Cryptography plays an integral role in secure communication and is usually the strongest link in the chain of security. Yet security problems abound in electronic communication: spyware, phishing, denial of service, and side-channel attacks are still major concerns. The main goal in this thesis is to consider how cryptographic techniques can be extended to offer greater defence against these non-traditional security threats. \n \nIn the first part of this thesis, we consider problems in classical cryptography. We introduce multi-factor password-authenticated key exchange which allows secure authentication and key agreement based on multiple short secrets, such as a long-term password and a one-time response; it can provide an enhanced level of assurance in higher security scenarios because a multi-factor protocol is designed to remain secure even if all but one of the factors has been compromised due to attacks such as phishing or spyware. Next, we consider the integration of denial of service countermeasures with key exchange protocols: by introducing a formal model for denial of service resilience that complements the extended Canetti-Krawczyk model for secure key agreement, we cover a wide range of existing denial of service attacks and prevent them by carefully using client puzzles. Additionally, we look at how side-channel attacks affect certain types of formulae used in elliptic curve cryptography, and demonstrate that information leaked during field operations such as addition, subtraction, and multiplication can be exploited by an attacker. \n \nIn the second part of this thesis, we examine cryptography in the quantum setting. We argue that quantum key distribution will have an important role to play in future information security infrastructures and will operate best when integrated with the powerful public key infrastructures that are used today. Finally, we present a new look at quantum money and describe a quantum coin scheme where the coins are not easily counterfeited, are locally verifiable, and can be transferred to another party.
Cryptography plays an integral role in secure communication and is usually the strongest link in the chain of security. Yet security problems abound in electronic communication: spyware, phishing, denial of service, and side-channel attacks are still major concerns. The main goal in this thesis is to consider how cryptographic techniques can be extended to offer greater defence against these non-traditional security threats. \n \nIn the first part of this thesis, we consider problems in classical cryptography. We introduce multi-factor password-authenticated key exchange which allows secure authentication and key agreement based on multiple short secrets, such as a long-term password and a one-time response; it can provide an enhanced level of assurance in higher security scenarios because a multi-factor protocol is designed to remain secure even if all but one of the factors has been compromised due to attacks such as phishing or spyware. Next, we consider the integration of denial of service countermeasures with key exchange protocols: by introducing a formal model for denial of service resilience that complements the extended Canetti-Krawczyk model for secure key agreement, we cover a wide range of existing denial of service attacks and prevent them by carefully using client puzzles. Additionally, we look at how side-channel attacks affect certain types of formulae used in elliptic curve cryptography, and demonstrate that information leaked during field operations such as addition, subtraction, and multiplication can be exploited by an attacker. \n \nIn the second part of this thesis, we examine cryptography in the quantum setting. We argue that quantum key distribution will have an important role to play in future information security infrastructures and will operate best when integrated with the powerful public key infrastructures that are used today. Finally, we present a new look at quantum money and describe a quantum coin scheme where the coins are not easily counterfeited, are locally verifiable, and can be transferred to another party.
Recent advances in quantum technology and the potential that practical quantum computers may become a reality in the future have led to renewed interest in developing cryptographic technologies that are secure against conventional and quantum attacks. Currently, virtually all asymmetric cryptographic schemes in use are threatened by the potential development of powerful quantum computers. Post-quantum cryptography is one of main the ways to combat this threat. Its security is based on the complexity of mathematical problems that are currently considered unsolvable efficiently, even with the help of quantum computers. The security of information systems is ensured through protection against various threats that use system vulnerabilities. Security protocols are the building blocks of secure communication. They implement security mechanisms to provide security services. Security protocols are considered abstract when analyzed, but may have additional vulnerabilities in implementation. This work contains a holistic study of security protocols. Basics of security protocols, taxonomy of attacks on security protocols and their implementation are considered, as well as various methods and models of protocol security analysis. In particular, the differences between information-theoretic and computational security, computational and symbolic models are specified. In addition, an overview of the computational security models for Authenticated Key Exchange (AKE) and Password Authentication Key Exchange (PAKE) protocols is provided. The most important security models for the AKE and PAKE protocols were also described. With the emergence of new technologies that may have different security requirements, as well as with increased opportunities for competition, there is always a need to develop new protocols. Thus, the purpose of this article is to review, classify, analyze, and research the vulnerabilities of information systems from classical, quantum, and special attacks, performed taking into account the forecast regarding the possibilities of attacks on post-quantum cryptographic transformations; studying security assessment models for existing cryptographic protocols, as well as reviewing and benchmarking security models and providing suggestions for protection against existing potential attacks.
