher PRESENT that is a stimulating first target, due to its simple algebraic structure. The proposed attacks have a number of interesting features: (1) they exploit the information leakages of all the cipher rounds, (2) in common implementation contexts (e.g. assuming a Hamming weight leakage model), they recover the block cipher keys after the observation of a single encryption, (3) these attacks can succeed in an unknown-plaintext/ciphertext adversarial scenario and (4) they directly defeat countermeasures such as boolean masking. Eventually, we argue that algebraic side-channel attacks can take advantage of any kind of physical leakage, leading to a new tradeoff between the robustness and informativeness of the side-channel information extraction.

This paper discusses the practical implementation of a novel security tool termed SIMPL system, which was introduced in [1]. SIMPL systems can be regarded as a public key version of physical unclonable functions (PUFs). Like the latter, a SIMPL system S is physically unique and nonreproducible, and implements an individual function FS. In opposition to a PUF, however, a SIMPL system S possesses a publicly known numerical description, which allows its digital simulation and prediction. At the same time, any such simulation must work at a detectably lower speed than the real-time behavior of S. As argued in [1], SIMPL systems have certain practicality and security advantages in comparison to PUFs, certificates of authenticity, physically obfuscated keys, and also to standard mathematical cryptotechniques. In [1], definitions, protocols, and optical implementations of SIMPL systems were presented. This manuscript focuses on concrete electrical, integrated realizations of SIMPL systems, and proposes two potential candidates: SIMPL systems derived from special SRAM-architectures (socalled “skew designs” of SRAM cells), and implementations based on Cellular Non-Linear Networks (CNNs).

We investigate the foundations of Physical Unclonable Functions from several perspectives. Firstly, we discuss formal and conceptual issues in the various current definitions of PUFs. As we argue, they have the effect that many PUF candidates formally meet no existing definition. Next, we present alternative definitions and a new formalism. It avoids asymptotic concepts like polynomial time, but is based on concrete time bounds and on the concept of a security experiment. The formalism splits the notion of a PUF into two new notions, Strong $t$-PUFs and Obfuscating $t$-PUFs. Then, we provide a comparative analysis between the existing definitions and our new notions, by classifying existing PUF implementations with respect to them. In this process, we use several new and unpublished machine learning results. The outcome of this comparative classification is that our definitions seem to match the current PUF landscape well, perhaps better than previous definitions. Finally, we analyze the security and practicality features of Strong and Obfuscating $t$-PUFs in concrete applications, obtaining further justification for the split into two notions.

This paper describes the design of a fast multi-core library for the cryptographic Tate pairing over supersingular elliptic curves. For the computation of the reduced modified Tate pairing over $\mathbb{F}_{3^{509}}$, we report calculation times of just $2.94$ ms and $1.87$ ms on the Intel Core2 and Intel Core i7 architectures, respectively. We also try to answer one important design question that surges: how many cores should be utilized for a given application?

This thesis studies the ring of boolean functions. A simple algorithm that performs similarly to the euclidian algorithm is derived, and it's performance for solving multivariate boolean equation systems is evaluated. Furthermore, the Raddum-Semaev equation system solving algorithm is put into geometric context, and the pseudo-euclidian algorithm is generalized to multivariate polynomial rings over arbitrary finite fields.

Abstract. The new hash function DIHA2 (Dynamic Input Hash Algorithm)is with the structure of Merkle-Damgard and is based on 64-bit computing.It oper- ates each 1024-bit block and outputts a 256-bit hash-value. For a 64-bit sub-block X[j](0 ≤ j ≤ 15) of each step, DIHA2 gets a dynamic mapping value of TLU (table look up,The table was 256-Byte only)and add it to operation of variables a, b, c, d,so as to eliminate the differential effect.At the same time DIHA2 sets 3 assistant register variables r1, r2, r3 to store the mapping value and resume loading 3 steps later, so as to be interleaving. DIHA2 therefore obtained strong avalanche effect than the others and can resist the sharp and serious attack of differential.

This paper presents a non-interactive verifiable secret sharing scheme (VSS) tolerating a dishonest majority based on data pre-distributed by a trusted authority. As an application of this VSS scheme we present very efficient unconditionally secure multiparty protocols based on pre-distributed data which generalize two-party computations based on linear pre-distributed bit commitments. The main results of this paper are a non-interactive VSS where the amount of data which needs to be pre-distributed to each player depends on the number of tolerable cheaters only, a simplified multiplication protocol for shared values based on pre-distributed random products, and non-interactive zero knowledge proofs for arbitrary polynomial relations. The security of the schemes are proved using the UC framework.

In this paper, we propose a combinatoric conjecture on binary string, on the premise that our conjecture is correct we mainly obtain two classes of functions which are both algebraic immunity optimal: the first class of functions are also bent, moreover, from this fact we conclude that the algebraic immunity of bent functions can take all possible values except one. The second class are balanced functions, which have optimal algebraic degree and the best nonlinearity up to now.

We show a method for compressing the ciphertext and reducing the computational cost of the Dolev-Dwork-Naor cryptosystem and related schemes without changing their other parameters nor reducing the original security levels.

Oblivious polynomial evaluation (OPE) consists of a two-party protocol where a sender inputs a polynomial $P$, and a receiver inputs a single value $i$. At the end of the protocol, the sender learns nothing and the receiver learns $P(i)$. This paper deals with the problem of oblivious polynomial evaluation under an information-theoretical perspective, which is based on recent definitions of Unconditional Security developed by Crépeau et al. (EUROCRYPT 2006). In this paper, we propose an information-theoretical model for oblivious polynomial evaluation relying on pre-distributed data, and prove very general lower bounds on the size of the pre-distributed data, as well as the size of the communications in any protocol. It is demonstrated that these bounds are tight by obtaining a round-optimal OPE protocol, which meets the lower bounds simultaneously. Some applications of the proposed model are provided, such as solutions for the ``Millionaire Problem'' and the ``Oblivious Equality Testing Problem''. We also present a natural generalization to OPE called oblivious linear functional evaluation.

