1,837 research outputs found
Server-Aided Revocable Predicate Encryption: Formalization and Lattice-Based Instantiation
Efficient user revocation is a necessary but challenging problem in many
multi-user cryptosystems. Among known approaches, server-aided revocation
yields a promising solution, because it allows to outsource the major workloads
of system users to a computationally powerful third party, called the server,
whose only requirement is to carry out the computations correctly. Such a
revocation mechanism was considered in the settings of identity-based
encryption and attribute-based encryption by Qin et al. (ESORICS 2015) and Cui
et al. (ESORICS 2016), respectively.
In this work, we consider the server-aided revocation mechanism in the more
elaborate setting of predicate encryption (PE). The latter, introduced by Katz,
Sahai, and Waters (EUROCRYPT 2008), provides fine-grained and role-based access
to encrypted data and can be viewed as a generalization of identity-based and
attribute-based encryption. Our contribution is two-fold. First, we formalize
the model of server-aided revocable predicate encryption (SR-PE), with rigorous
definitions and security notions. Our model can be seen as a non-trivial
adaptation of Cui et al.'s work into the PE context. Second, we put forward a
lattice-based instantiation of SR-PE. The scheme employs the PE scheme of
Agrawal, Freeman and Vaikuntanathan (ASIACRYPT 2011) and the complete subtree
method of Naor, Naor, and Lotspiech (CRYPTO 2001) as the two main ingredients,
which work smoothly together thanks to a few additional techniques. Our scheme
is proven secure in the standard model (in a selective manner), based on the
hardness of the Learning With Errors (LWE) problem.Comment: 24 page
Ad Hoc Multi-Input Functional Encryption
Consider sources that supply sensitive data to an aggregator. Standard encryption only hides the data from eavesdroppers, but using specialized encryption one can hope to hide the data (to the extent possible) from the aggregator itself. For flexibility and security, we envision schemes that allow sources to supply encrypted data, such that at any point a dynamically-chosen subset of sources can allow an agreed-upon joint function of their data to be computed by the aggregator. A primitive called multi-input functional encryption (MIFE), due to Goldwasser et al. (EUROCRYPT 2014), comes close, but has two main limitations:
- it requires trust in a third party, who is able to decrypt all the data, and
- it requires function arity to be fixed at setup time and to be equal to the number of parties.
To drop these limitations, we introduce a new notion of ad hoc MIFE. In our setting, each source generates its own public key and issues individual, function-specific secret keys to an aggregator. For successful decryption, an aggregator must obtain a separate key from each source whose ciphertext is being computed upon. The aggregator could obtain multiple such secret-keys from a user corresponding to functions of varying arity. For this primitive, we obtain the following results:
- We show that standard MIFE for general functions can be bootstrapped to ad hoc MIFE for free, i.e. without making any additional assumption.
- We provide a direct construction of ad hoc MIFE for the inner product functionality based on the Learning with Errors (LWE) assumption. This yields the first construction of this natural primitive based on a standard assumption.
At a technical level, our results are obtained by combining standard MIFE schemes and two-round secure multiparty computation (MPC) protocols in novel ways highlighting an interesting interplay between MIFE and two-round MPC
Equivalence-based Security for Querying Encrypted Databases: Theory and Application to Privacy Policy Audits
Motivated by the problem of simultaneously preserving confidentiality and
usability of data outsourced to third-party clouds, we present two different
database encryption schemes that largely hide data but reveal enough
information to support a wide-range of relational queries. We provide a
security definition for database encryption that captures confidentiality based
on a notion of equivalence of databases from the adversary's perspective. As a
specific application, we adapt an existing algorithm for finding violations of
privacy policies to run on logs encrypted under our schemes and observe low to
moderate overheads.Comment: CCS 2015 paper technical report, in progres
Forward-secure hierarchical predicate encryption
Secrecy of decryption keys is an important pre-requisite for security of any encryption scheme and compromised private keys must be immediately replaced. \emph{Forward Security (FS)}, introduced to Public Key Encryption (PKE) by Canetti, Halevi, and Katz (Eurocrypt 2003), reduces damage from compromised keys by guaranteeing confidentiality of messages that were encrypted prior to the compromise event. The FS property was also shown to be achievable in (Hierarchical) Identity-Based Encryption (HIBE) by Yao, Fazio, Dodis, and Lysyanskaya (ACM CCS 2004). Yet, for emerging encryption techniques, offering flexible access control to encrypted data, by means of functional relationships between ciphertexts and decryption keys, FS protection was not known to exist.\smallskip In this paper we introduce FS to the powerful setting of \emph{Hierarchical Predicate Encryption (HPE)}, proposed by Okamoto and Takashima (Asiacrypt 2009). Anticipated applications of FS-HPE schemes can be found in searchable encryption and in fully private communication. Considering the dependencies amongst the concepts, our FS-HPE scheme implies forward-secure flavors of Predicate Encryption and (Hierarchical) Attribute-Based Encryption.\smallskip Our FS-HPE scheme guarantees forward security for plaintexts and for attributes that are hidden in HPE ciphertexts. It further allows delegation of decrypting abilities at any point in time, independent of FS time evolution. It realizes zero-inner-product predicates and is proven adaptively secure under standard assumptions. As the ``cross-product" approach taken in FS-HIBE is not directly applicable to the HPE setting, our construction resorts to techniques that are specific to existing HPE schemes and extends them with what can be seen as a reminiscent of binary tree encryption from FS-PKE
Succinct Predicate and Online-Offline Multi-Input Inner Product Encryptions under Standard Static Assumptions
This paper presents expressive predicate encryption (PE) systems, namely non-zero
inner-product-predicate encryption (NIPPE) and attribute-based encryption (ABE) supporting monotone
span programs achieving best known parameters among existing similar schemes under well-studied
static complexity assumptions. Both the constructions are built in composite order bilinear
group setting and involve only 2 group elements in the ciphertexts. More interestingly, our NIPPE
scheme, which additionally features only 1 group element in the decryption keys, is the first to
attain succinct ciphertexts and decryption keys simultaneously. For proving selective security of
these constructions under the Subgroup Decision assumptions, which are the most standard static
assumptions in composite order bilinear group setting, we apply the extended version of the elegant
D´ej`a Q framework, which was originally proposed as a general technique for reducing the q-type
complexity assumptions to their static counter parts. Our work thus demonstrates the power of this
framework in overcoming the need of q-type assumptions, which are vulnerable to serious practical
attacks, for deriving security of highly expressive PE systems with compact parameters. We further
introduce the concept of online-offline multi-input functional encryption (OO-MIFE), which is a
crucial advancement towards realizing this highly promising but computationally intensive cryptographic
primitive in resource bounded and power constrained devices. We also instantiate our
notion of OO-MIFE by constructing such a scheme for the multi-input analog of the inner product
functionality, which has a wide range of application in practice. Our OO-MIFE scheme for multiinput
inner products is built in asymmetric bilinear groups of prime order and is proven selectively
secure under the well-studied k-Linear (k-LIN) assumption
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