A new model for a Client-Server Communication (CSC) system satisfying information theoretic security is proposed, and its fundamental properties are discussed. Our CSC allows n users to upload their respective messages to a server securely by using symmetric key encryptions with their own keys, and all ciphertexts are decrypted by the server. If we require all messages to be perfectly secure in CSC against the corrupted clients and adversaries without any keys, it is proved that a one time pad or more inefficient encryption must be used for each communication link between a client and the server. This means that, in order to realize more efficient CSC, it is necessary to leak out some information of each message. Based on these observations, we introduce a new model for such a secure CSC formally, and discuss its fundamental properties. In addition, we propose the optimal construction of CSC under several constraints on security parameters called security rates.
A new model for a Client-Server Communication (CSC) system satisfying information theoretic security is proposed, and its fundamental properties are discussed. Our CSC allows n users to upload their respective messages to a server securely by using symmetric key encryptions with their own keys, and all ciphertexts are decrypted by the server. If we require all messages to be perfectly secure in CSC against the corrupted clients and adversaries without any keys, it is proved that a one time pad or more inefficient encryption must be used for each communication link between a client and the server. This means that, in order to realize more efficient CSC, it is necessary to leak out some information of each message. Based on these observations, we introduce a new model for such a secure CSC formally, and discuss its fundamental properties. In addition, we propose the optimal construction of CSC under several constraints on security parameters called security rates.
Advancements in Internet of Things devices allow for the collection and analysis of big data. Moreover, cloud computing has emerged as an ideal platform for big data analysis because it quickly provides computing resources for analysis at scale. However, concerns exist regarding the privacy and security of this information. Secure computation is a technology that enables computation while maintaining data encryption. In this study, we present a new secure computation based on a client-server model, in which a set of servers perform computations using the inputs of multiple clients. We adopt the(k,n)threshold secret sharing approach, where an inputsis divided intonshares and can be recovered from shares with a thresholdk. However, conventional secure computation using(k,n)threshold secret sharing generally requires the conditionn≥2k−1and communication among multiple servers for each multiplication. To the best of our knowledge, no previous study has solved this problem completely. We demonstrate that processes that require communication can be concentrated in the preprocessing phase and propose a secure computation using(k,n)threshold secret sharing that does not require communication during the secure computation phase, even when multiplication is performed. Moreover, we show that the number of communications does not depend on the number of multiplications, unlike in conventional methods. As communication often requires more processing time than the actual secure computation, our method makes it possible to realize faster overall processing than conventional methods. We also present an extensive security analysis and experimental simulation of the proposed method. Our proposed method achieves information-theoretic security against semi-honest adversaries under certain conditions withn<2k−1.
Secure communication ensures the integrity and confidentiality of communication between connected devices. An information-theoretic approach to secure communication gives the strongest confidentiality guarantee by assuming that the attacker has unlimited computing power. The earliest formal model and definition of information-theoretic secure communication is by Shannon, who employed a secret key shared between communicating parties to provide confidentiality. An alternative elegant information-theoretic approach to secure communication views the natural characteristics of the environment (i.e., channel’s noise) as a resource to build security functionalities. This approach was first proposed by Wyner, and the corresponding secure communication model is called the wiretap channel model. These two approaches introduce two primary resources for providing information-theoretic secure communication: the shared secret key and physical properties of the communication medium. In this thesis, we study how to employ the above two resources for secure message transmission. We study this by using channel’s noise in the wiretap channel model. In this model, a sender is connected to the receiver and the adversary through two noisy channels. We propose a new wiretap encoding scheme with strong secrecy that provides perfect secrecy and reliability, asymptotically. The construction treats the noise in the adversary’s channel as a source of randomness that is extracted and used to hide the message from the adversary. We realize the wiretap channel model using cooperative jamming to evaluate the performance of wiretap codes in practice. We consider a model called keyed wiretap channel that unifies Wyner’s model with Shannon’s model of perfect secrecy for information systems, and propose a keyed encoding schemes with strong secrecy and other properties that are attractive in practice. We also study two-party information-theoretic secret key agreement when the two parties have access to samples of a common source of randomness and use a single message transmission to arrive at a shared random key. We propose a secret key agreement protocol in this setting, prove its security, and show its superior performance compared to other known protocols with the same properties. Finally, we propose an information-theoretic secret key agreement over a virtual wiretap channel created by cooperative jamming.
