Efficient Unlinkable Sanitizable Signatures from Signatures with Re-Randomizable Keys

In a sanitizable signature scheme the signer allows a designated third party, called the sanitizer, to modify certain parts of the message and adapt the signature accordingly. Ateniese et al. (ESORICS 2005) introduced this primitive and proposed five security properties which were formalized by Brzuska et al.~(PKC 2009). Subsequently, Brzuska et al. (PKC 2010) suggested an additional security notion, called unlinkability which says that one cannot link sanitized message-signature pairs of the same document. Moreover, the authors gave a generic construction based on group signatures that have a certain structure. However, the special structure required from the group signature scheme only allows for inefficient instantiations. Here, we present the first efficient instantiation of unlinkable sanitizable signatures. Our construction is based on a novel type of signature schemes with re-randomizable keys. Intuitively, this property allows to re-randomize both the signing and the verification key separately but consistently. This allows us to sign the message with a re-randomized key and to prove in zero-knowledge that the derived key originates from either the signer or the sanitizer. We instantiate this generic idea with Schnorr signatures and efficient $\Sigma$-protocols, which we convert into non-interactive zero-knowledge proofs via the Fiat-Shamir transformation. Our construction is at least one order of magnitude faster than instantiating the generic scheme of Brzuska et al. with the most efficient group signature schemes

Prioritization, security and relay selection in network coded multiple access relay networks

Wireless communication is undoubtedly one of the most significant advancements by the mankind for improving quality of life. Information is transmitted from one point to another via electromagnetic waves. After Shannon\u27s landmark paper ``A Mathematical Theory of Communication\u27\u27 in 1948, significant advancements have occurred in providing reliable point to point wireless communication. With ever growing need for reliable high speed links, Cooperative communication and Network coding have emerged as viable technologies to bridge the gap. In today\u27s wireless network, different users have different demands for reliability based on their respective application. In this context, we propose flexible network coding scheme to adapt to user needs. We develop coding rules which achieve maximal diversity of the system, yet provide differentiated class of service to the users. The proposed scheme can be adjusted to accommodate the dynamic changes in quality of service(QoS) demand of users. Second we consider the issue of security in multiple access relay network. Security has always been a primary concern in wireless networks due to it broadcast nature of transmission. The intermediate relay nodes in a wireless network could be modified by adversary to transmit corrupted information. We propose a novel iterative packet recycling methodology which gives performance improvement over traditional approach of discarding received corrupted packets at the destination. Finally, we consider the problem of choosing relay for transmission. We propose a novel selection scheme which provides balanced relay utilization and reduces relay switching rate compared to the traditional selection algorithms. This cuts down energy wastage at the relay and improves the overall network lifetime

Multi-Theorem Preprocessing NIZKs from Lattices

Non-interactive zero-knowledge (NIZK) proofs are fundamental to modern cryptography. Numerous NIZK constructions are known in both the random oracle and the common reference string (CRS) models. In the CRS model, there exist constructions from several classes of cryptographic assumptions such as trapdoor permutations, pairings, and indistinguishability obfuscation. Notably absent from this list, however, are constructions from standard lattice assumptions. While there has been partial progress in realizing NIZKs from lattices for specific languages, constructing NIZK proofs (and arguments) for all of NP from standard lattice assumptions remains open. In this work, we make progress on this problem by giving the first construction of a multi-theorem NIZK for NP from standard lattice assumptions in the preprocessing model. In the preprocessing model, a (trusted) setup algorithm generates proving and verification keys. The proving key is needed to construct proofs and the verification key is needed to check proofs. In the multi-theorem setting, the proving and verification keys should be reusable for an unbounded number of theorems without compromising soundness or zero-knowledge. Existing constructions of NIZKs in the preprocessing model (or even the designated-verifier model) that rely on weaker assumptions like one-way functions or oblivious transfer are only secure in a single-theorem setting. Thus, constructing multi-theorem NIZKs in the preprocessing model does not seem to be inherently easier than constructing them in the CRS model. We begin by constructing a multi-theorem preprocessing NIZK directly from context-hiding homomorphic signatures. Then, we show how to efficiently implement the preprocessing step using a new cryptographic primitive called blind homomorphic signatures. This primitive may be of independent interest. Finally, we show how to leverage our new lattice-based preprocessing NIZKs to obtain new malicious-secure MPC protocols purely from standard lattice assumptions