International Association
for Cryptologic Research

Identity-based encryption (IBE) has been regarded as an attractive alternative to more conventional certificate-based public key systems.

It has recently attracted not only considerable research from the academic community, but also interest from the industry and standardization bodies. However, while key revocation is a fundamental requirement to any public key systems, not much work has been done in the identity-based setting. In this paper, we continue the study of revocable IBE (RIBE) initiated by Boldyreva, Goyal, and Kumar. Their proposal of a selective secure RIBE scheme, and a subsequent construction by Libert and Vergnaud in a stronger adaptive security model are based on a binary tree approach, such that their key update size is logarithmic in the number of users. We ask the question of whether or not the key update size could be further reduced by using a cryptographic accumulator. We show that, indeed, the key update material can be made constant with some small amount of auxiliary information, through a novel combination of the Lewko and Waters IBE scheme and the Camenisch, Kohlweiss, and Soriente pairing-based dynamic accumulator.

Existing work on \"rational cryptographic protocols\" treats each party (or coalition of parties) running the protocol as a selfish agent trying to maximize its utility. In this work we propose a fundamentally different approach that is better suited to modeling a protocol under attack from an external entity. Specifically, we consider a two-party game between an protocol designer and an external attacker. The goal of the attacker is to break security properties such as correctness or privacy, possibly by corrupting protocol participants; the goal of the protocol designer is to prevent the attacker from succeeding.

We lay the theoretical groundwork for a study of cryptographic protocol design in this setting by providing a methodology for defining the problem within the traditional simulation paradigm. Our framework provides ways of reasoning about important cryptographic concepts (e.g., adaptive corruptions or attacks on communication resources) not handled by previous game-theoretic treatments of cryptography. We also prove composition theorems that--for the first time--provide a sound way to design rational protocols assuming \"ideal communication resources\" (such as broadcast or authenticated channels) and then instantiate these resources using standard cryptographic tools.

Finally, we investigate the problem of secure function evaluation in our framework, where the attacker has to pay for each party it corrupts. Our results demonstrate how knowledge of the attacker\'s incentives can be used to

In 2007, Camenisch, Neven and Shelat proposed an adaptive oblivious transfer (OT) in which a sender has $N$ messages, of which a receiver can adaptively choose to receive $k$ one-after-the-other.

In this paper, we show that the scheme has a drawback that the sender can only serve a single receiver only once. The drawback results from the deterministic encryption used. To fix it, we suggest to replace the deterministic encryption with a probabilistic encryption. The OT scheme adopts the paradigm of ``encryption and proof of knowledge\" in order to force the sender to keep the consistency of the transferred messages. We remark that the paradigm is unnecessary. In most reasonable applications of OT, the transferred messages must be recognizable for the receiver or the sender is willing to disclose some messages to the receiver. This property has been explicitly specified in the earlier works by Rabin, Even, Goldreich and Lempel.

We construct an efficient information-theoretically non-mall\\-eable code in the split-state model for one-bit messages. Non-malleable codes were introduced recently by Dziembowski, Pietrzak and Wichs (ICS 2010), as a general tool for storing messages securely on hardware that can be subject to tampering attacks. Informally, a code $(Enc : \\cal M \\rightarrow \\cal L \\times \\cal R, Dec : \\cal L \\times \\cal R \\rightarrow \\cal M)$ is {\\em non-malleable in the split-state model} if any adversary, by manipulating {\\em independently} $L$ and $R$ (where $(L,R)$ is an encoding of some message $M$), cannot obtain an encoding of a message $M\'$ that is not equal to $M$ but is ``related\'\' $M$ in some way. Until now it was unknown how to construct an information-theoretically secure code with such a property, even for $\\cal M = \\{0,1\\}$. Our construction solves this problem. Additionally, it is leakage-resilient, and the amount of leakage that we can tolerate can be an arbitrary fraction $\\xi < {1}/{4}$ of the length of the codeword. Our code is based on the inner-product two-source extractor, but in general it can be instantiated by any two-source extractor that has large output and has the property of being {\\em flexible}, which is a new notion that we define.

We also show that the non-malleable codes for one-bit messages have an equivalent, perhaps simpler characterization, namely such codes can be defined as follows: if $M$ is chosen uniformly from $\\{0,1\\}$ then the probability (in the experiment described above) that the output message $M\'$ is not equal to $M$ can be at most $1/2 + \\epsilon$.

We revisit the question of whether cryptographic protocols can replace correlated equilibria mediators in two-player strategic games. This problem was first addressed by Dodis, Halevi and Rabin (CRYPTO 2000), who suggested replacing the mediator with a secure protocol and proved that their solution is stable in the Nash equilibrium (NE) sense, provided that the players are computationally bounded.

