Abstract. Manual authentication is a recently proposed model of communication motivated by the settings where the only trusted infrastructure is a low bandwidth authenticated channel, possibly realized by the aid of a human, that connects the sender and the receiver who are otherwise connected through an insecure channel and do not have any shared key or public key infrastructure. A good example of such scenarios is pairing of devices in Bluetooth. Manual authentication systems are studied in computational and information theoretic security model and protocols with provable security have been proposed. In this paper we extend the results in information theoretic model in two directions. Firstly, we extend a single receiver scenario to multireceiver case where the sender wants to authenticate the same message to a group of receivers. We show new attacks (compared to single receiver case) that can launched in this model and demonstrate that the single receiver lower bound 2 log(1/ɛ)+O(1) on the bandwidth of manual channel stays valid in the multireceiver scenario. We further propose a protocol that achieves this bound and provides security, in the sense that we define, if up to c receivers are corrupted. The second direction is the study of non-interactive protocols in unconditionally secure model. We prove that unlike computational security framework, without interaction a secure authentication protocol requires the bandwidth of the manual channel to be at least the same as the message size, hence non-trivial protocols do not exist. Key words: manual channel, (interactive) multireceiver authentication, security. 1
Abstract. Manual authentication is a recently proposed model of communication motivated by the settings where the only trusted infrastructure is a low bandwidth authenticated channel, possibly realized by the aid of a human, that connects the sender and the receiver who are otherwise connected through an insecure channel and do not have any shared key or public key infrastructure. A good example of such scenarios is pairing of devices in Bluetooth. Manual authentication systems are studied in computational and information theoretic security model and protocols with provable security have been proposed. In this paper we extend the results in information theoretic model in two directions. Firstly, we extend a single receiver scenario to multireceiver case where the sender wants to authenticate the same message to a group of receivers. We show new attacks (compared to single receiver case) that can launched in this model and demonstrate that the single receiver lower bound 2 log(1/ɛ)+O(1) on the bandwidth of manual channel stays valid in the multireceiver scenario. We further propose a protocol that achieves this bound and provides security, in the sense that we define, if up to c receivers are corrupted. The second direction is the study of non-interactive protocols in unconditionally secure model. We prove that unlike computational security framework, without interaction a secure authentication protocol requires the bandwidth of the manual channel to be at least the same as the message size, hence non-trivial protocols do not exist. Key words: manual channel, (interactive) multireceiver authentication, security. 1
An authentication protocol is a procedure by which a sender tries to convey n bits of information, which we call an input message, to a receiver. An adversary controls the network over which the sender and the receiver talk and may change any message before it reaches its destination. This paper deals with the simple scenario where two parties, Alice and Bob, communicate and want to assure that the message received by Bob is the one sent by Alice. Unconditional cryptographic security cannot be generated simply from scratch, but must be based on some given primitive to start with. In this paper, we introduce a message authentication protocol with unconditional cryptographic security. More precisely, we give protocols using a weakly secret key allowing for the exchange of authenticated messages.
In secure multi-party computation n parties jointly evaluate an n-variate function f in the presence of an adversary which can corrupt up till t parties. Almost all the works that have appeared in the literature so far assume the presence of authenticated channels between the parties. This assumption is far from realistic. Two directions of research have been borne from relaxing this (strong) assumption: (a) The adversary is virtually omnipotent and can control all the communication channels in the network, (b) Only a partially connected topology of authenticated channels is guaranteed and adversary controls a subset of the communication channels in the network. This work introduces a new setting for (unconditional) secure multiparty computation problem which is an interesting intermediate model with respect to the above well studied models from the literature (by sharing a salient feature from both the above models). We consider the problem of (unconditional) secure multi-party computation when 'some' of the communication channels connecting the parties can be corrupted passively as well as actively. For this setting, some honest parties may be connected to several other honest parties via corrupted channels and may not be able to authentically communicate with them. Such parties may not be assured the canonical guarantees of correctness or privacy. We present refined definitions of security for this new intermediate model of unconditional multiparty computation. We show how to adapt protocols for (Unconditional) secure multiparty computation to realize the definitions and also argue the tightness of the results achieved by us.
