Abstract. The notion of authenticator, proposed by Bellare et al., is to transform a protocol secure in the authenticated-link model to a new one secure in the unauthenticated-link model. This notion admits a modular design and analysis of cryptographic protocols and thus greatly simplifies the underlying tasks. However, all previous authenticators are constructed via a so called MT-authenticator. This kind of authenticator authenticates each message independently. Thus, the round complexity of the resulting protocol is amplified by a multiplicative factor. In this paper, we propose two efficient authenticators which authenticate the protocol as a whole and the round complexity of the resulting protocol increases only by at most an additively small number. We also construct a very efficient key exchange protocol. Our protocol is provably secure under the general cryptographic assumption (especially without a concrete hardness assumption such as DDH or RSA). Of an independent interest, our security proof lies in the emulation based ideal-real model, instead of the widely adopted (seemingly weaker) SKsecurity. To our knowledge, this is the first protocol of its kind. It is worth mentioning that all our constructions are obtained by improving the related protocols of Bellare et al. [1]. 1
Abstract. The notion of authenticator, proposed by Bellare et al., is to transform a protocol secure in the authenticated-link model to a new one secure in the unauthenticated-link model. This notion admits a modular design and analysis of cryptographic protocols and thus greatly simplifies the underlying tasks. However, all previous authenticators are constructed via a so called MT-authenticator. This kind of authenticator authenticates each message independently. Thus, the round complexity of the resulting protocol is amplified by a multiplicative factor. In this paper, we propose two efficient authenticators which authenticate the protocol as a whole and the round complexity of the resulting protocol increases only by at most an additively small number. We also construct a very efficient key exchange protocol. Our protocol is provably secure under the general cryptographic assumption (especially without a concrete hardness assumption such as DDH or RSA). Of an independent interest, our security proof lies in the emulation based ideal-real model, instead of the widely adopted (seemingly weaker) SKsecurity. To our knowledge, this is the first protocol of its kind. It is worth mentioning that all our constructions are obtained by improving the related protocols of Bellare et al. [1]. 1
We consider the problem of authenticated group key exchange among n parties communicating over an insecure public network. A number of solutions to this problem have been proposed; however, all prior provably-secure solutions do not scale well and, in particular, require O (n) rounds. Our main contribution is the first scalable protocol for this problem along with a rigorous proof of security in the standard model under the DDH assumption; our protocol uses a constant number of rounds and requires only O (1) \full modular exponentiations per user. Toward this goal (and adapting work of Bellare, Canetti, and Krawczyk), we first present an efficient compiler that transforms any ngroup key-exchange protocol secure against a passive eavesdropper to an authenticated protocol which is secure against an active adversary who controls all communication in the network. This compiler adds only one round and O (1) communication (per user) to the original scheme. We then prove secure | against a passive adversary | a variant of the two-round group key-exchange protocol of Burmester and Desmedt. Applying our compiler to this protocol results in a provably-secure three-round protocol for authenticated group key exchange which also achieves forward secrecy.
Nowadays many crucial network applications rely on the existence of a confidential channel established by authenticated key exchange (AKE) protocols over public networks.With the rapid development of cyber technology, novel attacks to cryptosystem emerge in an endless stream.This has also led to the development of AKE solutions to provide increasingly stronger security guarantees.In this thesis we focus on provision of practical constructions for AKE protocols which are provably secure in a strong sense without resorting to random oracles.We first we present three new efficient compilers to generically turn passively secure key exchange protocols (KE) into authenticated key exchange protocols (AKE) where security also holds in the presence of active adversaries.Our compilers are not only a useful tool for the design of new AKE systems with many additional security properties in a modular and less error-prone fashion, but they also help to relax the assumptions on existing, practical key exchange mechanisms which are not known to be provably secure AKE protocols.Security of our compilers is shown in a strong modified CK model where the adversary is allowed to reveal either long-term secret key or state information of the protocol participants.On the second, we study the open problem on constructing eCK secure two party AKE protocol without random oracles and NAXOS trick.A generic construction satisfying those requirements is given based on well-known cryptographic primitives following the guideline of efficiency.Then a concrete protocol is proposed which is the first eCK secure protocol in the standard model under both standard assumptions and post-specified peer setting (i.e.without knowing any cryptographic information about its communication peer).Both proposed schemes can be more efficiently implemented with secure device than previous works which are eCK secure in the standard model, where the secure device might be normally used to store the long-term private key and to implement codes of protocol which need to be resilience of state leakage.Finally, we generalize our one-round two party AKE construction to group case that yields an efficient tripartite AKE protocol based on bilinear maps and a candidate group AKE protocol (with more than three members) based on multilinear maps.Up to now they are the first solutions which can be proved secure in the g-eCK model without random oracles.Meanwhile we make first step to show how to simplify the security proof for one-round group AKE under g-eCK model.i
Abstract. Deniable authentication is a technique that allows one party to send messages to another while the latter can not prove to a third party the fact of communication. In this paper, we first formalize a natural notion of deniable security and naturally extend the basic authenticator theorem by Bellare et al. [2] to the setting of deniable authentication. Of independent interest, this extension is achieved by defining a deniable MT-authenticator via a game. This game is essentially borrowed from the notion of universal composition [8] although we do not assume any result or background about it. Then we construct two deniable MT-authenticators: uncontrollable random oracle based and the PKI based, both of which are just 3-round protocols. The second construction assumes the receiver owns a secret key. Such a setup assumption is very popular in the real world. (Without this assumption), all the previous protocols do not have a widely satisfiable performance when applied in the Internet-like environment. Finally, as our application, we obtain key exchange protocols that is deniably secure in the real world. 1
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.
Abstract. Cryptographic protocol design in a two-party setting has often ignored the possibility of simultaneous message transmission by each of the two parties (i.e., using a duplex channel). In particular, most protocols for two-party key exchange have been designed assuming that parties alternate sending their messages (i.e., assuming a bidirectional half-duplex channel). However, by taking advantage of the communication characteristics of the network it may be possible to design protocols with improved latency. This is the focus of the present work. We present a number of provably-secure protocols for two-party authenticated key exchange (AKE) which require only a single round. Our first protocol provides key independence only, and is analyzed in the random oracle model. This scheme matches the most efficient AKE protocols among those found in the literature. Our second scheme additionally provides forward secrecy, and is also analyzed in the random oracle model. Our final protocol provides the same strong security guarantees, but is proven secure in the standard model. This scheme is only slightly less efficient (from a computational perspective) than the previous ones. These last two schemes are the first provably-secure one-round protocols for authenticated 2-party key exchange which provide forward secrecy.
So far, all solutions proposed for authenticated key agreement combine key agreement and authentication into a single cryptographic protocol. However, in many important application scenarios, key agreement and entity authentication are clearly separated protocols. This fact enables efficient attacks on the naïve combination of these protocols. In this paper, we propose new compilers for two-party key agreement and authentication, which are provably secure in the standard Bellare-Rogaway model. The constructions are generic: key agreement is executed first and results (without intervention of the adversary) in a secret session key on both sides. This key (or a derived key) is handed over, together with a transcript of all key exchange messages, to the authentication protocol, where it is combined with the random challenge(s) exchanged during authentication.