We consider session key agreement (SKA) protocols operating in a public key infrastructure, with pre-specified peers, that take no session ID as input, and output only a session key. Despite much work on SKA, we argue that there is no good definition of security for this (very natural) protocol syntax. The difficulty is that in this setting the adversary may not be able to tell which processes share a key, and thus which session keys may be revealed without trivializing the security condition. We consider security against adversaries that control all network traffic, can register arbitrary public keys, and can retrieve session keys. We do not attempt to mitigate damage from hardware failures, such as session-state compromise, as we aim to improve our understanding of this simpler setting. We give two natural but substantially different game based definitions of security and prove that they are equivalent. Such proofs are rare for SKA. The bulk of this proof consists of showing that, for secure protocols, only compatible processes can be made to share a key. This property is very natural but surprisingly subtle. For comparison, we give a version of our definition in which processes output session IDs and we give strong theorems relating these two
We consider session key agreement (SKA) protocols operating in a public key infrastructure, with pre-specified peers, that take no session ID as input, and output only a session key. Despite much work on SKA, we argue that there is no good definition of security for this (very natural) protocol syntax. The difficulty is that in this setting the adversary may not be able to tell which processes share a key, and thus which session keys may be revealed without trivializing the security condition. We consider security against adversaries that control all network traffic, can register arbitrary public keys, and can retrieve session keys. We do not attempt to mitigate damage from hardware failures, such as session-state compromise, as we aim to improve our understanding of this simpler setting. We give two natural but substantially different game based definitions of security and prove that they are equivalent. Such proofs are rare for SKA. The bulk of this proof consists of showing that, for secure protocols, only compatible processes can be made to share a key. This property is very natural but surprisingly subtle. For comparison, we give a version of our definition in which processes output session IDs and we give strong theorems relating these two
Abstract. We study the provable security of identity-based (ID-based) key agreement protocols. Although several published protocols have been proven secure in the random oracle model, only a weak adversarial model is considered – the adversary is not allowed to ask Session-Key Reveal queries that will allow the adversary to learn previously established session keys. Recent research efforts devoted to providing a stronger level of security require strong assumptions, such as assuming that the simulator has access to a non-existential computational or decisional oracle. In this work, we propose an ID-based key agreement protocol and prove its security in the widely accepted indistinguishability-based model of Canetti and Krawczyk. In our proof, the simulator does not require access to any non-existential computational or decisional oracle. We then extend our basic protocol to support ad-hoc anonymous key agreement with bilateral privacy. To the best of our knowledge, this is the first protocol of its kind as previously published protocols are for fixed group and provide only unilateral privacy (i.e., only one of the protocol participants enjoy anonymity).
Two-Party Authenticated key agreement protocol is essential for setting up the common session key for parties to establish secure channel for network communication in the open environment. Most of proposed schemes require expensive bilinear pairing operation and only provide limited security attributes. So such protocols are unsuitable for real-world applications that require stronger sense of secrecy. In this paper, we first analysis two schemes recently proposed by Cao etc., and then present a one-round ID-based authenticated key agreement protocol with more desired security attributes, which need no pairing operations while still achieve nice computational efficiency. These security attributes include known-key secrecy, perfect forward secrecy, PKG forward secrecy, key-compromise impersonation resilience, unknown key-share resilience and no key control. Moreover, it captures the enhanced security attributes known session-specific temporary key information secrecy, which means that even the ephemeral secret key were compromised, the adversary can hardly acquire the agreed session key.