Today's information society relies on cryptography to achieve security goals such as confidentiality, integrity, authentication, and non-repudiation for digital communications. Here, public-key cryptosystems play a pivotal role to share encryption keys and create digital signatures. However, quantum computers threaten the security of traditional public-key cryptosystems as they can tame computational problems underlying the schemes, i.e., discrete logarithm and integer factorization. The prospective arrival of capable-enough quantum computers already threatens today's secret communication in terms of their long-term secrecy when stored to be later decrypted. Therefore, researchers strive to develop and deploy alternative schemes.In this work, we evaluate a key exchange protocol based on combining public-key schemes with physical-layer security, anticipating the prospect of quantum attacks: If a powerful quantum attacker cannot immediately obtain a private key, legitimate parties have a window of short-term secrecy to perform a physical-layer jamming key exchange (JKE) to establish a long-term shared secret. Thereby, the protocol constraints the computation time available to the attacker to break the employed public-key cryptography. In this paper, we outline the protocol, discuss its security, and point out challenges to be resolved.
Key exchange protocols, first introduced by Diffie and Hellman in 1976, are one of the most widely-deployed cryptographic protocols. They allow two parties, that have never interacted before, to establish shared secrets. These shared cryptographic keys may subsequently be used to establish a secure communication channel. Use cases include the classic client-server setting that is for example at play when browsing the internet, but also chats via end-to-end-encrypted instant messaging applications. Security-wise, we generally demand of key exchange protocols to achieve key secrecy and authentication. While, informally, authentication ensures that the communicating parties have confidence in the identity of their peers, key secrecy ensures that any shared cryptographic key that is established via the key exchange protocol is only known to the participants in the protocol and can be used securely in cryptographic protocols, i.e., is sufficiently random. In 1993, Bellare and Rogaway gave a first formalization of key exchange protocol security that captures these properties with respect to powerful adversaries with full control over the network. Their model constitutes the basis of the many subsequent treatments of authenticated key exchange security, including the models presented in this thesis. The common methodological approach underlying all of these formalizations is the provable security paradigm, which has become a standard tool in assessing the security of cryptographic protocols and primitives. So-called security models specify the expected security guarantees of the scheme in question with regards to a well-defined class of adversaries. Proofs that validate these security claims do so by reducing the security of the overall scheme to the security of the underlying cryptographic primitives and hardness assumptions. However, advances in computational power and more sophisticated cryptanalytic capabilities often render exactly these components insecure. Especially the advent of quantum computers will have a devastating effect on much of today's public key cryptography. This is especially true for key exchange protocols since they rely crucially on public-key algorithms. In this thesis, our focus in future-proofing key exchange protocols is two-fold. First, we focus on extending security models for key exchange protocols to capture the (un)expected break of cryptographic primitives and hardness assumptions. The aim is to gain assurances with respect to future adversaries and to investigate the effects of primitive failures on key exchange protocols. More specifically, we explore how key exchange protocols can be safely transitioned to new, post-quantum secure algorithms with hybrid techniques. Hybrids combine classical and post-quantum algorithms such that the overall key agreement scheme remains secure as long as one of the two base schemes remains secure. For this, we introduce security notions for key encapsulation mechanisms that account for adversaries with varying levels of quantum capabilities and present three new constructions for hybrid key encapsulation mechanisms. Our hybrid designs are practice-inspired and for example capture draft proposals for hybrid modes in the Transport Layer Security (TLS) protocol, which is one of the most widely-deployed cryptographic protocols that enables key agreement. Furthermore, our notion of breakdown resilience for key exchange protocols allows to gauge the security of past session keys in the event of a failure of a cryptographic component in the key exchange. We exercise our model on variants of the post-quantum secure key exchange protocol NewHope by Alkim et al. Thereby, we confirm the intuition that, in order to