Power analysis attacks against embedded secret key cryptosystems are widely studied since the seminal paper of Paul C. Kocher, Joshua Jaffe and Benjamin Jun in 1998 where has been introduced the powerful Differential Power Analysis. The strength of DPA is such that it became necessary to develop sound and efficient countermeasures. Nowadays embedded cryptographic primitives usually integrate one or several of these countermeasures (e.g. masking techniques, asynchronous designs, balanced dynamic dual-rail gates designs, noise adding, power consumption smoothing, etc. ...). This document presents new power analysis attacks based on linear approximations of the target cipher. This new type of attacks have several advantages compared to classical DPA-like attacks: first they can use multiple intermediate values by query (i.e. power trace) allowing to reduce data complexity to a minimum, secondly they can be applied on parts of the symmetric cipher that are practically unreachable by DPA-like attacks and finally they can be mounted on an unknown cipher implementation.

We propose a dealer-free dynamic secret sharing scheme where the threshold and the secret can be changed multiple times to arbitrary values after the scheme's initialization. Our motivation is to tackle the following problems. In a threshold scheme, the sensitivity of the secret as well as the number of players may fluctuate due to various reasons, e.g., the structure of the players' organization might be changed. A possible solution to this problem is to modify the threshold and/or change the secret. Moreover, a common problem with almost all secret sharing schemes is that they are ``one-time'', meaning that the secret and shares are known to everyone after secret recovery. This problem could be resolved if the dealer shares various secrets at the beginning, but a better solution is to dynamically generate new secrets in the absence of the dealer. As our contribution, the well-known re-sharing techniques are analyzed in both passive and active adversary models. Subsequently, our solution for a dealer-free dynamic secret sharing scheme is provided. Finally, a new secret sharing protocol, as an applications of our dynamic scheme, is proposed.

We present a symbolic framework for refinement and composition of security protocols. The framework uses the notion of ideal functionalities. These are abstract systems which are secure by construction and which can be combined into larger systems. They can be separately refined in order to obtain concrete protocols implementing them. Our work builds on ideas from computational models such as the universally composable security and reactive simulatability frameworks. The underlying language we use is the applied pi calculus which is a general language for specifying security protocols. In our framework we can express the different standard flavours of simulation-based security which happen to all coincide. We illustrate our framework on an authentication functionality which can be realized using the Needham-Schroeder-Lowe protocol. For this we need to define an ideal functionality for asymmetric encryption and its realization. We also show a joint state result for this functionality which allows composition (even though the same key material is reused) using a tagging mechanism.

At Asiacrypt’99, Vaudenay modified the structure in the IDEA cipher to a new scheme, which they called as the Lai-Massey scheme. It is proved that 3-round Lai-Massey scheme is sufficient for pseudorandomness and 4-round Lai-Massey scheme is sufficient for strong pseudorandomness. But the author didn’t point out whether three rounds and four rounds are necessary for the pseudorandomness and strong pseudorandomness of the Lai-Massey Scheme. In this paper we find a two round pseudorandomness distinguisher and a three-round strong pseudorandomness distinguisher, thus prove that three rounds is necessary for the pseudorandomness and four rounds is necessary for the strong pseudorandomness.

In this paper, first we point out some flaws in the existing indifferentiability simulations of the pf-MD and the NMAC constructions, and provide new differentiable attacks on the hash functions based these schemes. Afterthat, the indifferentiability of the 20 collision resistant PGV hash functions, which are padded under the pf-MD, the NMAC/HMAC and the chop-MD constructions, are reconsidered. Moreover, we disclose that there exist 4 PGV schemes can be differentiable from a random oracle with the pf-MD among 16 indifferentiable PGV schemes proven by Chang et al. Finally, new indifferentiability simulations are provided for 20 collision-resistant PGV schemes. The simulations exploit that 20 collision-resistant PGV hash functions, which implemented with the NMAC/HMAC and the chop-MD, are indifferentiable from a random oracle. Our result implies that same compression functions under MD variants might have the same security bound with respect to the collision resistance, but quite different in the view of indifferentiability.

In this paper we propose a new MPKC, called PPS, based on (i) the 2-layer nonlinear piece in hand method, (ii) PMI, and (iii) STS. The PPS is a specific MPKC obtained by applying the 2-layer nonlinear piece in hand method to STS, in the manner that the rank and randomness of the lower rank steps in the original secret polynomial vector of STS are enhanced by adding a perturbation polynomial vector and moreover PMI is used in the auxiliary part. The PPS overcomes the drawbacks of the three schemes by the advantage of the three schemes themself. Thus, PPS can be thought to be immune simultaneously from the algebraic attacks, such as the Groebner bases attacks, from the rank attacks, and from the differential attacks.

Consider a model of secret sharing schemes with cheaters. We say that a secret sharing scheme is error decodable if we can still recover the secret $s$ correctly from a noisy share vector $({\tt share}_1', \cdots, {\tt share}_n')$. In this paper, we first prove that there exists an error decodable secret sharing scheme if and only if the adversary structure $\Gamma$ satisfies a certain condition called $Q^3$. Next for any $\Gamma$ which satisfies $Q^3$, we show an error decodable secret sharing scheme such that the decoding algorithm runs in polynomial-time in $|S|$ and the size of a linear secret sharing scheme (monotone span program) which realzes $\Gamma$. We finally show an applicaiton to $1$-round Perfectly Secure Message Transmission schemes (PSMT).

We study two-round authenticated message exchange protocols consisting of a single request and a single response, with the realistic assumption that the responder is long-lived and has bounded memory. We first argue that such protocols necessarily need elements such as timestamps to be secure. We then present such a protocol and prove that it is correct and computationally secure. In our model, the adversary provides the initiator and the responder with the payload of their messages, which means our protocol can be used to implement securely any service based on authenticated message exchange. We even allow the adversary to read and reset the memory of the principals and to use, with very few restrictions, the private keys of the principals for signing the payloads or parts thereof. The latter corresponds to situations in which the keys are not only used by our protocol. We use timestamps to secure our protocol, but only assume that each principal has access to a local clock.