Authenticated key exchange protocols are ubiquitous in modern-day life. They are used to secure numerous types of data exchange, ranging from online banking to instant messaging conversations. In this thesis, we extend the state of the art for authenticated key exchange security, following the three themes of realistic, strong and provable. First, we tackle the theme of provable security. We develop generic techniques, applicable to a wide range of security models, to assist in proofs of security of cryptographic schemes. Specifically, a "game hopping" proof is a powerful technique to prove strong statements about the security of a protocol. However, the proof technique often involves esoteric and repetitive steps. We demystify the methodology and prove generic theorems to reduce the length and complexity of a wide class of game hopping proofs. Next, we use this new framework to analyse strong and previously unstudied formal security guarantees of authenticated key exchange protocols. These protocols are able to achieve strong security guarantees because they share and update state across sessions. Using our framework, we are able to push the state of the art by defying the folklore of what was previously thought provable of authenticated key exchange protocols. Specifically, it was previously thought that no protocol can achieve a security guarantee about communication with a peer if the peer was already fully compromised. We show that this is not true by formally defining a new concept, which we name post-compromise security. We capture an intuitive "self-healing" property of protocols. We then construct and formally prove protocols can achieve this strong security notion. Finally, we leverage all of this work to analyse realistic protocols. Signal, which is used to encrypt messages in WhatsApp, Facebook Messenger and Google Allo amongst others, is one such protocol we consider. Despite being used by billions of people, Signal was completely unstudied in the academic literature prior to this work. Two reasons for this were that Signal had no documentation at the time and it did not fit into existing models: it shares state across sessions and has over ten different types of key, most of which continually update. Despite its complexity, we are able to use the other work in this thesis to give the first ever analysis and proof of security of Signal. Overall, this thesis aims to bridge the gap between theory and practice in the current state of the art of authenticated key exchange protocols.
Abstract. Secure communication is a fundamental cryptographic primitive. Typically, security is achieved by relying on an existing credential infrastructure, such as a PKI or passwords, for identifying the end points to each other. But what can be obtained when no such credential infrastructure is available? Clearly, when there is no pre-existing credential infrastructure, an adversary can mount successful “man in the middle ” attacks by modifying the communication between the legitimate endpoints. Still, we show that not all is lost, as long as the adversary’s control over the communication is not complete: We present relatively efficient key exchange and secure session protocols that provide the full guarantee of secure communication as long as the adversary fails to intercept even a single message between the legitimate endpoints. To obtain this guarantee we strengthen the notion of key exchange to require that the keys exchanged in any two sessions are independent of each other as long as each session has at least one honest endpoint, even if both sessions has an adversarial endpoint. We call this notion credential-free key exchange. We then strengthen the existing notion of secure session protocols to provide the above guarantee given a CFKE (existing definitions and constructions are insufficient for this purpose). We provide two alternative definitions and constructions of CFKE, a game-based one with a construction in the RO model, and a UC one with a construction in the CRS model.
The main goal of cryptography is to enable secure communication over a public channel; often a secret shared among the communicating parties is used to achieve this. The process by which these parties agree on such a shared secret is called key agreement. In this thesis, we focus on two-party key agreement protocols in the public-key setting and study the various methods used to establish and validate public keys. We pay particular attention to certificateless key agreement schemes and attempt to formalize a relevant notion of security. To that end, we give a possible extension of the existing extended Canetti-Krawzcyk security model applicable to the certificateless setting. We observe that none of the certificateless protocols we have seen in the literature are secure in this model; it is an open question whether such schemes exist. We analyze several published certificateless key agreement protocols, demonstrating the existence of key compromise impersonation attacks and even a man-in-the-middle attack in one case, contrary to the claims of the authors. We also briefly describe weaknesses exhibited by these protocols in the context of our suggested security model.
Key establishment is the process whereby two or more parties derive a shared \nsecret, typically used for subsequent confidential communication. However, \nidentifying the exact security requirements for key establishment protocols is \na non-trivial task. This thesis compares, extends and merges existing security \ndefinitions and models for key establishment protocols. \n \nThe primary focus is on two-party key agreement schemes in the public-key \nsetting. On one hand new protocols are proposed and analyzed in the existing \nCanetti-Krawzcyk model. On the other hand the thesis develops a security model \nand novel definition that capture the essential security attributes of the \nstandardized Unified Model key agreement protocol. These analyses lead to the \ndevelopment of a new security model and related definitions that combine and \nextend the Canetti-Krawzcyk pre- and post- specified peer models in terms of \nprovided security assurances. \n \nThe thesis also provides a complete analysis of a one-pass key establishment \nscheme. There are security goals that no one-pass key establishment scheme can \nachieve, and hence the two-pass security models and definitions need to be \nadapted for one-pass protocols. The analysis provided here includes \nthe description of the required modification to the underlying security model. \nFinally, a complete security argument meeting these altered conditions is \npresented as evidence supporting the security of the one-pass scheme. \n \nLastly, validation and reusing short lived key pairs are related to \nefficiency, which is a major objective in practice. The thesis considers the \nformal implication of omitting validation steps and reusing short lived key \npairs. The conclusions reached support the generally accepted cryptographic \nconventions that incoming messages should not be blindly trusted and extra \ncare should be taken when key pairs are reused.