Computer Security – ESORICS 2016

Direct-recording electronic (DRE) voting systems have been used in several countries including United States, India, and the Netherlands to name a few. A common flaw that was discovered by the security researchers was that the votes were stored sequentially according to the time they were cast, which allows an attacker to break the anonymity of the voters. Subsequent research pointed out the connection between vote storage and the privacy property history-independence. In a weakly history-independent data structure, every possible sequence of operations consistent with the current set of items is equally likely to have occurred. In a strongly history-independent data structure, items must be stored in a canonical way, i.e., for any set of items, there is only one possible memory representation. Strong history-independence implies weak history-independence but considerably constrains the design choices of the data structures. In this work, we present and analyze an efficient hash table data structure that simultaneously achieves the following properties:

To our knowledge, our hash table construction is the first data structure that combines history-independence and protection against a form of timing attacks.

Opaque constructs have developed into a commonly used primitive in obfuscation, watermarking, and tamper-proofing schemes. However, most prior work has based the resilience of these primitives on a poorly defined reduction to a known \(\mathcal {NP}\)-complete problem. There has been little scrutiny of the adversarial model and little discussion of how to generate instances that are always hard. In this paper, we offer what we believe to be the first complete algorithm for generating resilient opaque constructs against static analysis. We base their resilience on the complexity of 3SAT instances with cn clauses for \(c=6\) and n distinct variables. We draw on existing theoretical bounds to show that these instances always require exponential time to defeat under formal notions of resolution complexity.

This paper also explores in-depth the security of opaque constructs in real-world settings. We argue that the common theoretical model used in prior work (as well as our resilient opaque construction scheme) is too optimistic. It does not offer practical obfuscation against an adversary who tolerates some small false positive rate. We offer a heuristic-based attack to demonstrate this issue. Our results suggest that opaque constructs should be viewed with a high degree of skepticism until they can be proven secure under more useful theoretical models.

Password-protected secret sharing (PPSS) schemes allow a user to publicly share its high-entropy secret across different servers and to later recover it by interacting with some of these servers using only his password without requiring any authenticated data. In particular, this secret will remain safe as long as not too many servers get corrupted. However, servers are not always reliable and the communication can be altered. To address this issue, a robust PPSS should additionally guarantee that a user can recover his secret as long as enough servers provide correct answers, and these are received without alteration. In this paper, we propose new robust PPSS schemes which are significantly more efficient than the existing ones. Our contributions are two-fold: First, we propose a generic technique to build a Robust Gap Threshold Secret Sharing Scheme (RGTSSS) from some threshold secret sharing schemes. In the PPSS construction, this allows us to drop the verifiable property of Oblivious Pseudorandom Functions (OPRF); Then, we use this new approach to design two new robust PPSS schemes that are quite efficient, from two OPRFs. They are proven in the random-oracle model, just because our RGTSSS construction requires random non-malleable fingerprints, which is provided by an ideal hash function.

With the rise of practical Secure Multi-party Computation (MPC) protocols, compilers have been developed that create Boolean or Arithmetic circuits for MPC from functionality descriptions in a high-level language. Previous compilers focused on the creation of size-minimal circuits. However, many MPC protocols, such as GMW and SPDZ, have a round complexity that is dependent on the circuit’s depth. When deploying these protocols in real world network settings, with network latencies in the range of tens or hundreds of milliseconds, the round complexity quickly becomes a significant performance bottleneck.

In this work, we present ShallowCC, a compiler extension that creates depth minimized Boolean circuits from ANSI-C. We first introduce novel optimized building blocks that are up to 50 % shallower than previous constructions. Second, we present multiple high- and low-level depth minimization techniques and implement these in the existing CBMC-GC compiler. Our experiments show significant depth reductions over hand-optimized constructions (for some applications up to 2.5\(\times \)), while maintaining a circuit size that is competitive with size-minimizing compilers. Evaluating exemplary functionalities in a GMW framework, we show that depth reductions lead to significant speed-ups in any real-world network setting. For an exemplary biometric matching application we report a \(400\times \) speed-up in comparison with a circuit generated from a size-minimizing compiler.

Existing systems for secure computation require programmers to express the program to be securely computed as a circuit, or in a domain-specific language that can be compiled to a form suitable for applying known protocols. We propose a new system that can securely execute native MIPS code with no special annotations. Our system allows programmers to use a language of their choice to express their programs, together with any off-the-shelf compiler to MIPS; it can be used for secure computation of “legacy” MIPS code as well.