We provide the first proof of security for Abreast-DM, one of the oldest and most well-known constructions for turning a block cipher with $n$-bit block length and $2n$-bit key length into a 2n-bit cryptographic hash function. In particular, we prove that when Abreast-DM is instantiated with AES-256, i.e. a block cipher with 128-bit block length and 256-bit key length, any adversary that asks less than 2^124.42 queries cannot find a collision with success probability greater than 1/2. Surprisingly, this about 15 years old construction is one of the few constructions that have the desirable feature of a near-optimal collision resistance guarantee. We generalize our techniques used in the proof of Abreast-DM to a huge class of double block length (DBL) hash functions that we will call Cyclic-DM. Using this generalized theorem we are able to derive several DBL constructions that lead to compression functions that even have a higher security guarantee and are more efficient than Abreast-DM. Furthermore we give DBL constructions that have the highest security guarantee of all DBL compression functions currently known in literature. We also provide an analysis of preimage resistance for Cyclic-DM compression functions. Note that this work has been already presented at Dagstuhl '09.

In this paper, we compare the implementation costs of various SHA-3 candidates on low-cost 8-bit CPUs by estimating RAM/ROM requirements of them. As a first step toward this kind of research, in our comparison, we make reasonable estimations of RAM/ROM requirements of them which can be done without implementation.

Although SHA-1 has been theoretically broken for some time now, the task of finding a practical collision is yet to be completed. Using some new approaches to differential analysis, we were able to find a new differential path which can be used in a collision attack with complexity of $O(2^{52})$. This is currently the lowest complexity attack on SHA-1.

The elliptic curve method (ECM) is one of the best factorization methods available. It is possible to use hyperelliptic curves instead of elliptic curves but it is in theory slower. We use special hyperelliptic curves and Kummer surfaces to reduce the complexity of the algorithm. Our implementation GMP-HECM is faster than GMP-ECM for factoring big numbers.

Datatype-Preserving Encryption (DTP) enables encryption of values within a certain character set into ciphertext restricted to the same set, while still keeping data length. This is in contrast to conventional block cipher modes which produce binary data, i e each encrypted character may have an arbitrary value, possibly outside the original character set, often accompanied with a length expansion caused by padding. Format-Controlling Encryption (FCE) is an extension to DTP, for which data length still is kept, but the output character range is allowed to be larger, though not covering the range of all possible values (i e binary data). With FCE it is possible to handle certain DTP limitations, like limited key rotation and integrity support.

SMS4 is a 32-round unbalanced Feistel block cipher with its block size and key size being 128 bits. As a fundamental block cipher used in the WAPI standard, the Chinese national standard for WLAN, it has been widely implemented in Chinese WLAN industry. In this paper, we present a modified branch-and-bound algorithm which can be used for searching multiple linear characteristics for SMS4-like unbalanced Feistel block ciphers. Furthermore, we find a series of 5-round iterative linear characteristics of SMS4 when applying the modified algorithm in SMS4. Then based on each 5-round iterative linear characteristic mentioned above, an 18-round linear characteristic of SMS4 can be constructed, thus leading to a list of 18-round linear characteristics of SMS4. According to the framework of Biryukov $et\ al.$ from Crpto 2004, a key recovery attack can be mounted on 22-round SMS4 by utilizing the above multiple linear characteristics. As a matter of fact, our result has much lower data complexity than the previously best known cryptanalytic result on 22-round SMS4, which is also the previously best known result on SMS4.

This paper theoretically discusses a novel security tool termed {\it SIMPL system}, which can be regarded as a public key version of physical unclonable functions (PUFs). Like the latter, a SIMPL system $S$ is physically unique and non-reproducible, and implements an individual function $F_S$. In opposition to a PUF, however, a SIMPL system $S$ possesses a publicly known numerical description $D(S)$, which allows its digital simulation and prediction. At the same time, it is required that any digital simulation of a SIMPL system $S$ must work at a detectably lower speed than its real-time behavior. In other words, the holder of a SIMPL system $S$ can evaluate a publicly known, publicly computable function $F_S$ faster than anyone else. This feature, so we argue in this paper, allows a number of improved practicality and security features. Once implemented successfully, SIMPL systems would have specific advantages over PUFs, certificates of authenticity, physically obfuscated keys, and also over standard mathematical cryptotechniques.

Zhou et al proposed a quantum encryption scheme based on quantum computation in 2006. Each qubit of the ciphertext is constrained to two pairs of conjugate states. So its implementation is feasible with the existing technology. But it is inefficient since it entails six key bits to encrypt one message bit, and the resulting ciphertext for one message bit consists of three qubits. In addition, its security can not be directly reduced to the well-known BB84 protocol. In this paper, we revisit it using the technique developed in BB84 protocol. The new scheme entails only two key bits to encrypt one message bit. The resulting ciphertext is just composed of two qubits. It saves about a half cost without the loss of security. Moreover, the encryption scheme is probabilistic rather than deterministic.

Many recent key exchange (KE) protocols have been proven secure in the CK, CK-HMQV, or eCK security models. The exact relation between these security models, and hence between the security guarantees provided by the protocols, is unclear. First, we show that the CK, CK-HMQV, and eCK security models are formally incomparable. Second, we show that these models are also practically incomparable, by providing for each model attacks that are not considered by the other models. Our analysis enables us to find several previously unreported flaws in existing protocol security proofs. We identify the causes of these flaws and show how they can be avoided.

A system of Boolean equations is called sparse if each equation depends on a small number of variables. Finding efficiently solutions to the system is an underlying hard problem in the cryptanalysis of modern ciphers. In this paper we study new properties of the Agreeing Algorithm, which was earlier designed to solve such equations. Then we show that mathematical description of the Algorithm is translated straight into the language of electric wires and switches. Applications to the DES and the Triple DES are discussed. The new approach, at least theoretically, allows a faster key-rejecting in brute-force than with Copacobana.

Format-preserving encryption (FPE) encrypts a plaintext of some specified format into a ciphertext of identical format—for example, encrypting a valid credit-card number into a valid creditcard number. The problem has been known for some time, but it has lacked a fully general and rigorous treatment.We provide one, starting off by formally defining FPE and security goals for it.We investigate the natural approach for achieving FPE on complex domains, the “rank-then-encipher” approach, and explore what it can and cannot do. We describe two flavors of unbalanced Feistel networks that can be used for achieving FPE, and we prove new security results for each. We revisit the cycle-walking approach for enciphering on a non-sparse subset of an encipherable domain, showing that the timing information that may be divulged by cycle walking is not a damaging thing to leak.