We consider secure two-party computation in the client-server model. In our scenario, two adversaries operate separately but simultaneously, each of them corrupting one of the parties and a restricted subset of servers that they interact with. We model security in this setting via the local universal composability framework introduced by Canetti and Vald and show that informationtheoretically secure two-party computation is possible if and only if there is always at least one server which remains uncorrupted. Moreover, in our protocols each of the servers only needs to communicate with the two clients, i.e. no messages are exchanged directly between servers. This communication pattern is minimal.
A hybrid encryption scheme consists of a public-key part called the key encapsulation mechanism (KEM), that is used to generate and establish a shared secret key between two parties, and a (symmetric) secret-key part called the data encapsulation mechanism (DEM) that encrypts the data using the shared key. Hybrid encryption schemes are widely used for securing Internet communication. In this paper, we initiate the study of hybrid encryption in preprocessing model which assumes access to initial correlated variables by all parties (including the eavesdropper), and define information-theoretic KEM (iKEM) that together with a (computationally) secure DEM, results in a hybrid encryption scheme in preprocessing model. We define security of each building block, and prove a composition theorem that guarantees (computational) q-chosen-plaintext attack (CPA) security of the hybrid encryption system if the iKEM and the DEM satisfy q-chosen-encapsulation attack security and one-time security, respectively. We show that an iKEM can be realized by an information-theoretic one-way secret key agreement (OW-SKA) protocol where a single message is transmitted from Alice to Bob, with a new security definition that allowsqqueries to Alice. Using a OW-SKA that satisfies this new definition of security, effectively allows the established secret key to be used with a onetime symmetric key encryption system that can be implemented, for example, by XORing the output of a (computationally) secure pseudorandom generator with the message, to provide secure encryption ofqarbitrary messages (polynomially bounded length). We discuss our results and directions for future work.
In this paper, we consider encryption systems with two-out-of-two threshold decryption, where one of the parties (the client) initiates the decryption and the other one (the server) assists. Existing threshold decryption schemes disclose to the server the ciphertext that is being decrypted. We give a construction, where the identity of the ciphertext is not leaked to the server, and the client's privacy is thus preserved. While showing the security of this construction, we run into the issue of defining the security of a scheme with blindly assisted decryption. We discuss previously proposed security definitions for similar cryptographic functionalities and argue why they do not capture the expected meaning of security. We propose an ideal functionality for the encryption with server-supported blind threshold decryption in the universal composability model, carefully balancing between the meaning of privacy, and the ability to implement it. We construct a protocol and show that it is a secure implementation of the proposed functionality in the random oracle model.
The server-aided model for multiparty computation (MPC) was introduced to capture a real-world scenario where clients wish to off-load the heavy computation of MPC protocols to dedicated servers. A rich body of work has studied various trade-offs between security guarantees (e.g., semi-honest vs malicious), trust assumptions (e.g., the threshold on corrupted servers), and efficiency. However, all existing works make the assumption that all clients must agree on employing the same servers, and accept the same corruption threshold. In this paper, we challenge this assumption and introduce a new paradigm for server-aided MPC, where each client can choose their own set of servers and their own threshold of corrupted servers. In this new model, the privacy of each client is guaranteed as long as their own threshold is satisfied, regardless of the other servers/clients. We call this paradigm per-party private server-aided MPC to highlight both a security and efficiency guarantee: (1) per-party privacy, which means that each party gets their own privacy guarantees that depend on their own choice of the servers; (2) per-party complexity, which means that each party only needs to communicate with their chosen servers. Our primary contribution is a new theoretical framework for server-aided MPC. We provide two protocols to show feasibility, but leave it as a future work to investigate protocols that focus on concrete efficiency.