A Distributed Secret Sharing Protocol (DSSP) enables a dealer to distribute multiple secrets among a set of users by placing shares on storage nodes scattered across a public network, so that each user can reconstruct exactly one secret by accessing its designated nodes. Two security notions are considered: weak secrecy, which prevents a user from learning any other individual secret, and perfect secrecy, which prevents a user from gaining any information about the collection of all other secrets. We study perfectly-secure graph-based DSSPs, where each user accesses exactly two storage nodes. We determine the exact optimal storage overhead for trees, stars, paths, cycles, and complete graphs, and provide matching constructions. For arbitrary graphs, we present a decomposition construction that uses protocols on simpler subgraphs as building blocks. Our results show that perfect secrecy has no additional storage cost over weak secrecy for acyclic graphs, while it strictly increases the overhead for graphs containing cycles.

Hierarchical Key Assignment Schemes (HKAS) are cryptographic tools that enforce access control in partially ordered hierarchies, allowing each class to derive the keys of all classes below it. All existing schemes assume that the hierarchy structure is publicly known. In many modern applications, however, the topology of the access hierarchy is itself sensitive information that should be protected. We study the problem of extending existing HKAS to hide the hierarchy topology from adversaries who corrupt a subset of classes, without redesigning the underlying schemes from scratch. We formalize this goal through new security definitions capturing graph-hiding and topology privacy, and we propose a simple generic compiler that transforms any HKAS satisfying a natural anonymizability condition into one that achieves these stronger guarantees. Our main technical contribution is the identification of this condition, which requires that the joint distribution of the public values of the scheme be computationally indistinguishable from independently simulated random values. We show that this condition is not implied by the standard indistinguishability (IND-ST) security notion alone, by exhibiting a concrete counterexample based on the Akl-Taylor scheme. On the positive side, we verify that the Encryption Based Construction (EBC) and the Dynamic Encryption Based Construction (DEBC) of De Santis et al. both satisfy our condition and can therefore be made topology-private through our lightweight transformation.

In this paper we consider evolving 2-threshold secret sharing schemes. In such schemes, the number of participants grows over time and is potentially unbounded, any two participants reconstruct the secret, and no single participant can figure out any partial information about it. They are referred to as (2,infinity) -threshold secret sharing schemes. The cost of a (2,infinity) -threshold secret sharing scheme can be measured as the maximum, over all possible n >= 2 , of the ratio between the sum of the lengths of the shares for the first n participants and the sum of the lengths of the shares for a (standard) optimal (2,n) -threshold secret sharing scheme. It is known that such a cost measure is lower bounded by 3/2 . Moreover, currently, the best known (2,infinity) -threshold secret sharing scheme has cost 1.59375. Our contribution improves the state-of-the-art in several ways:center dot We describe a new (2,infinity) -threshold secret sharing scheme whose cost is 1.5859375, improving on the previous best known scheme. center dot Motivated by the fact that in some applications one knows a lower bound on the number of participants, we generalize the cost measure, by considering the maximum over all possible n >= z0 , where z0 is any integer greater than or equal to 2. center dot We provide constructions of optimal schemes for the generalized cost measure and through a theoretical analysis we prove some interesting properties for the lower bound of the cost. center dot By using algorithmic techniques, for reasonably small cases, we exhaustively study the problem of finding tight lower bounds. In particular, we obtain a lower bound of 1.534375, improving the lower bound of 3/2 . We close the paper summarizing our findings and discussing some open issues.

Hierarchical structures are frequently used to manage access to sensitive data in various contexts, ranging from organizational settings to IoT networks. A Hierarchical Key Assignment Scheme (HKAS) is designed to cryptographically enforce access control in hierarchical structures. It operates by assigning secrets and encryption keys to a set of classes within a partially ordered hierarchy. This approach ensures that the secret of a higher-level class can be used to efficiently derive keys for all classes positioned at a lower level in the hierarchy. In this paper, we introduce a novel cryptographic primitive that we name HKAS with Key Rotation (KR-HKAS). This extension enhances the current HKAS framework by enabling a provably secure mechanism for periodically rotating both encryption keys and secrets, without necessitating a complete setup reset. This proactive approach effectively mitigates the risk of security breaches due to compromised cryptographic material, aligning with the best security practice.