Our system uses oblivious RAM for fetching instructions and performing load/store operations in memory, and garbled universal circuits for the execution of a MIPS CPU in each instruction step. We also explore various optimizations based on an offline analysis of the MIPS code to be executed, in order to minimize the overhead of executing each instruction while still maintaining security.

Service providers collect and process more user data then ever, while users of these services remain oblivious to the actual processing and utility of the processed data to the service providers. This leads users to put less trust in service providers and be more reluctant to share data. Transparency logging is about service providers continuously logging descriptions of the data processing on their users’ data, where each description is intended for a particular user.

We propose Insynd, a new cryptographic scheme for privacy-preserving transparency logging. Insynd improves on prior work by (1) increasing the utility of all data sent through the scheme thanks to our publicly verifiable proofs: one can disclose selected events without having to disclose any long term secrets; and (2) enabling a stronger adversarial model: Inysnd can deal with an untrusted server (such as commodity cloud services) through the use of an authenticated data structure named Balloon. Finally, our publicly available prototype implementation shows greatly improved performance with respect to related work and competitive performance for more data-intensive settings like secure logging.

Since hundreds of certificate authorities (CAs) can issue browser-trusted certificates, it can be difficult for domain owners to detect certificates that have been fraudulently issued for their domain. Certificate Transparency (CT) is a recent standard by the Internet Engineering Task Force (IETF) that aims to construct public logs of all certificates issued by CAs, making it easier for domain owners to monitor for fraudulently issued certificates. To avoid relying on trusted log servers, CT includes mechanisms by which monitors and auditors can check whether logs are behaving honestly or not; these mechanisms are primarily based on Merkle tree hashing and authentication proofs. Given that CT is now being deployed, it is important to verify that it achieves its security goals. In this work, we define four security properties of logging schemes such as CT that can be assured via cryptographic means, and show that CT does achieve these security properties. We consider two classes of security goals: those involving security against a malicious logger attempting to present different views of the log to different parties or at different points in time, and those involving security against malicious monitors who attempt to frame an honest log for failing to include a certificate in the log. We show that Certificate Transparency satisfies these security properties under various assumptions on Merkle trees all of which reduce to collision resistance of the underlying hash function (and in one case with the additional assumption of unforgeable signatures).

Bug-bounty programs have the potential to harvest the efforts and diverse knowledge of thousands of white hat hackers. As a consequence, they are becoming increasingly popular as a key part of the security culture of organizations. However, bug-bounty programs can be riddled with myriads of invalid vulnerability-report submissions, which are partially the result of misaligned incentives between white hats and organizations. To further improve the effectiveness of bug-bounty programs, we introduce a theoretical model for evaluating approaches for reducing the number of invalid reports. We develop an economic framework and investigate the strengths and weaknesses of existing canonical approaches for effectively incentivizing higher validation efforts by white hats. Finally, we introduce a novel approach, which may improve efficiency by enabling different white hats to exert validation effort at their individually optimal levels.

We consider the problem of optimal investment in cyber-security by an enterprise. Optimality is measured with respect to the overall (1) monetary cost of implementation, (2) negative side-effects of cyber-security controls (indirect costs), and (3) mitigation of the cyber-security risk. We consider “passive” and “reactive” threats, the former representing the case where attack attempts are independent of the defender’s plan, the latter, where attackers can adapt and react to an implemented cyber-security defense. Moreover, we model in three different ways the combined effect of multiple cyber-security controls, depending on their degree of complementarity and correlation. We also consider multi-stage attacks and the potential correlations in the success of different stages. First, we formalize the problem as a non-linear multi-objective integer programming. We then convert them into Mixed Integer Linear Programs (MILP) that very efficiently solve for the exact Pareto-optimal solutions even when the number of available controls is large. In our case study, we consider 27 of the most typical security controls, each with multiple intensity levels of implementation, and 37 common vulnerabilities facing a typical SME. We compare our findings against expert-recommended critical controls. We then investigate the effect of the security models on the resulting optimal plan and contrast the merits of different security metrics. In particular, we show the superior robustness of the security measures based on the “reactive” threat model, and the significance of the hitherto overlooked role of correlations.

