For a secure communication through the public network, cryptographic encryption must be applied. The key establishment issue is the most important part in cryptographic encryption. Only the legitimate principals who have the key can encrypt messages and decrypt cryptograms. A secure multi-party communication held through the public network can only take place after a multi-party key establishment protocol has been executed. The multi-party key establishment protocol is designed so that the principals can receive a common key and use it to securely exchange the messages.\nIn this thesis, the secure multi-party key establishment protocols are discussed. W e start from investigating properties of two-party key establishment protocols such as motivation of using session key, adversaries in the protocols, attacks of the protocol and security analysis of the protocol. Then we give the requirements of a multi-party key establishment protocol and summarize about what should be considered in designing a multi-party key establishment protocol. After that we review some of the previous proposed multi-party key establishment protocol. The existing multi-party key establishment protocols are either impractical ( not efficient enough) or missing some important security goals such as authentication of principals. Three new protocols are proposed in this thesis. The first protocol addresses how to hold a conference for members of two hierarchical groups in an efficient way. The message broadcasting is used to reduce the cost of secure message transport.\nThe second and the third protocols manage the authentication problem by using a secret sharing scheme. We replace the principal authentication with the group authentication so that the computational overhead and communication overhead will be reduced especially in reuse of the protocol for new communication. Again, message broadcasting is used for replacing the expansive secure communication channel.\nPart of this thesis appeared in ACSC'99(Australian Computer Science Conference) and ACISP'99(Australian Conference of Information Security and Privacy).
For a secure communication through the public network, cryptographic encryption must be applied. The key establishment issue is the most important part in cryptographic encryption. Only the legitimate principals who have the key can encrypt messages and decrypt cryptograms. A secure multi-party communication held through the public network can only take place after a multi-party key establishment protocol has been executed. The multi-party key establishment protocol is designed so that the principals can receive a common key and use it to securely exchange the messages.\nIn this thesis, the secure multi-party key establishment protocols are discussed. W e start from investigating properties of two-party key establishment protocols such as motivation of using session key, adversaries in the protocols, attacks of the protocol and security analysis of the protocol. Then we give the requirements of a multi-party key establishment protocol and summarize about what should be considered in designing a multi-party key establishment protocol. After that we review some of the previous proposed multi-party key establishment protocol. The existing multi-party key establishment protocols are either impractical ( not efficient enough) or missing some important security goals such as authentication of principals. Three new protocols are proposed in this thesis. The first protocol addresses how to hold a conference for members of two hierarchical groups in an efficient way. The message broadcasting is used to reduce the cost of secure message transport.\nThe second and the third protocols manage the authentication problem by using a secret sharing scheme. We replace the principal authentication with the group authentication so that the computational overhead and communication overhead will be reduced especially in reuse of the protocol for new communication. Again, message broadcasting is used for replacing the expansive secure communication channel.\nPart of this thesis appeared in ACSC'99(Australian Computer Science Conference) and ACISP'99(Australian Conference of Information Security and Privacy).
This chapter considers key establishment protocols and related cryptographic techniques which provide shared secrets between two or more parties, typically for subsequent use as symmetric keys for a variety of cryptographic purposes including encryption, message authentication, and entity authentication. The main focus is two-party key establishment, with the aid of a trusted third party in some cases. While many concepts extend naturally to multi-party key establishment including conference keying protocols, such protocols rapidly become more complex, and are considered here only briefly, as is the related area of secret sharing. Broader aspects of key management, including distribution of public keys, certificates, and key life cycle issues, are deferred to Chapter 13 .
Cryptography grew to be much more than "the study of secret writing". Modern cryptography is concerned with establishing properties such as privacy, integrity and authenticity in protocols for secure communication and computation. This comes at a price: Cryptographic tools usually introduce an overhead, both in terms of communication complexity (that is, number and size of messages transmitted) and computational efficiency (that is, time and memory required). As in many settings communication between the parties involved is the bottleneck, this thesis is concerned with improving communication complexity in cryptographic protocols. One direction towards this goal is scalable cryptography: In many cryptographic schemes currently deployed, the security degrades linearly with the number of instances (e.g. encrypted messages) in the system. As this number can be huge in contexts like cloud computing, the parameters of the scheme have to be chosen considerably larger - and in particular depending on the expected number of instances in the system - to maintain security guarantees. We advance the state-of-the-art regarding scalable cryptography by constructing schemes where the security guarantees are independent of the number of instances. This allows to choose smaller parameters, even when the expected number of instances is immense. - We construct the first scalable encryption scheme with security against active adversaries which has both compact public keys and ciphertexts. In particular, we significantly reduce the size of the public key to only about 3% of the key-size of the previously most efficient scalable encryption scheme. (Gay,Hofheinz, and Kohl, CRYPTO, 2017) - We present a scalable structure-preserving signature scheme which improves both in terms of public-key and signature size compared to the previously best construction to about 40% and 56% of the sizes, respectively. (Gay, Hofheinz, Kohl, and Pan, EUROCRYPT, 2018) Another important area of cryptography is secure multi-party computation, where the goal is to jointly evaluate some function while keeping each party’s input private. In traditional approaches towards secure multi-party computation either the communication complexity scales linearly in the size of the function, or the computational efficiency is poor. To overcome this issue, Boyle, Gilboa, and Ishai (CRYPTO, 2016) introduced the notion of homomorphic secret sharing. Here, inputs are shared between parties such that each party does not learn anything about the input, and such that the parties can locally evaluate