Alphabetic codes and binary search trees are combinatorial structures that abstract search procedures in ordered sets endowed with probability distributions. In this paper, we design new linear-time algorithms to construct alphabetic codes, and we show that the obtained codes are not too far from being optimal. Moreover, we exploit our results on alphabetic codes to provide new bounds on the average cost of optimal binary search trees. Our results improve on the best-known bounds on the average cost of optimal binary search trees present in the literature.

The rapid progress of Internet technology has led to a strong increase in the use of online social networks for disseminating information on the Internet. In this scenario, it is crucial to establish approaches that can effectively reduce the diffusion of false information (fake news) that can potentially cause harm to society. A defensive approach, based on integer-order differential equations, has been recently developed to analyze the effects of verification and blocking of users for containing the spread of fake news. Starting from it, we introduce a novel fractional model providing a more accurate, powerful, and realistic representation of the transmission of fake news messages. The model aims to predict the spread of such messages, by better considering the effect of the system’s status evolution over time. The use of fractional differential equations to schematize the propagation of fake news results in incorporating a greater amount of memory information and better considering hereditary properties of the system of interest, also capturing its hidden nonlinear dynamics, mainly related to fractality and multiscale nature.

A one-message unilateral entity authentication scheme allows one party, called the prover , to authenticate himself, i.e., to prove his identity, to another party, called the verifier , by sending a single authentication message . We consider schemes where the prover and the verifier do not share any secret information, such as a password, in advance. We propose the first theoretical characterization for one-message unilateral entity authentication schemes, by formalizing the security requirements for such schemes with respect to different kinds of passive and active adversarial behaviours. In particular, we consider both static and adaptive adversaries for each kind of attack (passive/active). Afterwards, we explore the relationships between the security notions resulting from different adversarial behaviours for one-message unilateral entity authentication schemes. Finally, we propose three different constructions for one-message unilateral entity authentication schemes and we analyze their security with respect to the different definitions introduced in this paper.

In this paper, we introduce an information-theoretic approach to the access control problem within a scenario where a trusted central authority is tasked with user registration, and a set of guards is responsible for granting anonymous access to a restricted resource. More precisely, we consider access schemes with centralized user registration, where a trusted authority is responsible for the generation of access tokens assigned to users, while preserving user anonymity with respect to the guards. We first propose an information-theoretic model for anonymous access schemes with centralized user registration, then we show a lower bound on the size of the private information that each guard has to store. Finally, we propose a simple and optimal construction for anonymous access schemes with centralized registration.

Distributed Secret Sharing is a (multi) secret sharing model in which the shares are distributed over storage nodes of a network and each participant is able to reconstruct a specific secret by accessing a subset of the storage nodes. In this work, we provide new Distributed (multi) Secret Sharing Protocols for a specific class of access structures, namely those that can be described with a graph. The protocols improve on previous results allowing a faster encoding and decoding phase while maintaining optimal storage requirements. Moreover, our protocols can manage any kind of graph, while previous protocols have been designed only for complete graphs, and we provide a complete characterization of graph-based protocols. We also prove some tight bounds on the size of the information held in the storage nodes and communication complexity by using an information-theoretic approach. Finally, we also introduce a computationally secure technique for the general case that allows improvements in the size of the needed disk space if secrecy is computational, that is, if the scheme is robust against resource-bounded adversaries.

A Distributed Secret Sharing Protocol ( DSSP for short) allows a dealer to share multiple secrets among a set of users by storing the shares on storage nodes distributed over a public insecure network. Users can later download the shares from the nodes they can access, in order to reconstruct the secrets (each user is allowed to reconstruct exactly one secret). In this paper we propose three contributions: we first carefully analyze the security requirements for DSSPs and we show that if the shared secrets are statistically dependent, then no DSSP satisfying either the weak secrecy requirement or the perfect secrecy one can exist. Then, we propose two new definitions of security for DSSPs, which also take into account the statistical dependencies among the secrets. Afterwards, we consider DSSPs for a specific class of access structures, i.e., those which can be represented by a graph. In particular, we propose a protocol which can be used to share independent secrets having different sizes. The protocol can manage any kind of graph and generalizes previous protocols for secrets having the same sizes, while maintaining optimal storage requirements.