Inner state of a stream cipher is said to be as large as necessary but at the same time as small as possible. Trivium, a hardware oriented stream cipher, has been selected for the final portfolio of the eSTREAM project. It offers a security level of 80 bits while it has 288 internal state bits. Owing to its simple algebraic structure, it has been proved experimentally that Trivium can provide only a marginal security level of 80 bits. This article presents some modified versions of Trivium to increase its security level from 80 bits. Our objective is to give a better security level with the same number of internal states without changing much the elegant and simple design philosophy of Trivium. The focus is to make its algebraic structure intricate enough to resist the algebraic attack with guess and determine approach, which can recover its secret internal state bits. We have proposed two possible modifications that can increase its security level without any increase in the number of AND gates. Maximov and Biryukov have proposed a tweaked version of Trivium (Trivium/128) (11), with additional AND gates, to increase the security level to 128 bits. In this article, two other modifications with additional product terms proven to have a better security margin than Trivium/128 are also presented.

We give an efficient decision procedure that, on input two (acyclic) cryptographic expressions making arbitrary use of an encryption scheme and a (length doubling) pseudorandom generator, determines (in polynomial time) if the two expressions produce computationally indistinguishable distributions for any pseudorandom generator and encryption scheme satisfying the standard security notions of pseudorandomness and indistinguishability under chosen plaintext attack. The procedure works by mapping each expression to a symbolic pattern that captures, in a fully abstract way, the information revealed by the expression to a computationally bounded observer. We then prove that if any two (possibly cyclic) expressions are mapped to the same pattern, then the associated distributions are indistinguishable. At the same time, if the expressions are mapped to different symbolic patterns and do not contain encryption cycles, there are secure pseudorandom generators and encryption schemes for which the two distributions can be distinguished with overwhelming advantage.

In this paper we describe a cryptanalysis of MST_3, a public key cryptosystem based on non-commutative groups recently proposed by Lempken, Magliveras, van Trung and Wei.

We study the necessary and sufficient assumptions for universally composable (UC) computation, both in terms of setup and computational assumptions. We look at the common reference string model, the common random string model and the key-registration authority (KRA) model, and provide new result for all of them. Perhaps most interestingly we show that: - For even the minimal meaningful KRA, where we only assume that the secret key is a value which is hard to compute from the public key, one can UC securely compute any poly-time functionality if there exists a passive secure oblivious-transfer protocol for the stand-alone model. Since a KRA where the secret keys can be computed from the public keys is useless, and some setup assumption is needed for UC secure computation, this establishes the best we could hope for the KRA model: any non-trivial KRA is sufficient for UC computation. - We show that in the KRA model one-way functions are sufficient for UC commitment and UC zero-knowledge. These are the first examples of UC secure protocols for non-trivial tasks which do not assume the existence of public-key primitives. In particular, the protocols show that non-trivial UC computation is possible in Minicrypt.

This paper proposes the use of Cellular Non-Linear Networks (CNNs) as physical uncloneable functions (PUFs). We argue that analog circuits offer higher security than existing digital PUFs and that the CNN paradigm allows us to build large, unclonable, and scalable analog PUFs, which still show a stable and repeatable input--output behavior. CNNs are dynamical arrays of locally-interconected cells, with a cell dynamics that depends upon the interconnection strengths to their neighbors. They can be designed to evolve in time according to partial differential equations. If this equation describes a physical phenomenon, then the CNN can simulate a complex physical system on-chip. This can be exploited to create electrical PUFs with high relevant structural information content. To illustrate our paradigm at work, we design a circuit that directly emulates nonlinear wave propagation phenomena in a random media. It effectively translates the complexity of optical PUFs into electrical circuits. %&, leading to better practicality, while maintaining or even improving the security properties of their optical counterparts.

In 2008, Hashimoto and Sakurai proposed a new efficient signature scheme, which is a non-commutative ring version of Shamir’s birational permutation signature scheme. Shamir’s scheme is a generalization of the OSS (Ong-Schnorr-Shamir) signature scheme and was broken by Coppersmith et al. using its linearity and commutativity. The HS (Hashimoto-Sakurai) scheme is expected to be secure against the attack of Coppersmith et al. since the scheme is based on the noncommutative structure. In this paper, we propose an attack against the HS scheme. Our proposed attack is practical under the condition that its step size and the number of steps are small. More precisely, we firstly show that the HS scheme is essentially a commutative scheme, that is, the HS scheme can be reduced to some commutative birational permutation signature scheme. Then we apply Patarin-like attack against the commutative birational permutation signature scheme. We discuss efficiency of our attack by using some experimental results. Furthermore the commutative scheme obtained from the HS scheme is the Rainbow-type signature scheme. We also discuss the security of the Rainbow-type signature scheme, and propose an efficient attack against some class of the Rainbow-type signature scheme.

We introduce a new attack model for collusion secure codes, and analyze the collusion resistance of two version of the Tardos code in this model, both for binary and non-binary alphabets. The model allows to consider signal processing and averaging attacks via a set of symbol detection error rates. The false positive rate is represented as a single number; the false negative rate is a function of the false positive rate and of the number of symbols mixed by the colluders. We study two versions of the q-ary Tardos code in which the accusation method has been modified so as to allow for the detection of multiple symbols in the same content segment. The collusion resilience of both variants turns out to be comparable. For realistic attacker strengths the increase in code length is modest, demonstrating that the modified Tardos code is effective in the new model.

This paper presents efficient formulas for computing cryptographic pairings on the curve y^2 = c*x^3 + 1 over fields of large characteristic. We provide examples of pairing-friendly elliptic curves of this form which are of interest for efficient pairing implementations.