functions on the shares. Homomorphic secret sharing implies secure computation where the communication complexity only depends on the size of the inputs, which is typically much smaller than the size of the function. A different approach towards efficient secure computation is to split the protocol into an input-independent preprocessing phase, where long correlated strings are generated, and a very efficient online phase. One example for a useful correlation are authenticated Beaver triples, which allow to perform efficient multiplications in the online phase such that privacy of the inputs is preserved and parties deviating the protocol can be detected. The currently most efficient protocols implementing the preprocessing phase require communication linear in the number of triples to be generated. This results typically in high communication costs, as the online phase requires at least one authenticated Beaver triple per multiplication. We advance the state-of-the art regarding efficient protocols for secure computation with low communication complexity as follows. - We construct the first homomorphic secret sharing scheme for computing arbitrary functions in NC 1 (that is, functions that are computably by circuits with logarithmic depth) which supports message spaces of arbitrary size, has only negligible correctness error, and does not require expensive multiplication on ciphertexts. (Boyle, Kohl, and Scholl, EUROCRYPT, 2019) - We introduce the notion of a pseudorandom correlation generator for general correlations. Pseudorandom correlation generators allow to locally extend short correlated seeds into long pseudorandom correlated strings. We show that pseudorandom correlation generators can replace the preprocessing phase in many protocols, leading to a preprocessing phase with sublinear communication complexity. We show connections to homomorphic secret sharing schemes and give the first instantiation of pseudorandom correlation generators for authenticated Beaver triples at reasonable computational efficiency. (Boyle, Couteau, Gilboa, Ishai, Kohl, and Scholl, CRYPTO, 2019)
Introduction/purpose: The problem of efficient distribution of cryptographic keys in communication systems has existed since its first days and is especially emphasized by the emergence of mass communication systems. Defining and implementing efficient protocols for symmetric cryptographic keys establishment in such circumstances is of great importance in raising information security in cyberspace. Methods: Using the methods of Information Theory and Secure Multiparty Computation, protocols for direct establishment of cryptographic keys between communication parties have been defined. Results: The paper defines two new approaches to the problem of establishing cryptographic keys. The novelty in the protocol defined in the security model based on information theory is based on the source of common randomness, which in this case is the EEG signal of each subject participating in the communication system. Experimental results show that the amount of information leaking to the attacker is close to zero. A novelty in the second case, which provides security with keys at the level of computer security by applying Secure Multiparty Computation, is in the new application field, namely generation and distribution of symmetric cryptographic keys. It is characteristic of both approaches that within the framework of formal theories, it is possible to draw conclusions about their security characteristics in a formal way. Conclusions: The paper describes two new approaches for establishing cryptographic keys in symmetric cryptographic systems with experimental results. The significance of the proposed solutions lies in the fact that they enable the establishment of secure communication between communication parties from end to end, avoiding the influence of a trusted third party. In that way, the achieved communication level security significantly increases in relation to classical cryptographic systems.
Key agreement protocols are used to establish common keys between two or more entities. The established key can then be used to assure confidentiality of exchanged messages through encryption. Additionally, authenticated key agreement protocols offer implicit authentication. In this paper, new authenticated key agreement protocols are proposed and their security properties are investigated. We start by examining the two-party case and then develop extensions to three parties as well as extensions to any group of k parties. The proposed protocols try to keep exchanged messages and computations minimal, while offering adequate security guarantees. Finally, we develop a variant of our multi-party key agreement protocol tailored for secure group communications among mobile devices.
The paper proposes a multi-party key exchange scheme with Perfect Forward Secrecy (PFS) which ensures that a session key derived from a set of long-term public and private keys used in the protocol to authenticate, does not compromise the secrecy of session key. In this protocol, each group establishes connection by communicating through a trusted third party. The trusted party acts as a group controller generates a public key and broadcasts it among the groups. Each group generate their public keys and send to trusted party. After receiving all the keys the group controller shares it among all the parties to generate the session key. Similarly, each group manager of individual group communicates with the members of the corresponding group. The security of the proposed scheme is analysed rigorously and proved to be resistant against non-repudiation attack, replay attack, chosen cipher attack, man-in-the middle attack. The scheme has wider applications such as, sending encrypted e-mail in a military environment to multiple users, securing submitted bids information in online tender, contract signing by many organizations, etc.
Over the last few years, researchers have introduced many group key agreement protocols, aiming to construct new efficient schemes in terms of both computation and communication. Group key agreement protocols aim to derive a common session key among a group of users, which will be used later in order to achieve secure communication between them. In this thesis, we provide a complete analysis of the protocols that have already been introduced in the literature from 2010 until now and require only one round. The main goal of this analysis is to evaluate these protocols in both communication and computation cost, in order to provide a comprehensive comparative analysis. The second part of the thesis is to analyze and evaluate the multi key encapsulation mechanisms that exist in the literature. A multi key encapsulation mechanism is the encryption technique by which a symmetric cryptographic algorithm is combined with an asymmetric one. To be more specific, such a mechanism uses an asymmetric cryptographic algorithm, as a method to encrypt the session key, while the key is derived by a symmetric cryptographic algorithm. Finally, we propose two new group key agreement protocols which use the multi key encapsulation mechanisms, and compare them with existing group key agreement protocols. These constructions are based on a generic methodology for creating a group key exchange protocol by using multi key encapsulation mechanisms. For this reason, emphasis has been given to both one round group key agreement protocols and multi key encapsulation mechanisms, in order to examine the possibility of combining them