Digital currencies like Bitcoin rely on cryptographic primitives to operate. However, past experience shows that cryptographic primitives do not last forever: increased computational power and advanced cryptanalysis cause primitives to break frequently, and motivate the development of new ones. It is therefore crucial for maintaining trust in a cryptocurrency to anticipate such breakage.

We present the first systematic analysis of the effect of broken primitives on Bitcoin. We identify the core cryptographic building blocks and analyze the ways in which they can break, and the subsequent effect on the main Bitcoin security guarantees. Our analysis reveals a wide range of possible effects depending on the primitive and type of breakage, ranging from minor privacy violations to a complete breakdown of the currency. Our results lead to several observations on, and suggestions for, the Bitcoin migration plans in case of broken or weakened cryptographic primitives.

Nearly all verifiable e-voting schemes require trustworthy authorities to perform the tallying operations. An exception is the DRE-i system which removes this requirement by pre-computing all encrypted ballots before the election using random factors that will later cancel out and allow the public to verify the tally after the election. While the removal of tallying authorities significantly simplifies election management, the pre-computation of ballots necessitates secure ballot storage, as leakage of precomputed ballots endangers voter privacy. In this paper, we address this problem and propose DRE-ip (DRE-i with enhanced privacy). Adopting a different design strategy, DRE-ip is able to encrypt ballots in real time in such a way that the election tally can be publicly verified without decrypting the cast ballots. As a result, DRE-ip achieves end-to-end verifiability without tallying authorities, similar to DRE-i, but with a significantly stronger guarantee on voter privacy. In the event that the voting machine is fully compromised, the assurance on tallying integrity remains intact and the information leakage is limited to the minimum: only the partial tally at the time of compromise is leaked.

Protocols for secure electronic voting are of increasing societal importance. Proving rigorously their security is more challenging than many other protocols, which aim at authentication or key exchange. One of the reasons is that they need to be secure for an arbitrary number of malicious voters. In this paper we identify a class of voting protocols for which only a small number of agents needs to be considered: if there is an attack on vote privacy then there is also an attack that involves at most 3 voters (2 honest voters and 1 dishonest voter).

In the case where the protocol allows a voter to cast several votes and counts, e.g., only the last one, we also reduce the number of ballots required for an attack to 10, and under some additional hypotheses, 7 ballots. Our results are formalised and proven in a symbolic model based on the applied pi calculus. We illustrate the applicability of our results on several case studies, including different versions of Helios and Prêt-à-Voter, as well as the JCJ protocol. For some of these protocols we can use the ProVerif tool to provide the first formal proofs of privacy for an unbounded number of voters.

Motivated by this, we address the following question: “is it possible to create non-trivial efficient smart contracts using the standard transactions only?” We answer this question affirmatively, by constructing efficient Zero-Knowledge Contingent Payment protocol for a large class of NP-relations. This includes the relations for which efficient sigma protocols exist. In particular, our protocol can be used to sell a factorization (p, q) of an RSA modulus \(n=pq\), which is an example that we implemented and tested its efficiency in practice.

As another example of the “smart contract without scripts” we show how our techniques can be used to implement the contract called “trading across chains”.

Recent research into automotive security has shown that once a single vehicle component is compromised, it is often possible to take full control of the vehicle. This paper proposes LeiA, a lightweight authentication protocol for the Controller Area Network (CAN). This protocol allows critical vehicle Electronic Control Units (ECUs) to authenticate each other providing compartmentalisation and preventing a number of attacks e.g., where a compromised CD player is able to accelerate the vehicle. LeiA is designed to run under the stringent time and bandwidth constraints of automotive applications and is backwards compatible with existing vehicle infrastructure. The protocol is suitable to be implemented using lightweight cryptographic primitives yet providing appropriate security levels by limiting the usage of every key in the system. The security of LeiA is proven under the unforgeability assumption of the MAC scheme under chosen message attacks (uf-cma).

Automatic service discovery is essential to realizing the full potential of the Internet of Things (IoT). While discovery protocols like Multicast DNS, Apple AirDrop, and Bluetooth Low Energy have gained widespread adoption across both IoT and mobile devices, most of these protocols do not offer any form of privacy control for the service, and often leak sensitive information such as service type, device hostname, device owner’s identity, and more in the clear.