We show that there exist two-player games for which no cryptographic protocol can implement the mediator in a sequentially rational way; that is, without introducing empty threats. This explains why all solutions so far were either sequentially unstable, or were restricted to a limited class of correlated equilibria (specifically, those that do not dominate any NE, and hence playing them does not offer a clear advantage over playing any NE).

In the context of computational NE, we classify necessary and sufficient cryptographic assumptions for implementing a mediator that allows to achieve a given utility profile of a correlated equilibrium. The picture that emerges is somewhat different than the one arising in semi-honest secure two-party computation. Specifically, while in the latter case every functionality is either \"complete\" (i.e., implies Oblivious Transfer) or \"trivial\" (i.e., can be securely computed unconditionally), in the former there exist some \"intermediate\" utility profiles whose implementation is equivalent to the existence of one-way functions.

We show that the class of polynomial-size branching programs can be obfuscated according to a virtual black-box notion akin to that of Barak et al. [Crypto 01], in an idealized black-box group model

over pseudo-free groups. This class is known to lie between $NC^1$ and $P$ and includes most interesting cryptographic algorithms. The construction is rather simple and is based on Kilian\'s randomization

technique for Barrington\'s branching programs.

The black-box group model over pseudo-free groups is a strong idealization. In particular, in a pseudo-free group, the group operation can be efficiently performed, while finding surprising relations between group elements is intractable.

%inverses or linking between different representations of the same group element are infeasible. A black-box representation of the group provides an ideal interface which permits prescribed group operations, and nothing else. Still, the algebraic structure and security requirements appear natural and potentially realizable. They are also unrelated to the specific function to be obfuscated.

Our modeling should be compared with the recent breakthrough obfuscation scheme of Garg et al. [FOCS 2013]: While the high level structure is similar, some important details differ. It should be stressed however that, unlike Garg et al., we do not provide a candidate concrete instantiation of our abstract structure.

The Trusted Platform Module (TPM) is designed to enable trustworthy computation and communication over open networks. The TPM provides a way to store cryptographic keys and other sensitive values in its shielded memory and act as \\emph{Root of Trust for Storage} (RTS). The TPM interacts with applications via a predefined set of commands (an API). In this paper, we give an abstraction model for the TPM 2.0 specification concentrating on Protected Storage part. With identification and formalization of their secrecy properties, we devise a type system with asymmetric cryptographic primitives to statically enforce and prove their security.

After a series of works on RC4 cryptanalysis in last few years (published in flagship cryptology conferences and journals), the most significant (and also very recent) attack on the cipher has been the discovery of vulnerabilities in the SSL/TLS protocol, by AlFardan, Bernstein, Paterson, Poettering and Schuldt. They ran extensive computations to identify significant short-term single-byte keystream biases of RC4, and utilized that knowledge in the attack. The biases identified by AlFardan et al. consist of earlier known biases of RC4, as well as some newly discovered ones.

In this paper, we attempt at proving the new, unproved or partially proved biases amongst the above-mentioned ones. The theoretical proofs of these biases not only assert a scientific justification, but also discover intricate patterns and operations of the cipher associated with these biases. For example, while attempting the proof of a bias of the first output byte towards 129, we observe that this bias occurs prominently only for certain lengths of the secret key of RC4. In addition, our findings reveal that this bias may be related to the old and unsolved problem of ``anomalies\'\' in the distribution of the state array after the Key Scheduling Algorithm. In this connection, we prove the anomaly in $S_0[128] = 127$, a problem open for more than a decade.

Other than proving the new biases, we also complete the proof for the extended keylength dependent biases in RC4, a problem attempted and partially solved by Isobe, Ohigashi, Watanabe and Morii in FSE 2013. Our new proofs and observations in this paper, along with the connection to the older results, provide a comprehensive view on the state-of-the-art literature in RC4 cryptanalysis.

We present an efficient algorithm that can distinguish the keystream of WPA from that of a generic instance of RC4 with a packet complexity of $O(N^2)$, where $N$ denotes the size of the internal permutation of RC4. In practice, our distinguisher requires approximately $2^{19}$ packets; thus making it the best known distinguisher of WPA to date. This is a significantly improved distinguisher than the previous WPA distinguisher identified by Sepehrdad, Vaudenay and Vuagnoux in Eurocrypt 2011, which requires more than $2^{40}$ packets in practice. The motivation of our distinguisher arises from the recent observations on WPA by AlFardan, Bernstein, Paterson, Poettering and Schuldt, and this work puts forward an example how an experimental bias may lead to an efficient theoretical distinguisher.

Broadcast encryption is conventionally formalized as broadcast encapsulation in which, instead of a cipher-

text, a session key is produced, which is required to be indistinguishable from random. Such a scheme can

provide public encryption functionality in combination with a symmetric encryption through the hybrid en-

cryption paradigm. The Boneh-Gentry-Waters scheme of 2005 proposed a broadcast scheme with constant-size

ciphertext. It is one of the most efficient broadcast encryption schemes regarding overhead size. In this work we

consider the improved scheme of Phan-Pointcheval-Shahandashi-Ste

er [PPSS12] which provides an adaptive

CCA broadcast encryption scheme. These two schemes may be tweaked to use bilinear pairings[DGS].