Here we present practical differential $q$-multicollisions for AES-256, which can be tested on any implementation of AES-256. In our paper "Distinguisher and Related-Key Attack on the Full AES-256" $q$-multicollisions are found with complexity $q\cdot 2^{67}$. We relax conditions on the plaintext difference $\Delta_P$ allowing some bytes to vary and find multicollisions for 13 and 14 round AES with complexity $q\cdot 2^{37}$. Even with the relaxation there is still a large complexity gap between our algorithm and the lower bound that we have proved in Lemma 1. Moreover we believe that in practice finding even two fixed-difference collisions for a good cipher would be very challenging.

In this paper we construct a chosen-key distinguisher and a related-key attack on the full 256-bit key AES. We define a notion of {\em differential $q$-multicollision} and show that for AES-256 $q$-multicollisions can be constructed in time $q\cdot 2^{67}$ and with negligible memory, while we prove that the same task for an ideal cipher of the same block size would require at least $O(q\cdot 2^{\frac{q-1}{q+1}128})$ time. Using similar approach and with the same complexity we can also construct $q$-pseudo collisions for AES-256 in Davies-Meyer hashing mode, a scheme which is provably secure in the ideal-cipher model. We have also computed partial $q$-multicollisions in time $q\cdot 2^{37}$ on a PC to verify our results. These results show that AES-256 can not model an ideal cipher in theoretical constructions. Finally, we extend our results to find the first publicly known attack on the full 14-round AES-256: a related-key distinguisher which works for one out of every $2^{35}$ keys with $2^{120}$ data and time complexity and negligible memory. This distinguisher is translated into a key-recovery attack with total complexity of $2^{131}$ time and $2^{65}$ memory.

We observe that finding invalid signatures in batches of signatures that fail batch verification is an instance of the classical group testing problem. We present and compare new sequential and parallel algorithms for finding invalid signatures based on group testing algorithms. Of the five new algorithms, three show improved performance for many parameter choices, and the performance gains are especially notable when multiple processors are available.

Protecting an implementation against Side Channel Analysis for Reverse Engineering (SCARE) attacks is a great challenge and we address this challenge by presenting a first proof of concept. White-box cryptography has been developed to protect programs against an adversary who has full access to their software implementation. It has also been suggested as a countermeasure against side channel attacks and we examine here these techniques in the wider perspective of SCARE. We consider that the adversary has only access to the cryptographic device through its side channels and his goal is to recover the specifications of the algorithm. In this work, we focus on FPGA (Field-Programmable Gate Array) technologies and examine how to thwart SCARE attacks by implementing a block cipher following white-box techniques. The proposed principle is based on changing dynamically the implementations. It is illustrated by an example on the Noekeon cipher and feasibility in different FPGAs is studied.

BLAKE is a new hash family proposed for SHA-3. The core of compression function reuses the core function of ChaCha. A round-dependent permutation is used as message schedule. BLAKE is claimed to achieve full diffusion after 2 rounds. However, message words can be controlled on the first several founds. By exploiting properties of message permutation, we can attack 2.5 reduced rounds. The results do not threat the security claimed in the specification.

Zeng and Keitel proposed an arbitrated quantum signature scheme In 2002. Recently, Curty and Lütkenhaus pointed out that the protocol is not operationally specified. In a reply, Zeng gave more details of the scheme. The author also claimed that the scheme is suitable for unknown messages. In this letter, we remark that the invented scenario in the original scheme is artificial. This is because its security entirely depends on the presence of a trustworthy arbitrator. Moreover, the claim that the original scheme is suitable for unknown messages is not sound.

In this paper, we present a classification of classes of elliptic curves defined over cubic extension of finite fields with odd characteristics, which have coverings over the finite fields therefore can be attacked by the GHS attack. We then show the density of these weak curves with hyperelliptic and non-hyperelliptic coverings respectively. In particular, we shown for elliptic curves defined in Legendre forms, about half of them are weak.

Signcryption is a cryptographic primitive that provides authentication and confidentiality simultaneously in a single logical step. It is often required that multiple senders have to signcrypt a single message to a certain receiver. Obviously, it is inefficient to signcrypt the messages separately. An efficient alternative is to go for multi-signcryption. The concept of multi-signcryption is similar to that of multi-signatures with the added property - confidentiality. Recently, Jianhong et al. proposed an identity based multi-signcryption scheme. They claimed that their scheme is secure against adaptive chosen ciphertext attack and it is existentially unforgeable. In this paper, we show that their scheme is not secure against chosen plaintext attack and is existentially forgeable, we also provide a fix for the scheme and prove formally that the improved scheme is secure against both adaptive chosen ciphertext attack and existential forgery.

EDON-$\mathcal{R}$ is one of the candidate hash functions for the ongoing NIST competition for the next cryptographic hash standard called SHA-3. Its construction is based on algebraic properties of non-commutative and non-associative quasigroups of orders $2^{256}$ and $2^{512}$. In this paper we are giving some of our results in investigation of the randomness and regularity of reduced EDON-$\mathcal{R}$ compression functions over quasigroups of order $2^{8}$ and $2^{16}$. Our experiments show that the Bellare-Khono balance of EDON-$\mathcal{R}$ compression function is high. Actually, for the reduced EDON-$\mathcal{R}$ with quasigroups of order $2^8$ we show that the compression function is perfectly balanced, while with quasigroups of order $2^{16}$ the Belare-Khono balance is $\mu(R_{16}) = 0.99985$.

Let $p$ be a prime and $\F_p$ the finite field with $p$ elements. We show how, when given an irreducible bivariate polynomial $f \in \F_p[X,Y]$ and approximations to $(v_0,v_1) \in \F_p^2$ such that $f(v_0,v_1)=0 $, one can recover $(v_0,v_1)$ efficiently, if the approximations are good enough. This result has been motivated by the predictability problem for non-linear pseudorandom number generators and, other potential applications to cryptography.