To address the need for better privacy in both the IoT and the mobile landscape, we develop two protocols for private service discovery and private mutual authentication. Our protocols provide private and authentic service advertisements, zero round-trip (0-RTT) mutual authentication, and are provably secure in the Canetti-Krawczyk key-exchange model. In contrast to alternatives, our protocols are lightweight and require minimal modification to existing key-exchange protocols. We integrate our protocols into an existing open-source distributed applications framework, and provide benchmarks on multiple hardware platforms: Intel Edisons, Raspberry Pis, smartphones, laptops, and desktops. Finally, we discuss some privacy limitations of the Apple AirDrop protocol (a peer-to-peer file sharing mechanism) and show how to improve the privacy of Apple AirDrop using our private mutual authentication protocol.

Mesh networked low-end embedded devices are increasingly used in various scenarios, including industrial control, wireless sensing, robot swarm communication, or building automation. Recently, more and more software vulnerabilities in embedded systems are disclosed, as they become appealing targets for cyber attacks. In order to patch these systems, an efficient and secure code update mechanism is required. However, existing solutions are unable to provide verifiable code updates for networked commodity low-end embedded devices. This work presents a novel code update scheme which verifies and enforces the correct installation of code updates on all devices in the network. After update distribution and installation, devices mutually attest and verify each others’ software state. Devices being in an untrustworthy state are excluded from the network. In this way, the scheme enforces software integrity as well as software up-to-dateness on all devices in the network. Issuing a secure code update, the network operator is able to learn the identity of all trustworthy and all untrustworthy devices. We demonstrate that the proposed scheme is applicable to a wide range of existing commodity low-end embedded systems. Furthermore, we show that the scheme is practically usable in networks with tens of thousands of devices.

The Internet of Things (IoT) is composed of a wide range of heterogeneous network devices that communicate with their users and the surrounding devices. The secure communications between these devices are still essential even with little or no previous knowledge about each other and regardless of their resource capabilities. This particular context requires appropriate security mechanisms which should be well-suited for the heterogeneous nature of IoT devices, without pre-sharing a secret key for each secure connection.

In this work, we first propose a novel symmetric cipher proxy re-encryption scheme. Such a primitive allows a user to delegate her decryption rights to another with the help of a semi-trusted proxy, but without giving this latter any information on the transmitted messages and the user’s secret keys. We then propose AKAPR, an Authenticated Key Agreement mediated by a Proxy Re-encryptor for IoT. The mechanism permits any two highly resource-constrained devices to establish a secure communication with no prior trust relationship. AKAPR is built upon our proposed proxy re-encryption scheme. It has been proved by ProVerif to provide mutual authentication for participants while preserving the secrecy of the generated session key. In addition, the scheme benefits from the lightness of our proxy re-encryption algorithm as it requires no expensive cryptographic operations such as pairing or modular exponentiation.

We conceptually design, formally verify and experimentally evaluate a sophisticated information control mechanism for a relational database instance. The mechanism reacts on access requests for data publishing or query answering with a granularity of either the whole instance or individual tuples. The reaction is based on a general read access permission for the instance combined with user-specific exceptions expressed as prohibitions regarding particular pieces of information declared in a confidentiality policy. These prohibitions are to be enforced in the sense that the user should neither be able to get those pieces directly nor by rational reasoning exploiting the interaction history and background knowledge about both the database and the control mechanism. In an initial off-line phase, the control mechanism basically determines instance-independent weakening templates for individual tuples and generates a policy-compliant weakened view on the stored instance. During the system-user interaction phase, each request to receive data of the database instance is fully accepted but redirected to the weakened view.

Large-scale datasets of consumer behavior might revolutionize the way we gain competitive advantages and increase our knowledge in the respective domains. At the same time, valuable datasets pose potential privacy risks that are difficult to foresee. In this paper we study the impact that the prices from consumers’ purchase histories have on the consumers’ location privacy. We show that using a small set of low-priced product prices from the consumers’ purchase histories, an adversary can determine the country, city, and local retail store where the transaction occurred with high confidence. Our paper demonstrates that even when the product category, precise time of purchase, and currency are removed from the consumers’ purchase history (e.g., for privacy reasons), information about the consumers’ location is leaked. The results are based on three independent datasets containing thousands of low-priced and frequently-bought consumer products. The results show the existence of location privacy risks when releasing consumer purchase histories. As such, the results highlight the need for systems that hide transaction details in consumer purchase histories.