This document details our choices for the implementation of the PPSS scheme. We provide a complete golden sequence

of the protocol with efficient pairings (Tate, Ate and Optimal Ate). We target a 128-bit security

level, hence we use a BN-curve [BN06]. The aim of this work is to contribute to the use and the standardization of

PPSS scheme and pairings in concrete systems.

Mechanisms for secure mobile communication can be enablers for novel applications in the area of cooperative work. In this context, this article exemplarily investigates an emergency management setting. An efficient support of emergency communication is of high practical importance, but has specific challenges: unpredictable local crisis situations harden the establishment of communication structures, legal requirements dictate the use of end-to-end secure and documentable approaches, while end users demand user-friendliness and privacy protection.

Dealing with these challenges, the contribution of this article is two-fold. Firstly, together with emergency practioners, we follow a participatory design approach. We define security requirements, patterns for efficient communication and derive a design proposal. Secondly, we devise a novel approach to multilaterally end-to-end secure, user-friendly attribute-based messaging for one-to-many communication. It builds on a hybrid encryption technique, which efficiently combines ciphertext-policy attribute-based encryption, location-based encryption and symmetric encryption. The hybrid encryption approach supports dynamic location attributes as user-friendly selectors for targeted messaging with dynamic groups of mobile and anonymous receivers. The achieved security of the approach and concrete application scenarios are discussed.

Quantum Readout PUFs (QR-PUFs) have been proposed as

a technique for remote authentication of objects. The security

is based on basic quantum information theoretic principles

and the assumption that the adversary cannot efficiently implement arbitrary unitary transformations.

We analyze the security of QR-PUF schemes in the case where

each challenge consists of precisely $n$ quanta and the dimension $K$ of the Hilbert space is larger than $n^2$.

We consider a class of attacks where the adversary first tries to learn as much as possible about the challenge and then bases his response on his estimate of the challenge.

For this class of attacks we derive an upper bound on the adversary\'s success probability as a function of $K$ and~$n$.

We put forward a new approach for the design of efficient multiparty protocols:

1. Design a protocol for a small number of parties (say, 3 or 4)

which achieves security against a single corrupted party. Such

protocols are typically easy to construct as they may employ

techniques that do not scale well with the number of corrupted

parties.

2. Recursively compose with itself to obtain an efficient n-party

protocol which achieves security against a constant fraction of

corrupted parties.

The second step of our approach combines the player emulation

technique of Hirt and Maurer (J. Cryptology, 2000) with

constructions of logarithmic-depth formulae which compute

threshold functions using only constant fan-in threshold gates.

Using this approach, we simplify and improve on previous results

in cryptography and distributed computing. In particular:

- We provide conceptually simple constructions of efficient

protocols for Secure Multiparty Computation (MPC) in the

presence of an honest majority, as well as broadcast protocols

from point-to-point channels and a 2-cast primitive.

- We obtain new results on MPC over blackbox groups and other

algebraic structures.

The above results rely on the following complexity-theoretic

contributions, which may be of independent interest:

- We show that for every integers j,k such that m = (k-1)/(j-1)

is an integer, there is an explicit (poly(n)-time) construction

of a logarithmic-depth formula which computes a good

approximation of an (n/m)-out-of-n threshold function using only

j-out-of-k threshold gates and no constants.

- For the special case of n-bit majority from 3-bit majority

gates, a non-explicit construction follows from the work of

Valiant (J. Algorithms, 1984). For this special case, we provide

an explicit construction with a better approximation than for the

general threshold case, and also an exact explicit construction

based on standard complexity-theoretic or cryptographic

assumptions.

This short note describes a key-recovery attack against a multivariate quadratic cryptosystem proposed by Huang, Liu, and Yang (PKC 2012). Our attack is running lattice-basis reduction algorithms on a lattice constructed from the keys in the cryptosystem. The attack takes less than 20 minutes for the proposed parameter sets which are expected to be 80-bit and 128-bit security.

Proving bounds on the expected differential probability (EDP) of a characteristic over all keys has been a popular technique of arguing security for both block ciphers and hash functions. In fact, to a large extent, it was the clear formulation and elegant deployment of this very principle that helped Rijndael win the AES competition. Moreover, most SHA-3 finalists have come with explicit upper bounds on the EDP of a characteristic as a major part of their design rationale. However, despite the pervasiveness of this design approach, there is no understanding of what such bounds actually mean for the security of a primitive once a key is fixed --- an essential security question in practice.