This specification describes a modification of a candidate for SHA-3, named MeshHash. The first version had a flaw in it, it was possible to mount a second preimage attack [Tho08]. So MeshHash has not fulfilled the requirements for SHA-3 anymore and hence was conceded broken. Furthermore there was a bug in the reference implementation: The macro for rotation of a word computed an undefined value if it should rotate a word by 0 bit. But since the flaw can be easily fixed, which was already implemented in a preliminary version, it seems to be a good idea to publish MeshHash2 as a patch and see if it might be useful for further research or even usage. The patch uses a feedback, which increases the memory usage, but doesn't give more security against a straight forward collision attack, which was the reason it has been dropped from the preliminary version of MeshHash. This specification is the patched version of MeshHash, named MashHash2. It is a very flexible but conservative design with primarily security in mind and only secondarily speed. But it achieves about the same speed as the SHA-2 family and security up to 16320 bit. It can also be used in a keyed version as PRF or PRG and hence build a stream-cipher of it.

MDPL is a masked logic style that unites principles of dual-rail pre-charge as well as masked logic to achieve resistance against differential power analysis attacks. MDPL has received much attention and numerous papers discussing the security provided by MDPL as well as its weaknesses have been published. As a matter of fact, most of these papers are purely theoretical or provide evidence based on simulations. At present, it is unclear to what extent these concepts affect the security provided by MDPL in practice. We fill this gap and present results of an extensive case study of attacks against an MDPL prototype chip. We demonstrate successful DPA attacks and show that MDPL implementations, resistant to standard DPA attacks, can be broken in practice. Further, we show that the underlying concept of the folding attack, i.e. analysis of probability densities, indeed exposes MDPL's greatest weakness: the masking renders the circuit more vulnerable to attacks than a circuit with a fixed mask. In addition, our analysis leads to novel insights into the power consumption properties of MDPL in real silicon.

A broadcast protocol allows a sender to distribute a message through a point-to-point network to a set of parties, such that (i) all parties receive the same message, even if the sender is corrupted, and (ii) this is the sender's message, if he is honest. Broadcast protocols satisfying these properties are known to exist if and only if $t<n/3$, where $n$ denotes the total number of parties, and $t$ denotes the maximal number of corruptions. When a setup allowing signatures is available to the parties, then such protocols exist even for $t<n$. Broadcast is the probably most fundamental primitive in distributed cryptography, and is used in almost any cryptographic (multi-party) protocol. However, a broadcast protocol ``only'' satisfying the above properties might be insecure when being used in the context of another protocol. In order to be safely usable within other protocols, a broadcast protocol must satisfy a simulation-based security notion, which is secure under composition. In this work, we show that most broadcast protocols in the literature do not satisfy a (natural) simulation-based security notion. We do not know of any broadcast protocol which could be securely invoked in a multi-party computation protocol in the secure-channels model. The problem is that existing protocols for broadcast do not preserve the secrecy of the message while being broadcasted, and in particular allow the adversary to corrupt the sender (and change the message), depending on the message being broadcasted. For example, when every party should broadcast a random bit, the adversary could corrupt those parties that want to broadcast 0, and make them broadcast 1. More concretely, we show that simulatable broadcast in a model with secure channels is possible if and only if $t<n/3$, respectively $t \le n/2$ when a signature setup is available. The positive results are proven by constructing secure broadcast protocols.

Hash functions are widely used in Cryptography, and hardware implementations of hash functions are of interest in a variety of contexts such as speeding up the computations of a network server or providing authentication in small electronic devices such as RFID tags. Provably secure hash functions, the security of which relies on the hardness of a mathematical problem, are particularly appealing for security, but they used to be too inefficient in practice. In this paper, we study the efficiency in hardware of ZT', a provably secure hash function based on the Zémor-Tillich hash function. We consider three kinds of implementations targeting a high throughput and a low area in different ways. We first present a high-speed implementation of ZT' on FPGA that is nearly half as efficient as state-of-the-art SHA implementations in terms of throughput per area. We then focus on area reduction and present an ASIC implementation of ZT' with much smaller area costs than SHA-1 and even than SQUASH, which was specially designed for low-cost RFID tags. Between these two extreme implementations, we show that the throughput and area can be traded with a lot of flexibility. Finally, we show that the inherent parallelism of ZT' makes it particularly suitable for applications requiring high speed hashing of very long messages. Our work, together with existing reasonably efficient software implementations, shows that this variant of the Zémor-Tillich hash function is in fact very practical for a wide range of applications, while having a security related to the hardness of a mathematical problem and significant additional advantages such as scalability and parallelism.

Security devices are vulnerable to side-channel attacks that perform statistical analysis on data leaked from cryptographic computations. Higher-order (HO) attacks are a powerful approach to break protected implementations. They inherently demand multivariate statistics because multiple aspects of signals have to be analyzed jointly. However, all published works on HO attacks follow the approach to first apply a pre-processing function to map the multivariate problem to a univariate problem and then to apply established $1^{st}$ order techniques. We propose a novel and different approach to HO attacks, Multivariate Mutual Information Analysis (MMIA), that allows to directly evaluate joint statistics without pre-processing. While this approach can benefit from a good power model, it also works without an assumption. A thorough empirical evaluation of MMIA and established HO attacks confirms the overwhelming advantage of the new approach: MMIA is more efficient and less affected by noise. Most important and opposed to all published approaches, MMIA's measurement cost grows sub-exponentially with the attack order. As a consequence, the security provided by the masking countermeasure needs to be reconsidered as $3^{rd}$ and higher order attacks become very practical.

We analyze the relation between induction, co-induction and the presence of encryption cycles in the context of computationally sound symbolic equivalence of cryptographic expressions. Our main finding is that the use of co-induction in the symbolic definition of the adversarial knowledge allows to prove unconditional soundness results that do not require syntactic restrictions, like the absence of encryption cycles. Encryption cycles are relevant only to the extent that the key recovery function associated to acyclic expressions can be shown to have a unique fix-point. So, when a cryptographic expression has no encryption cycles, the inductive (least fix-point) and co-inductive (greatest fix-point) security definitions produce the same results, and the computational soundness of the inductive definitions for acyclic expressions follows as a special case of the soundness of the co-inductive definition.

We describe a new explicit function that given an elliptic curve $E$ defined over $\FF_{p^n}$, maps elements of $\FF_{p^n}$ into $E$ in \emph{deterministic} polynomial time and in a constant number of operations over $\FF_{p^n}$. The function requires to compute a cube root. As an application we show how to hash \emph{deterministically} into an elliptic curve.