Video surveillance has become prevalent both in public spaces, e.g. to prevent crimes, and in private areas, e.g. in order to assist the staff in assisted living communities. This leads to privacy concerns regarding the ability of third parties to create profiles and track individuals, possibly across several services.

Usually, techniques such as pixelation and silhouettes are used to anonymize individuals. However, no formal treatment of privacy for video data has been proposed and current anonymization techniques are simply “best practice”. To resolve this unsatisfactory state of affairs, we initiate a formal treatment of privacy in video data and propose a game-based notion for privacy in video data that is inspired by cryptographic security games.

We show for an exemplary video privacy scheme that this scheme satisfies our notion with good parameters. In order to evaluate these parameters, we conduct a user study where the users essentially play the role of the adversary in the privacy game. Our approach can be used as a blueprint to evaluate the privacy of other video privacy schemes.

Attacker models are a fundamental part of research on security of any system. For different application scenarios, suitable attacker models have to be chosen to allow comprehensive coverage of possible attacks. We consider Cyber-Physical Systems (CPS), that typically consist of networked embedded systems which are used to sense, actuate, and control physical processes. The physical layer aspects of such systems add novel attack vectors and opportunities for defenses, that require extended models of attackers’ capabilities. We develop a taxonomy to classify and compare attacker models in related work. We show that, so far, there are no commonly used attacker models for such CPS. In addition, concepts of what information belongs in an attacker model are widely different among the community. To address that problem, we develop a framework to classify attacker models and use it to review related work on CPS Security. Using our framework, we propose a set of attacker profiles and show that those profiles capture most types of attackers described in the related work. Our framework provides a more formal and standardized definition of attacker model for CPS, enabling the use of well-defined and uniform attacker models in the future.

Timed Intruder Models have been proposed for the verification of Cyber-Physical Security Protocols (CPSP) amending the traditional Dolev-Yao intruder to obey the physical restrictions of the environment. Since to learn a message, a Timed Intruder needs to wait for a message to arrive, mounting an attack may depend on where Timed Intruders are. It may well be the case that in the presence of a great number of intruders there is no attack, but there is an attack in the presence of a small number of well placed intruders. Therefore, a major challenge for the automated verification of CPSP is to determine how many Timed Intruders to use and where should they be placed. This paper answers this question by showing it is enough to use the same number of Timed Intruders as the number of participants. We also report on some preliminary experimental results in discovering attacks in CPSP.

Automatic restoration of control wireless networks based on dynamic cyber-physical systems has become a hot topic in recent years, since most of their elements tend to have serious vulnerabilities that may be exploited by attackers. In fact, any exploitation may rapidly extend to the entire control network due to its problem of non-locality, where control properties of a system and its structural controllability can disintegrate over time. Unfortunately, automated self-healing processes may become costly procedures in which the reliability of the strategies and the time-critical of any recovery of the control can become key factors to re-establish the control properties in due time. This operational need is precisely the aim of this paper, in which four reachability-based recovery strategies from a theoretical point of view are proposed so as to find the best option/s in terms of optimization, robustness and complexity. To do this, new definitions related to structural controllability in relation to the type of distribution of the network and its control load capacity are given in this paper, resulting in an interesting practical study.

Autocomplete injection can be exploited in multiple ways, including phishing, framing, illegitimate content-promotion and sometimes persistent cross-site scripting attacks. We evaluated the effectiveness of the attack with several experiments. Our results show the potential impact of the autocomplete injection attacks.

We analyze the security of Android KeyStore, a system service whose purpose is to shield users credentials and cryptographic keys. The KeyStore protects the integrity and the confidentiality of keys by using a particular encryption scheme. Our main results are twofold. First, we formally prove that the used encryption scheme does not provide integrity, which means that an attacker is able to undetectably modify the stored keys. Second, we exploit this flaw to define a forgery attack breaching the security guaranteed by the KeyStore. In particular, our attack allows a malicious application to make mobile apps to unwittingly perform secure protocols using weak keys. The threat is concrete: the attacker goes undetected while compromising the security of users. Our findings highlight an important fact: intuition often goes wrong when security is concerned. Unfortunately, system designers still tend to choose cryptographic schemes not for their proved security but for their apparent simplicity. We show, once again, that this is not a good choice, since it usually results in severe consequences for the whole underlying system.