In modern cryptography, information-theoretic security is formalized by means of some probability (e.g., success probability of adversary's guessing) or some information-theoretic measure (e.g., Shannon entropy), and the study on cryptographic protocols with information-theoretic security is one of effective applications of the probability theory, statistics, and information theory. In this paper, we study the blind authentication code (BA-code), a kind of information-theoretically secure authentication protocols, in which verifier's secret keys are not required. For realizing it, we utilize a unidirectional low-bandwidth auxiliary channel which is called a manual channel. Specifically, in this paper we propose a model, a security definition, and a construction of BA-codes in the manual channel model. Furthermore, we consider BA-codes in other models, i.e., the noisy channel model and the bounded storage model, in which no verifier's secret key is required.
A secure communication over an insecure channel without any prior exchanged key can be established with the help of an authentication step to exchange a public key and then using public-key cryptography such as RSA. In this work, we concentrate on message authentication protocols which require an extra authenticated channel. We also recall biometrics-based systems which are hard to implement and distance bounding-based authentication systems which are limited to closer communications. We sketch three generic attacks against any message authentication protocol, conclude on their maximal security, and study the security of some protocols. Finally, we propose a new protocol which achieves the same security level as that of SSH and GPG, but using much less authenticated bits.
Historically, cryptography was created and used to deliver messages between people in such a way that only the intended receiver would be able to read them. The two persons communicating trusted each other, but not the rest of the world - for instance, not the message bearer. But what happens if we assume instead that even the receiver of a message may not be trusted by the sender, while both still have an interest in communicating? This problem is addressed by secure multi-party computation, that studies communication and shared computation between mutually distrusting parties. In a world where most of the communication happens over the Internet, and where an ever-increasing amount of computation is outsourced, for example to the cloud, multi-party computation is a subject of crucial importance. However, unconditionally secure multi-party protocols have never found practical application: the lack of realistic noisy channel models, that are required to achieve security against computationally unbounded adversaries, prevents implementation over real-world, standard communication protocols.\n\nIn this thesis, we study the properties and characteristics of noisy channels, and we introduce new models based on real-world error sources that have never been studied in this context, such as delays. On the contrary, previous works mostly focused on theoretical noisy channels that have little in common with the reality of network communication. We show that the models we propose are suitable for multi-party computation by building on each of them a protocol for oblivious transfer, a fundamental cryptographic primitive that implies any secure computation. Thanks to the realism of our noisy channels, we are able to provide the first working implementation of unconditionally secure oblivious transfer over both wired and wireless networks and different Internet protocols, which we test thoroughly in an extensive set of experiments. Our constructions are not only more realistic, but also more flexible, as they allow larger ranges of error probabilities. This will eventually open the way for the widespread use of unconditionally secure multi-party computation.
Authentication is a well-studied area of classical cryptography: a sender A and a receiver B sharing a classical secret key want to exchange a classical message with the guarantee that the message has not been modified or replaced by a dishonest party with control of the communication line. In this paper we study the authentication of messages composed of quantum states. We give a formal definition of authentication in the quantum setting. Assuming A and B have access to an insecure quantum channel and share a secret, classical random key, we provide a non-interactive scheme that enables A to both encrypt and authenticate an m qubit message by encoding it into m+s qubits, where the error probability decreases exponentially in the security parameter s. The scheme requires a secret key of size 2m+O(s). To achieve this, we give a highly efficient protocol for testing the purity of shared EPR pairs. It has long been known that learning information about a general quantum state will necessarily disturb it. We refine this result to show that such a disturbance can be done with few side effects, allowing it to circumvent cryptographic protections. Consequently, any scheme to authenticate quantum messages must also encrypt them. In contrast, no such constraint exists classically. This reasoning has two important consequences: It allows us to give a lower bound of 2m key bits for authenticating m qubits, which makes our protocol asymptotically optimal. Moreover, we use it to show that digitally signing quantum states is impossible.