In this paper, we give a security proof for Abreast-DM in terms of collision resistance and preimage resistance. As old as Tandem-DM, the compression function Abreast-DM is one of the most well-known constructions for double block length compression functions. The bounds on the number of queries for collision resistance and preimage resistance are given by O(2^n). Based on a novel technique using query-response cycles, our security proof is simpler than those for MDC-2 and Tandem-DM. We also present a wide class of Abreast-DM variants that enjoy a birthday-type security guarantee with a simple proof.

In this paper, we present the pseudo-collision, pseudo-second-preimage and pseudo-preimage attacks on the SHA-3 candidate algorithm Luffa. The pseudo-collisions and pseudo-second-preimages can be found easily by computing the inverse of the message injection function at the beginning of Luffa. We explain in details the pseudo-preimage attacks. For Luffa-224/256, given the hash value, only 2 iteration computations are needed to get a pseudo-preimage. For Luffa-384, finding a pseudo-preimage needs about $2^{64}$ iteration computations with $2^{67}$ bytes memory by the extended generalized birthday attack. For Luffa-512, the complexity is $2^{128}$ iteration computations with $2^{132}$ bytes memory. It is noted that, we can find the pseudo-collision pairs and the pseudo-second images only changing a few different bits of initial values. That is directly converted to the forgery attack on NMAC in related key cases.

Since the first feasible collision differential was given for MD5 in 2004 by Wang et al, a lot of work has been concentrated on how to improve it, but the researches on how to select weak input differences for MD5 collision attack are only sporadically scattered in literature. This paper focuses on a reasonable selection of weak input differences for MD5 collision attack, tries to answer some questions such as, what techniques can be applied satisfying bit conditions? which step in the second round can be the latest to apply a search on free bits without violating previously satisfied conditions? what is the optimal characterization of feasible collision differential propagation for MD5, by which we can find more weak input differences? is there any collision differentials that exceed Wang et al's by some practical criteria? In this paper, a divide-and-conquer strategy is introduced with an optimal scheme of grouping the 64 steps of operation into five stages of independent condition fulfillment, and a feasible collision differential propagation is optimally characterized as a guide to select those 1~3-bit weak input differences, with their computational costs estimated. As a result, hundreds of thousands of weak input differences have been found, quite a number of which are superior to Wang et al's, for example, a 2-bit collision differential is able to find a collision within 2^10 MD5 compressions, a 1-MSB differential collision attack on MD5 is developed with a time complexity of 2^20.96 MD5 compressions, and a practical 1-block collision attack on MD5 is found to be possible. This paper also provides a rich resource of colliding messages with different weak input differences, therefore much greatly increase the probability of finding a second MD5 pre-image for an arbitrarily given message.

In [Solving Multiple Right Hand Sides linear equations. Designs, Codes and Cryptography, 49:147–160, 2008] Raddum and Semaev propose a technique to solve systems of polynomial equations over GF(2) as occurring in algebraic attacks on block ciphers. This approach is known as MRHS, and we present a special purpose architecture to implement MRHS in a dedicated hardware device. Our preliminary performance analysis of this Parallel Elimination Technique Supporting Nice Algebraic Key Elimination shows that the use of ASICs seems to enable significant performance gains over a software implementation of MRHS. The main parts of the proposed architecture are scalable, the limiting factor being mainly the available bandwidth for interchip communication. Our focus is on a design choice that can be implemented within the limits of available fab technology. The proposed design can be expected to offer a running time improvement in the order of several magnitudes over a software implementation. We do not make any claims about the practical feasibility of an attack against AES-128 with our design, as we do not see the necessary theoretical tools to be available: deriving reliable running time estimates for an algebraic attack with MRHS when being applied to a full-round version of AES-128 is still an open problem.

The Boneh-Boyen signature scheme is a pairing based short signature scheme which is provably secure in the standard model under the $q$-Strong Diffie-Hellman assumption. In this paper, we prove the converse of this statement, and show that forging Boneh-Boyen signatures is actually equivalent to solving the $q$-Strong Diffie-Hellman problem. Using this equivalence, we exhibit an algorithm which, on the vast majority of pairing-friendly curves, recovers Boneh-Boyen private keys in $O(p^{\frac{2}{5}+\varepsilon})$ time, using $O(p^{\frac{1}{5}+\varepsilon})$ signature queries. We present implementation results comparing the performance of our algorithm and traditional discrete logarithm algorithms such as Pollard's lambda algorithm and Pollard's rho algorithm. We also discuss some possible countermeasures and strategies for mitigating the impact of these findings.

A leakage-resilient cryptosystem remains secure even if arbitrary, but bounded, information about the secret key (or possibly other internal state information) is leaked to an adversary. Denote the length of the secret key by $n$. We show a signature scheme tolerating (optimal) leakage of up to $n-n^\epsilon$ bits of information about the secret key, and a more efficient one-time signature scheme that tolerates leakage of $(\frac{1}{4}-\epsilon) \cdot n$ bits of information about the signer's entire state. The latter construction extends to give a leakage-resilient $t$-time signature scheme. All these constructions are in the standard model under general assumptions.

We introduce a formal model for certificateless authenticated key exchange (CL-AKE) protocols. Contrary to what might be expected, we show that the natural combination of an ID-based AKE protocol with a public key based AKE protocol cannot provide strong security. We provide the first one-round CL-AKE scheme proven secure in the random oracle model. We introduce two variants of the Diffie-Hellman trapdoor introduced by \cite{DBLP:conf/eurocrypt/CashKS08}. The proposed key agreement scheme is secure as long as each party has at least one uncompromised secret. Thus, our scheme is secure even if the key generation centre learns the ephemeral secrets of both parties.

Cube testers are a generic class of methods for building disstinguishers, based on cube attacks and on algebraic property-testers. In this paper, we report on an efficient FPGA implementation of cube testers on the stream cipher Grain-128. Our best result (a distinguisher on Grain-128 reduced to 237 rounds, out of 256) was achieved after a computation involving 2^54 clockings of Grain-128, with a 256×32 parallelization. An extrapolation of our results with standard methods suggests the possibility of a distinguishing attack on the full Grain-128 in time 2^83, which is well below the 2^128 complexity of exhaustive search. We also describe the method used for finding good cubes (a simple evolutionary algorithm), and report preliminary results on Grain-v1 obtained with a bitsliced C implementation.