We define and implement a novel side-channel attack that exploits a smartphone's accelerometer to eavesdrop entire words that the device itself is reproducing through its loudspeakers. The proposed approach consists of two modules: (i) a deep learning-based system that, using a Convolutional Neural Network (CNN), learns to recognize a set of significant speech units, using the spectrogram representation of the corresponding acceleration signals; (ii) an evolutionary-based segmentation method that, given the accelerometer measurements corresponding to an input speech, finds the best way to split it so that the proposed CNN maintains a high classification performance on each of the segments obtained, guarantying the recognition of a significant percentage of words from the original speech. Results of experiments performed to assess the effectiveness of the proposed attack, show its ability to recognize a percentage of words which is higher for short speeches and diminishes as the speeches get longer. We experimented with speeches of lengths ranging from 5 to 60 s, obtaining a recognition percentage going from about 80% for the shortest speeches, down to about 54% for the longest ones.

In the context of the Internet of Things (IoT), the proliferation of identity spoofing threats has led to the need for the constant entity verification of devices. Recently, a formal framework has been proposed to study resistance to impersonation attacks for One-Message Unilateral Entity Authentication (OM-UEA) schemes, in which the prover continuously authenticates itself through the use of a sequence of authentication messages. Given the limited computing power of the parties (particularly the prover) and the often limited bandwidth channel, in the IoT scenario it is desirable to design unilateral entity authentication schemes that require the use of a single message per session and light computations. In this paper, we first show that OM-UEA schemes can be implemented through digital signatures and that a weak form of unforgeability is sufficient to achieve security against active adversaries. We then apply the signature scheme proposed by Yang et al. in ASIACCS 2020 to our framework, resulting in an OM-UEA scheme that requires minimal computational effort and low storage requirements for the prover. Inspired by this last construction, we propose an OM-UEA scheme based on the hardness of the discrete logarithm problem, which further improves the computational performance for the prover. Our findings offer feasible options for implementing secure continuous entity authentication in IoT applications.

In Distributed Secret Sharing schemes, secrets are encoded with shares distributed over multiple nodes of a network. Each involved party has access to a subset of the nodes and thus to a subset of the shares and is able to reconstruct a specific secret. Usually, these schemes are evaluated by measuring the required storage overhead, as well as the encoding and decoding complexities. In this paper, we provide new Distributed (multi) Secret Sharing Protocols for $(k,n)$ -threshold access structures that improve on previous results, characterized by nearly-optimal storage overhead, achieving both storage optimality and a better encoding/decoding complexity. The protocols are also simpler than previous ones and allow for easier encoding.

Over the last years, our lifestyle has been positively upset by the sudden advent of technology. The Internet of Things (IoT), offering universal and ubiquitous connectivity to both people and objects, revealed to be the silver bullet for enabling a vast number of previously unexpected applications. In particular, media streaming providers are growing in business and scope, and we can forecast that soon, video streaming will substitute TV broadcasting activities. With the increasing success of multi-user smart environments, empowered by new-generation smart devices and IoT architectures, multimedia contents (i.e., images and videos) need to be effectively accessed anytime and anywhere. Recent advances in computer vision technologies have made the development of intelligent monitoring systems for video surveillance and ambient-assisted living. Such a scenario permits better integration among technologies, multimedia content, and end-users. However, there are several challenges, and some are still open. More precisely, due to the sensitivity of some multimedia content (e.g., video-surveillance streams), it is paramount to preserve users’ privacy. Again, it is necessary to guarantee the integrity of usage rights during any multimedia transmission process, starting from the video encoding phase. In this way, the private content is disclosed only when the stream is decoded on the other endpoint, by the legitimate user. In this article, we present a secure video transmission strategy that can address the challenges mentioned above. The proposed strategy takes advantage of both watermarking and video scrambling techniques to make it possible for the secure and privacy-preserving transmission of multimedia streaming. Through our proposal, multimedia streaming is of low quality and thus unusable. However, it can be fully recovered and enjoyed only by authorized users. Finally, due to its low complexity and energy-efficiency, our proposal is particularly suitable for onboard implementations.