Ciphertext-policy attribute-based encryption (CP-ABE) is a highly promising solution for cloud computing, which has been widely applied to provide fine-grained access control in cloud storage services recently. However, for CP-ABE based cloud storage systems, if a decryption device appears on eBay described and advertised to be able to decrypt any ciphertexts with policies satisfied by an attribute set or even with a specific access policy only, no one can trace the malicious user(s) who built such a decryption device using their private key(s). This has been known as a major obstacle to deploying CP-ABE systems in real-world commercial applications. Due to the one-to-many encryption mechanism of CP-ABE, the same decryption privilege is shared by multiple users who have the same attributes. It is difficult to identity the malicious user(s) who built such a decryption device. To track people selling decryption devices on eBay efficiently, in this paper, we develop a new methodology for constructing traitor tracing functionality, and present the first black-box traceable CP-ABE (BT-CP-ABE) with short ciphertexts which are independent of the number of users \(\mathcal {N}\). The black-box traceability is public, fully collusion-resistant, and adaptively traceable against both key-like decryption black-box and policy-specific decryption black-box.

Our construction combines the conventional CP-ABE with Anonymous Hierarchical Identity-Based Encryption (A-HIBE) in a novel way, which is the first to construct the (underlying) traitor tracing system from A-HIBE. The resulting ciphertexts are independent of \(\mathcal {N}\) while the private keys are linear in \(\mathcal {N}\), which partially answers an open problem posed by Boneh and Waters [CCS 2006]. We believe this work is a constructive step towards efficient traitor tracing system with short ciphertexts and private keys. In particular, we believe that following the route of this work, any progress in A-HIBE (i.e., with shorter ciphertexts and private keys) may result in some progress in BT-CP-ABE and finally give a satisfactory solution to this open problem.

As a one-to-many public key encryption system, attribute-based encryption (ABE) enables scalable access control over encrypted data in cloud storage services. However, efficient user revocation has been a very challenging problem in ABE. To address this issue, Boldyreva, Goyal and Kumar [5] introduced a revocation method by combining the binary tree data structure with fuzzy identity-based encryption, in which a key generation center (KGC) periodically broadcasts key update information to all data users over a public channel. The Boldyreva-Goyal-Kumar approach reduces the size of key updates from linear to logarithm in the number of users, and it has been widely used in subsequent revocable ABE systems; however, it requires each data user to keep a private key of logarithmic size and all non-revoked data users to periodically update decryption keys for each new time period. To further optimize user revocation in ABE, in this paper, we propose a notion called server-aided revocable ABE (SR-ABE), in which almost all workloads of data users incurred by user revocation are delegated to an untrusted server and each data user only needs to store a key of constant size. We then define a security model for SR-ABE, and present a concrete SR-ABE scheme secure under this model. Interestingly, due to the key embedding gadget employed in the construction of SR-ABE, our SR-ABE scheme does not require any secure channels for key transmission, and also enjoys an additional property in the decryption phase, where a data user only needs to perform one exponentiation computation to decrypt a ciphertext.

Public-Index Predicate Encryption (PIPE) allows users to encrypt according to boolean predicates defined on arbitrary attributes. The expensive algebraic operations are the major efficiency obstacle for PIPE to be applied to mobile clouds. This paper proposes a general Online/Offline PIPE (OO-PIPE) framework to address this issue. First, we propose a generic transformation from a Large Universe PIPE (LU-PIPE) secure against chosen plaintext attack (CPA) to OO-PIPE in the same security model. The challenge is to generate ciphertext without the knowledge of the associated ciphertext attributes in the offline phase. We address the challenge by identifying an interesting attribute-malleability property in many LU-PIPE schemes. The property allows an encryptor to efficiently malleate a ciphertext associated with one ciphertext attribute to any assigned ciphertext attribute. Second, we design a generic transformation from CPA-secure LU-PIPE to OO-PIPE secure against adaptively chosen ciphertext attack (CCA2), assuming the underlying LU-PIPE has attribute-malleability and public-verifiability properties. The main obstacle here is that the online/offline mechanism endogenously implies forgery in the sense that a pre-computed ciphertext must be able to be efficiently malleated to the resulting ciphertext associated with a different ciphertext attribute and a plaintext, while any efficient valid ciphertext forgery is forbidden in CCA2 security. We circumvent this obstacle by employing a universally collision resistant Chameleon hash, namely, only the original encryptor can malleate the ciphertext to associate with different attributes and provide a hash collision of the ciphertext components.