Adaptively Secure Identity-Based Encryption from Lattices with Asymptotically Shorter Public Parameters

In this paper, we present two new adaptively secure identity-based encryption (IBE) schemes from lattices. The size of the public parameters, ciphertexts, and private keys are $\tilde{O}(n^2 \kappa^{1/d})$, $\tilde{O}(n)$, and $\tilde{O}(n)$ respectively. Here, $n$ is the security parameter, $\kappa$ is the length of the identity, and $d$ is a flexible constant that can be set arbitrary (but will affect the reduction cost). Ignoring the poly-logarithmic factors hidden in the asymptotic notation, our schemes achieve the best efficiency among existing adaptively secure IBE schemes from lattices. In more detail, our first scheme is anonymous, but proven secure under the LWE assumption with approximation factor $n^{\omega(1)}$. Our second scheme is not anonymous, but proven adaptively secure assuming the LWE assumption for all polynomial approximation factors. As a side result, based on a similar idea, we construct an attribute-based encryption scheme for branching programs that simultaneously satisfies the following properties for the first time: Our scheme achieves compact secret keys, the security is proven under the LWE assumption with polynomial approximation factors, and the scheme can deal with unbounded length branching programs

Group Signatures with Message-Dependent Opening: Formal Definitions and Constructions

This paper introduces a new capability for group signatures called message-dependent opening. It is intended to weaken the high trust placed on the opener; i.e., no anonymity against the opener is provided by an ordinary group signature scheme. In a group signature scheme with message-dependent opening (GS-MDO), in addition to the opener, we set up an admitter that is not able to extract any user’s identity but admits the opener to open signatures by specifying messages where signatures on the specified messages will be opened by the opener. The opener cannot extract the signer’s identity from any signature whose corresponding message is not specified by the admitter. This paper presents formal definitions of GS-MDO and proposes a generic construction of it from identity-based encryption and adaptive non-interactive zero-knowledge proofs. Moreover, we propose two specific constructions, one in the standard model and one in the random oracle model. Our scheme in the standard model is an instantiation of our generic construction but the message-dependent opening property is bounded. In contrast, our scheme in the random oracle model is not a direct instantiation of our generic construction but is optimized to increase efficiency and achieves the unbounded message-dependent opening property. Furthermore, we also demonstrate that GS-MDO implies identity-based encryption, thus implying that identity-based encryption is essential for designing GS-MDO schemes

On Cryptographic Building Blocks and Transformations

Cryptographic building blocks play a central role in cryptography, e.g., encryption or digital signatures with their security notions. Further, cryptographic building blocks might be constructed modularly, i.e., emerge out of other cryptographic building blocks. Essentially, one cryptographically transforms the underlying block(s) and their (security) properties into the emerged block and its properties. This thesis considers cryptographic building blocks and new cryptographic transformations

Hierarchical Functional Encryption

Functional encryption provides fine-grained access control for encrypted data, allowing each user to learn only specific functions of the encrypted data. We study the notion of \emph{hierarchical} functional encryption, which augments functional encryption with \emph{delegation} capabilities, offering significantly more expressive access control. We present a {\em generic transformation} that converts any general-purpose public-key functional encryption scheme into a hierarchical one without relying on any additional assumptions. This significantly refines our understanding of the power of functional encryption, showing (somewhat surprisingly) that the existence of functional encryption is equivalent to that of its hierarchical generalization. Instantiating our transformation with the existing functional encryption schemes yields a variety of hierarchical schemes offering various trade-offs between their delegation capabilities (i.e., the depth and width of their hierarchical structures) and underlying assumptions. When starting with a scheme secure against an unbounded number of collusions, we can support \emph{arbitrary} hierarchical structures. In addition, even when starting with schemes that are secure against a bounded number of collusions (which are known to exist under rather minimal assumptions such as the existence of public-key encryption and shallow pseudorandom generators), we can support hierarchical structures of bounded depth and width

