Trade-Off between Collusion Resistance and User Life Cycle in Self-Healing Key Distributions with t-Revocation

We solve the problem of resisting the collusion attack in the one-way hash chain based self-healing key distributions introduced by Dutta et al., coupling it with the prearranged life cycle based approach of Tian et al. that uses the same self-healing mechanism introduced in Dutta et al. Highly efficient schemes are developed compared to the existing works with the trade-off in pre-arranged life cycles on users by the group manager and a slight increase in the storage overhead. For scalability of business it is often necessary to design more innovation and flexible business strategies in certain business models that allow contractual subscription or rental, such as subscription of mobile connection or TV channel for a pre-defined period. The subscribers are not allowed to revoke before their contract periods (life cycles) are over. Our schemes fit into such business environment. The proposed schemes are proven to be computationally secure and resist collusion between new joined users and revoked users together with forward and backward secrecy. The security proof is in an appropriate security model. Moreover, our schemes do not forbid revoked users from rejoining in later sessions unlike the existing self- healing key distribution schemes

Attribute-Based Access Control for Inner Product Functional Encryption from LWE

The notion of functional encryption (FE) was proposed as a generalization of plain public-key encryption to enable a much more fine-grained handling of encrypted data, with advanced applications such as cloud computing, multi-party computations, obfuscating circuits or Turing machines. While FE for general circuits or Turing machines gives a natural instantiation of the many cryptographic primitives, existing FE schemes are based on indistinguishability obfuscation or multilinear maps which either rely on new computational hardness assumptions or heuristically claimed to be secure. In this work, we present new techniques directly yielding FE for inner product functionality where secret-keys provide access control via polynomial-size bounded-depth circuits. More specifically, we encrypt messages with respect to attributes and embed policy circuits into secret-keys so that a restricted class of receivers would be able to learn certain property about the messages. Recently, many inner product FE schemes were proposed. However, none of them uses a general circuit as an access structure. Our main contribution is designing the first construction for an attribute-based FE scheme in key-policy setting for inner products from well-studied Learning With Errors (LWE) assumption. Our construction takes inspiration from the attribute-based encryption of Boneh et al. from Eurocrypt 2014 and the inner product functional encryption of Agrawal et al. from Crypto 2016. The scheme is proved in a stronger setting where the adversary is allowed to ask secret-keys that can decrypt the challenge ciphertext. Doing so requires a careful setting of parameters for handling the noise in ciphertexts to enable correct decryption. Another main advantage of our scheme is that the size of ciphertexts and secret-keys depends on the depth of the circuits rather than its size. Additionally, we extend our construction in a much desirable multi-input variant where secret-keys are associated with multiple policies subject to different encryption slots. This enhances the applicability of the scheme with finer access control

Overview of Key Agreement Protocols

The emphasis of this paper is to focus on key agreement. To this aim, we address a self-contained, up-to-date presentation of key agreement protocols at high level. We have attempted to provide a brief but fairly complete survey of all these schemes

Key-Oblivious Encryption from isogenies and its application to Accountable Tracing Signatures.

Key-oblivious encryption (KOE) is a newly developed cryptographic primitive that randomizes the public keys of an encryption scheme in an oblivious manner. It has applications in designing accountable tracing signature (ATS) that facilitates the group manager to revoke the anonymity of traceable users in a group signature while preserving the anonymity of non-traceable users. Despite of its importance and strong application, KOE has not received much attention in the literature. In this work, we introduce the first isogeny-based KOE scheme. Isogeny is a fairly young post-quantum cryptographic field with sophisticated algebraic structures and unique security properties. Our KOE scheme is resistant to quantum attacks and derives its security from Commutative Supersingular Decisional Diffie-Hellman (CSSDDH), which is an isogeny based hard problem. More concretely, we have shown that our construction exhibits key randomizability, plaintext indistinguishability under key randomization and key privacy under key randomization in the standard model adapting the security framework of [KM15]. Furthermore, we have manifested instantiation of our scheme from cryptosystem based on Commutative Supersingular Isogeny Diffie-Hellman (CSIDH-512) [BKV19]. Additionally, we demonstrate the utility of our KOE scheme by leveraging it to construct an isogeny-based ATS scheme preserving anonymity under tracing, traceability, non-frameability, anonymity with accountability and trace obliviousness in the random oracle model following the security framework of [LNWX19]

Enhanced Outsider-anonymous Broadcast Encryption with Subset Difference Revocation