Most smartphones are equipped with an accelerometer sensor. There are numerous scenarios in which this sensor can be very useful. However it can also represent a privacy threat. Indeed, the measurement of the device vibrations can be exploited to detect private information. The attack can be favored by the fact that this specific sensor is normally not considered a “dangerous” one and also by the fact that the measurements of today’s sensors are quite accurate. Recently many research studies have focused on the task of inferring information from the accelerometer measurements. There are several settings that can be considered and several final goals; in this paper we consider the specific case of recognizing words that the device itself is reproducing through its loudspeakers. A recent paper has considered this scenario and has proposed a recognizer, based on Convolutional Neural Networks, for single digits, single letters and a small set of “hot words”. Following such a research direction, in this paper, we provide an improved recognizer for single letters and digits. We performed an evaluation study to assess the effectiveness of the proposed attack. Results show that the system outperforms the previous approach. We also propose a generalization whose goal is that of recognizing entire words, or even sentences, not by means of a dictionary, but by first recognizing syllables and then locate sequences of syllables that correspond to words. We provide preliminary results in this direction.

We propose a new design technique for constructing secret sharing schemes over a potentially infinite set of participants. Our findings leverage on a nice property of secret sharing schemes for finite sets of participants based on the Chinese remainder theorem: the possibility of providing shares of different sizes to participants. We successful apply the technique to the (3,∞)-threshold access structure. The scheme we exhibit improves over the best construction currently available. Most importantly, the idea underlying the technique is of independent interest. Hopefully, it could be employed for other access structures, and in other areas of secure computation for potentially infinite sets of players.

There is an increasing involvement of the Internet of Things (IoT) in many of our daily activities, with the aim of improving their efficiency and effectiveness. We are witnessing the advent of smart cities, in which IoT is exploited to improve the management of a city's assets, as well as smart factories, where IoT is paving the way for the forth industrial revolution. These applications and many other ones imply several non-functional requirements to be satisfied by the adopted IoT solution, where security assumes paramount importance. Secure communications among the IoT nodes are strongly needed due to the use of wireless technologies that are easy to eavesdrop, in order to steal valuable information. Accordingly, confidentiality is a fundamental prerequisite, but the existing solutions based on transport-level encryption are ineffective, while the ones with application-level encryption may be too expensive in terms of energy consumption. In this work, we propose a series of solutions and methods to achieve confidentiality with end-to-end guarantees, by using group-based keys within the context of a clustered and distributed key management framework. We have implemented such solutions on top of TinyOS, and assessed their achievable quality by means of the TOSSIM simulator.

We present the design of an outdoor position certification authority. Such an authority aims at certifying the geolocalisation of a mobile device equipped with a global navigation satellite system receiver. Such a receiver is capable of acquiring radio signals (low-level data) and navigation messages (high-level data) in outdoor environments coming from different constellations of global/regional satellite navigation systems and satellite-based augmentation systems. To date, this information is unreliable from a security point of view because it can be easily forged by malicious attackers through specialised spoofing techniques. An outdoor position certification authority defines a client/server architecture through which a user can certify his position by sending the geolocalisation information needed to verify it to one or more remote servers. There are several scenarios for which this service can be very useful and, with the advent of the internet of things age, devices that might require such a service will grow in number.

SummaryConventional privacy‐enforcement mechanisms, such as encryption‐based ones, are frequently used to prevent third‐party eavesdroppers to intercept confidential information exchanged between two or more parties. However, the use of such mechanisms can be perceivable and it alerts the involved intercepting entities that could devote some effort in trying to remove the protection, eg, by cracking the encryption keys used or by exploiting the vulnerabilities of the technological solution used to protect the data. Sometimes, from the security point of view, avoiding to draw the attention or suspect to intermediate intercepting entities, may be better than protecting a data in a conventional manner. In such direction, one of the most effective approaches is hiding the secret information to be exchanged inside other data, through steganographic techniques. In this work, we exploit, for this specific purpose, the hierarchical structure of a compressed archive, as well as the algorithms and parameters used to create and maintain such archive. It is important to point out that, by doing this, the secret information is in no way semantically related to the contents of the compressed archive. This can be extremely useful in many cloud‐based situations where several confidential data is moved across multiple independent data center, which are under the control of different and not always fully trusted authorities. The effectiveness of this proposal has been assessed by using a properly designed and implemented prototype, where extensive tests have been performed within the context of a proof‐of‐concept.