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The recent advent of cloud computing and the IoT has made it imperative to have efficient and secure cryptographic schemes for online data sharing. Data owners would ideally want to store their data/files online in an encrypted manner, and delegate decryption rights for some of these to users with appropriate credentials. An efficient and recently proposed solution in this regard is to use the concept of aggregation that allows users to decrypt multiple classes of data using a single key of constant size. In this paper, we propose a secure and dynamic key aggregate encryption scheme for online data sharing that operates on elliptic curve subgroups while allowing dynamic revocation of user access rights. We augment this basic construction to a generalized two-level hierarchical structure that achieves optimal space and time complexities, and also efficiently accommodates extension of data classes. Finally, we propose an extension to the generalized scheme that allows use of efficiently computable bilinear pairings for encryption and decryption operations. Each scheme is formally proven to be semantically secure. Practical experiments have been conducted to validate all claims made in the paper.

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In the energy sector, IoT manifests in the form of next-generation power grids that provide enhanced electrical stability, efficient power distribution, and utilization. The primary feature of a Smart Grid is the presence of an advanced bi-directional communication network between the Smart meters at the consumer end and the servers at the Utility Operators. Smart meters are broadly vulnerable to attacks on communication and physical systems. We propose a secure and operationally asymmetric mutual authentication and key-exchange protocol for secure communication. Our protocol balances security and efficiency, delegates complex cryptographic operations to the resource-equipped servers, and carefully manages the workload on the resource-constrained Smart meter nodes using unconventional lightweight primitives such as Physically Unclonable Functions. We prove the security of the protocol using well-established cryptographic assumptions. We implement the proposed scheme end-to-end in a Smart meter prototype using commercial-off-the-shelf products, a Utility server, and a credential generator as the trusted third party. Additionally, we demonstrate a physics-based attack named load modification attack on the Smart meter to demonstrate that merely securing the communication channel using authentication does not secure the meter, but requires further protections to ensure the correctness of the reported consumption. Hence, we propose a countermeasure to such an attack that goes side-by-side with our protocol implementation.

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We present the first practically realizable sidechannel assisted fault attack on PRESENT, that can retrieve the last round key efficiently using single nibble faults. The attack demonstrates how side-channel leakage can allow the adversary to precisely determine the fault mask resulting from a nibble fault injection instance. We first demonstrate the viability of such an attack model via side-channel analysis experiments on top of a laser-based fault injection setup, targeting a PRESENT-80 implementation on an ATmega328P microcontroller. Subsequently, we present a differential fault analysis (DFA) exploiting the knowledge of the output fault mask in the target round to recover multiple last round key nibbles independently and in parallel. Both analytically and through experimental evidence, we show that the combined attack can recover the last round key of PRESENT with 4 random nibble fault injections in the best case, and around 7-8 nibble fault injections in the average case. Our attack sheds light on a hitherto unexplored vulnerability of PRESENT and PRESENT-like block ciphers that use bit-permutations instead of maximum distance separable (MDS) layers for diffusion.