In this paper, we aim to bridge this fundamental gap. Our main result is a quantitative connection between a bound on the EDP of differential characteristics and the highest

number of input pairs that actually satisfy a characteristic for a fixed key. This is particularly important for the design of permutation-based hash functions such as sponge functions, where the EDP value itself is not informative for the absence of rekeying. We apply our theoretical result to revisit the security arguments of some prominent recent block ciphers and hash functions. For most of those, we have good news: a characteristic is followed by a small number of pairs only. For Keccak, though, currently much more rounds would be needed for our technique to guarantee any reasonable maximum number of pairs.

Thus, our work --- for the first time --- sheds light on the fixed-key differential behaviour of block ciphers in general and substitution-permutation networks in particular which has been a long-standing fundamental problem in symmetric-key cryptography.

Coppersmith described at Eurocrypt 96 a polynomial-time algorithm for finding small roots of univariate modular equations, based on lattice reduction. In this paper we describe the first improvement of the asymptotic complexity of Coppersmith\'s algorithm. Our method consists in taking advantage of Coppersmith\'s matrix structure, in order to apply LLL algorithm on a matrix whose elements are smaller than those of Coppersmith\'s original matrix. Using the $L^2$ algorithm, the asymptotic complexity of our method is $O(\\log^{6+\\epsilon} N)$ for any $\\epsilon > 0$, instead of $O(\\log^{8+\\epsilon} N)$ previously. Furthermore, we devise a method that allows to speed up the exhaustive search which is usually performed to reach Coppersmith\'s theoretical bound. Our approach takes advantage of the LLL performed to test one guess, to reduce complexity of the LLL performed for the next guess. Experimental results confirm that it leads to a considerable performance improvement.

In past years a body of data integrity checking techniques have been proposed for securing cloud data services. Most of these work assume that only the data owner can modify cloud-stored data. Recently a few attempts started considering more realistic scenarios by allowing multiple cloud users to modify data with integrity assurance. However, these attempts are still far from practical due to the tremendous computational cost on cloud users. Moreover, collusion between misbehaving cloud servers and revoked users is not considered. This paper proposes a novel data integrity checking scheme characterized by multi-user modification, collusion resistance and a constant computational cost of integrity checking for cloud users, thanks to our novel design of polynomial-based authentication tags and proxy tag update techniques. Our scheme also supports public checking and efficient user revocation and is provably secure. Numerical analysis and extensive experimental results show the efficiency and scalability of our proposed scheme.

Searching an object from a large set is a tedious task. Radio Frequency IDentification (RFID) technology helps us to search the desired object efficiently. In this technology, a small chip called RFID tag, that contains the identification information about an object is attached to the same object. In general, a set of objects are attached with RFID tags. To find out a particular object preserving the possible security requirements, the RFID reader requests the tag in desired object to respond with its encrypted identification information. Since there is a response only from the tag in desired object the adversary gets the knowledge of existence of the desired object. Fake response from tag in undesired objects may fool the adversary. However, computation for fake responses is

an overhead. In this paper, we propose a search technique which has a negligible amount of computation for fake responses. Multiple tags in the same object increases the detection probability and also the probability of success in search process. Our aim is to search a particular object efficiently preserving the possible security requirements amid various resource limitations in low-cost RFID tag.

In Radio Frequency Identification (RFID) technology, an adversary

may access classified information about an object tagged with RFID tag. Therefore, authentication is a necessary requirement. Use of multiple tags in an object increases the detection probability and simultaneously ensures availability of multiple resources in the form of memory and computability. Authentication process in multi-tag arrangement may increase the traffic between reader and object and/or decrease the detection probability. Therefore the challenge is to keep intact the detection probability without increasing the traffic. Existence of multiple number of tags helps to distribute the authentication responsibility for an object among multiple number of tags. In this paper, we assume that an object is attached with multiple number of active tags and in each session a randomly selected tag is responsible for authentication process. The detection probability is intact since an active tag within the range of reader can be an inter-mediator.

The GHS attack is known as a method to map the discrete logarithm problem(DLP) in the Jacobian of a curve C_{0} defined over the d degree extension k_{d} of a finite field k to the DLP in the Jacobian of a new curve C over k which is a covering curve of C_{0}. Such curves C_{0}/k_{d} can be attacked by the GHS attack and index calculus algorithms. In this paper, we will classify all elliptic curves and hyperelliptic curves C_{0}/k_{d} of genus 2, 3 which possess (2,...,2) covering C/k of \\Bbb{P}^1 under the isogeny condition (i.e. g(C)=d \\cdot g(C_{0})) in odd characteristic case. Our main approach is analysis of ramification points and representation of the extension of Gal(k_{d}/k) acting on the covering group cov(C/\\Bbb{P}^1). Consequently, all explicit defining equations of such curves C_0/k_d and existential conditions of a model of C over k are provided.