This paper considers the construction and analysis of pseudo-random functions (PRFs) with specific reference to modes of operations of a block cipher. In the context of message authentication codes (MACs), earlier independent work by Bernstein and Vaudenay show how to reduce the analysis of relevant PRFs to some probability calculations. In the first part of the paper, we revisit this result and use it to prove a general result on constructions which use a PRF with a ``small'' domain to build a PRF with a ``large'' domain. This result is used to analyse two new parallelizable PRFs which are suitable for use as MAC schemes. The first scheme, called {\iPMAC}, is based on a block cipher and improves upon the well-known PMAC algorithm. The improvements consist in faster masking operations and the removal of a design stage discrete logarithm computation. The second scheme, called {\VPMAC}, uses a keyed compression function rather than a block cipher. The only previously known compression function based parallelizable PRF is called the protected counter sum (PCS) and is due to Bernstein. {\VPMAC} improves upon PCS by requiring lesser number of calls to the compression function. The second part of the paper takes a new look at the construction and analysis of modes of operations for authenticated encryption (AE) and for authenticated encryption with associated data (AEAD). Usually, the most complicated part in the security analysis of such modes is the analysis of authentication security. Previous work by Liskov, Rivest and Wagner and later Rogaway had suggested that this analysis is simplified by using a primitive called a tweakable block cipher (TBC). In contrast, we take a direct approach. We prove a general result which shows that the authentication security of an AE scheme can be proved from the privacy of the scheme and by showing a certain associated function to be a PRF. Two new AE schemes \sym{PAE} and \sym{PAE}-1 are described and analysed using this approach. In particular, it is shown that the authentication security of \sym{PAE} follows easily from the security of {\iPMAC}. As a result, no separate extensive analysis of the authentication security of \sym{PAE} is required. An AEAD scheme can be obtained by combining an AE scheme and an authentication scheme and it has been suggested earlier that a TBC based approach simplifies the analysis. Again, in contrast to the TBC based approach, we take a direct approach based on a simple masking strategy. Our idea uses double encryption of a fixed string and achieves the same effect of mask separation as in the TBC based approach. Using this idea, two new AEAD schemes \sym{PAEAD} and \sym{PAEAD}-1 are described. An important application of AEAD schemes is in the encryption of IP packets. The new schemes offer certain advantages over previously well known schemes such as the offset codebook (OCB) mode. These improvements include providing a wider variety of easily reconfigurable family of schemes, a small speed-up, a smaller size decryption algorithm for hardware implementation and uniform processing of only full-block messages.

A new construction of block cipher based tweakable enciphering schemes (TES) is described. The major improvement over existing TES is that the construction uses only the encryption function of the underlying block cipher. Consequently, this leads to substantial savings in the size of hardware implementation of TES applications such as disk encryption. This improvement is achieved without loss in efficiency of encryption and decryption compared to the best previously known schemes.

We revisit the problem of constructing a protocol for performing authenticated encryption with associated data (AEAD). A technique is described which combines a collision resistant hash function with a protocol for authenticated encryption (AE). The technique is both simple and generic and does not require any additional key material beyond that of the AE protocol. Concrete instantiations are shown where a 256-bit hash function is combined with some known single-pass AE protocols employing either 128-bit or 256-bit block ciphers. This results in possible efficiency improvement in the processing of the header.

We address one of the foundational problems in cryptography: the bias of coin-flipping protocols. Coin-flipping protocols allow mutually distrustful parties to generate a common unbiased random bit, guaranteeing that even if one of the parties is malicious, it cannot significantly bias the output of the honest party. A classical result by Cleve [STOC '86] showed that for any two-party $r$-round coin-flipping protocol there exists an efficient adversary that can bias the output of the honest party by $\Omega(1/r)$. However, the best previously known protocol only guarantees $O(1/\sqrt{r})$ bias, and the question of whether Cleve's bound is tight has remained open for more than twenty years. In this paper we establish the optimal trade-off between the round complexity and the bias of two-party coin-flipping protocols. Under standard assumptions (the existence of oblivious transfer), we show that Cleve's lower bound is tight: we construct an $r$-round protocol with bias $O(1/r)$.

In this note we first characterize the class of Montgomery curves with B=1 by the simplicity of their transformation into short Weierstrass form and explicitly determine their torsion points of order 2 and 4. We then consider two ``verifiably random'' elliptic curves in the SECG standards and show that they are B=1 Montgomery curves that can also be simply transformed into Edwards form over their ground fields.

We propose a lightweight RFID authentication protocol that supports forward and backward security. The only cryptographic mechanism that this protocol uses is a pseudo-random number generator (PRNG) that is shared with the backend Server. Authentication is achieved by exchanging a few numbers (3 or 5) drawn from the PRNG. The protocol is optimistic with constant lookup time, and can be easily adapted to prevent online man-in-the-middle relay attacks. Security is proven in the UC security framework.

Efficient zero-knowledge proofs of knowledge (ZK-PoK) are basic building blocks of many practical cryptographic applications such as identification schemes, group signatures, and secure multiparty computation. Currently, first applications that critically rely on ZK-PoKs are being deployed in the real world. The most prominent example is Direct Anonymous Attestation (DAA), which was adopted by the Trusted Computing Group (TCG) and implemented as one of the functionalities of the cryptographic Trusted Platform Module (TPM) chip. Implementing systems using ZK-PoK turns out to be challenging, since ZK-PoK are, loosely speaking, significantly more complex than standard crypto primitives, such as encryption and signature schemes. As a result, implementation cycles of ZK-PoK are time-consuming and error-prone, in particular for developers with minor or no cryptographic skills. In this paper we report on our ongoing and future research vision with the goal to bring ZK-PoK to practice by making them accessible to crypto and security engineers. To this end we are developing compilers and related tools that support and partially automate the design, implementation, verification and secure implementation of ZK-PoK protocols.