This paper puts forward an efficient broadcast encryption in public key setting employing ternary tree subset difference method for revocation. It provides outsider anonymity disabling the revoked users from getting any information of message and concealing the set of subscribed users from the revoked users. Our approach utilizes composite order bilinear group setting and exhibits significant improvement in the broadcast efficiency. The proposed scheme compares favourably over the existing similar schemes in standard model. The public key and secret key sizes are poly-logarithmic while the ciphertext size is sub linear in total number of users. Our scheme achieves selective security against chosen plaintext attack in the standard model under reasonable assumptions

Cost-Effective Private Linear Key Agreement With Adaptive CCA Security from Prime Order Multilinear Maps and Tracing Traitors

Private linear key agreement (PLKA) enables a group of users to agree upon a common session key in a broadcast encryption (BE) scenario, while traitor tracing (TT) system allows a tracer to identify conspiracy of a troop of colluding pirate users. This paper introduces a key encapsulation mechanism in BE that provides the functionalities of both PLKA and TT in a unified cost-effective primitive. Our PLKA based traitor tracing offers a solution to the problem of achieving full collusion resistance property and public traceability simultaneously with significant efficiency and storage compared to a sequential improvement of the PLKA based traitor tracing systems. Our PLKA builds on a prime order multilinear group setting employing indistinguishability obfuscation (iO) and pseudorandom function (PRF). The resulting scheme has a fair communication, storage and computational efficiency compared to that of composite order groups. Our PLKA is adaptively chosen ciphertext attack (CCA)-secure and based on the hardness of the multilinear assumption, namely, the Decisional Hybrid Diffie-Hellman Exponent (DHDHE) assumption in standard model and so far a plausible improvement in the literature. More precisely, our PLKA design significantly reduces the ciphertext size, public parameter size and user secret key size. We frame a traitor tracing algorithm with shorter running time which can be executed publicly

Secure and Efficient Multi-Key FHE Scheme Supporting Multi-bit Messages from LWE Preserving Non-Interactive Decryption

We consider multi-key fully homomorphic encryption (multi-key FHE) which is the richest variant of fully homomorphic encryption (FHE) that allows complex computation on encrypted data under different keys. Since its introduction by Lopez-Alt, Tromer and Vaikuntanathan in 2012, numerous proposals have been presented yielding various improvements in security and efficiency. However, most of these multi-key FHE schemes encrypt a single-bit message. Constructing a multi-key FHE scheme encrypting multi-bit messages have been notoriously difficult without loosing efficiency for homomorphic evaluation and ciphertext extension under additional keys. In this work, we study multi-key FHE that can encrypt multi-bit messages. Motivated by the goals of improving the efficiency, we propose a new construction with non-interactive decryption and security against chosen-plaintext attack (IND-CPA) from the standard learning with errors (LWE) assumption. We consider a binary matrix as plaintext instead of a single-bit. Our approach supports efficient homomorphic matrix addition and multiplication. Another interesting feature is that our technique of extending a ciphertext under additional keys yields significant reduction in the computational overhead. More interestingly, when contrasted with the previous multi-key FHE schemes for multi-bit messages, our candidates exhibits favorable results in the length of the secret key, public key and ciphertext preserving non-interactive decryption. Keywords: lattice based cryptosystem, multi-key fully homomorphic encryption, learning with errors, multi-bit message

Chosen Ciphertext Secure Functional Encryption from Constrained Witness PRF

Functional encryption generates sophisticated keys for users so that they can learn specific functions of the encrypted message. We provide a generic construction of chosen ciphertext attacks (CCA) secure public-key functional encryption (PKFE) for all polynomial-size circuits. Our PKFE produces succinct ciphertexts that are independent of the size and depth of the circuit class under consideration. We accomplish our goal in two steps. First, we define a new cryptographic tool called constrained witness pseudorandom function (CWPRF) which is motivated by combining WPRF of Zhandry (TCC 2016) and constrained PRF of Boneh and Waters (ASIACRYPT 2013). More specifically, CWPRF computes pseudorandom values associated with NP statements and generates constrained keys for boolean functions. We can recompute the pseudorandom value corresponding to a particular statement either using a public evaluation key with a valid witness for the statement or applying a constrained key for a function that satisfies the statement. We construct CWPRF by coupling indistinguishability obfuscation (iO) and CPRF supporting all polynomial-size functions. In the second and main technical step, we show a generic construction of a CCA secure PKFE for all circuits utilizing our CWPRF. It has been observed that obtaining PKFE supporting all circuits is already a complex task and iO-based constructions of PKFEs are only proven to be chosen plaintext attacks (CPA) secure. On the other hand, existing CCA secure functional encryption schemes are designed for specific functions such as equality testing, membership testing, linear function etc. We emphasize that our construction presents the first CCA secure PKFE for all circuits along with succinct ciphertexts