guard against adversaries that only have access to quantum computing power in the (more distant) future, it is sufficient to use classically-secure authentication mechanisms alongside post-quantum key agreement to achieve authenticated key exchange. As with any mathematical statement, theorems in the provable security paradigm are only as valid as the underlying assumptions. A careful consideration of any newly made assumption is thus essential to ensure the meaningfulness of the statement itself and make the assumption a viable tool for future analyses. Thus, secondly, we systematically classify the PRF-ODH assumption, a complexity-theoretic hardness assumption that has been used in key exchange security analyses of such prominent protocols as TLS, Signal, and Wireguard. In particular, we give a unified, parametrized definition of the assumption encompassing different variants that are present in the literature. We relate the resulting parametrized notions in terms of their strength and show where these assumptions fit in the collection of well-understood related hardness assumptions. We finally sketch our result on the impossibility of instantiating this assumption in the standard model, thereby disposing of the uncertainty in the community whether PRF-ODH is in fact a standard model assumption, i.e., removes the usage of some idealized assumptions in key exchange protocol proofs.
Cryptography is the technique of masking the message by means of any encryption method so that intended entity can decrypt the contents. Cryptanalysis is the art of retrieving plain text from cipher text with no prior information about technique or key used for encrypting the message. An intruder can attack in many ways to access the communication channel like impersonating, non-repudiation, denial of services, modification of data, threatening confidentiality and breaking availability of services. Thus, the security of any cryptographic schemes lies on the strength of particular hard problem like the integer factorization problem, the discrete logarithm problem in the multiplicative subgroup and on an elliptic curve. Nonetheless, with increased speed and memory of computers and stronger cryptanalytic algorithms, the two problems discrete logarithm problem in the multiplicative subgroup and the integer factorization problem, earlier considered to be strong and hard can be cracked in sub exponential time. Elliptic curve DLP has advantages over the other two: it generates shorter keys and is still an exponentially hard problem for classical and traditional computers. The circumstances will change if stronger attacks are discovered or fast computers are invented. Hence, researchers are predicting, quantum computers could solve Elliptic Curve Discrete Logarithm Problem in polynomial time. Researchers are working in the area to identify hard problems for quantum computers. Thus, the focus of the papers is on various available classical and quantum cryptographic schemes and its hardness with respect to classical and quantum computers respectively.
Certificateless public-key cryptography has conquered both the certificate management problem in the traditional public-key cryptography and the key escrow problem in the ID-based public-key cryptography. Certificateless authenticated key exchange (CLAKE) protocol is an important primitive of the certificateless public-key cryptography. A CLAKE protocol is employed to provide both mutual authentication and establishing a session key between two participators. Indeed, all conventional public-key cryptographies have encountered a new kind of attack, named “side-channel attacks”. Fortunately, leakage-resilient cryptography is a flexible approach to withstand such attacks. However, the design of leakage-resilient CLAKE (LR-CLAKE) protocols is not studied. In the article, by extending the well-known extended-Canetti-Krawczyk (eCK) model, we present the security notions (adversary model) of LR-CLAKE protocols, called continual-leakage-resilient eCK (CLReCK) model. The first LR-CLAKE protocol withstanding side-channel attacks is proposed. By employing the proof technique of the generic bilinear group (GBG) model, we formally prove the security of our protocol in the CLReCK model.
The realm of this thesis is cryptographic protocol theory in the quantum world. We study the security of quantum and classical protocols against adversaries that are assumed to exploit quantum effects to their advantage. Security in the quantum world means that quantum computation does not jeopardize the assumption, underlying the protocol construction. But moreover, we encounter additional setbacks in the security proofs, which are mostly due to the fact that some well-known classical proof techniques are forbidden by certain properties of a quantum environment. Interestingly, we can exploit some of the very same properties to the benefit of quantum cryptography. Thus, this work lies right at the heart of the conflict between highly potential effects but likewise rather demanding conditions in the quantum world.