Data Sharing on Untrusted Storage with Attribute-Based Encryption

Storing data on untrusted storage makes secure data sharing a challenge issue. On one hand, data access policies should be enforced on these storage servers; on the other hand, confidentiality of sensitive data should be well protected against them. Cryptographic methods are usually applied to address this issue -- only encrypted data are stored on storage servers while retaining secret key(s) to the data owner herself; user access is granted by issuing the corresponding data decryption keys. The main challenges for cryptographic methods include simultaneously achieving system scalability and fine-grained data access control, efficient key/user management, user accountability and etc. To address these challenge issues, this dissertation studies and enhances a novel public-key cryptography -- attribute-based encryption (ABE), and applies it for fine-grained data access control on untrusted storage. The first part of this dissertation discusses the necessity of applying ABE to secure data sharing on untrusted storage and addresses several security issues for ABE. More specifically, we propose three enhancement schemes for ABE: In the first enhancement scheme, we focus on how to revoke users in ABE with the help of untrusted servers. In this work, we enable the data owner to delegate most computation-intensive tasks pertained to user revocation to untrusted servers without disclosing data content to them. In the second enhancement scheme, we address key abuse attacks in ABE, in which authorized but malicious users abuse their access privileges by sharing their decryption keys with unauthorized users. Our proposed scheme makes it possible for the data owner to efficiently disclose the original key owner\u27s identity merely by checking the input and output of a suspicious user\u27s decryption device. Our third enhancement schemes study the issue of privacy preservation in ABE. Specifically, our proposed schemes hide the data owner\u27s access policy not only to the untrusted servers but also to all the users. The second part presents our ABE-based secure data sharing solutions for two specific applications -- Cloud Computing and Wireless Sensor Networks (WSNs). In Cloud Computing cloud servers are usually operated by third-party providers, which are almost certain to be outside the trust domain of cloud users. To secure data storage and sharing for cloud users, our proposed scheme lets the data owner (also a cloud user) generate her own ABE keys for data encryption and take the full control on key distribution/revocation. The main challenge in this work is to make the computation load affordable to the data owner and data consumers (both are cloud users). We address this challenge by uniquely combining various computation delegation techniques with ABE and allow both the data owner and data consumers to securely mitigate most computation-intensive tasks to cloud servers which are envisaged to have unlimited resources. In WSNs, wireless sensor nodes are often unattendedly deployed in the field and vulnerable to strong attacks such as memory breach. For securing storage and sharing of data on distributed storage sensor nodes while retaining data confidentiality, sensor nodes encrypt their collected data using ABE public keys and store encrypted data on storage nodes. Authorized users are given corresponding decryption keys to read data. The main challenge in this case is that sensor nodes are extremely resource-constrained and can just afford limited computation/communication load. Taking this into account we divide the lifetime of sensor nodes into phases and distribute the computation tasks into each phase. We also revised the original ABE scheme to make the overhead pertained to user revocation minimal for sensor nodes. Feasibility of the scheme is demonstrated by experiments on real sensor platforms

Identity-Based Key Aggregate Cryptosystem from Multilinear Maps

The key-aggregate cryptosystem~(KAC) proposed by Chu et al. in 2014 offers a solution to the flexible access delegation problem in shared data environments such as the cloud. KAC allows a data owner, owning $N$ classes of encrypted data, to securely grant access to any subset $S$ of these data classes among a subset $\hat{S}$ of data users, via a single low overhead \emph{aggregate key} $K_{\mathcal{S}}$. Existing constructions for KAC are efficient in so far they achieve constant size ciphertexts and aggregate keys. But they resort to a public parameter that has size linear in the number of data classes $N$, and require $O(M\u27M)$ secure channels for distribution of aggregate keys in a system with $M\u27$ data owners and $M$ data users. In this paper, we propose three different multilinear-map based KAC constructions that have at most polylogarithmic overhead for both ciphertexts and public parameters, and generate constant size aggregate keys. We further demonstrate how the aggregate keys may be efficiently broadcast among any arbitrary size subset of $M$ data users using only $O(M\u27+M)$ secure channels, in a system with $M\u27$ data owners. Our constructions are secure in the generic multilinear group model and are fully collusion resistant against any number of colluding parties. In addition, they naturally give rise to \emph{identity based} secure access delegation schemes

Unbounded Dynamic Predicate Compositions in Attribute-Based Encryption

We present several transformations that combine a set of attribute-based encryption (ABE) schemes for simpler predicates into a new ABE scheme for more expressive composed predicates. Previous proposals for predicate compositions of this kind, the most recent one being that of Ambrona et.al. at Crypto\u2717, can be considered static (or partially dynamic), meaning that the policy (or its structure) that specifies a composition must be fixed at the setup. Contrastingly, our transformations are dynamic and unbounded: they allow a user to specify an arbitrary and unbounded-size composition policy right into his/her own key or ciphertext. We propose transformations for three classes of composition policies, namely, the classes of any monotone span programs, any branching programs, and any deterministic finite automata. These generalized policies are defined over arbitrary predicates, hence admitting modular compositions. One application from modularity is a new kind of ABE for which policies can be ``nested\u27\u27 over ciphertext and key policies. As another application, we achieve the first fully secure completely unbounded key-policy ABE for non-monotone span programs, in a modular and clean manner, under the q-ratio assumption. Our transformations work inside a generic framework for ABE called symbolic pair encoding, proposed by Agrawal and Chase at Eurocrypt\u2717. At the core of our transformations, we observe and exploit an unbounded nature of the symbolic property so as to achieve unbounded-size policy compositions