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IEEE Conference on Communications and Network Security

Secret Key and Private Key Capacities Over an Unauthenticated Public Channel

Wenwen Tu, Lifeng Lai

Published Jun 1, 2019• 2 citations
Publisher

Abstract

In this paper, the problem of key generation over an unauthenticated public channel in a network setup is considered. The considered model consists of four terminals: two legitimate terminals who need to share a key, a helper and an active adversary. The legitimate terminals and the helper are allowed to exchange messages over a public noiseless channel, to which the adversary has full access and may modify messages exchanged through this public channel. We investigate two types of keys: secret key (secure from the adversary only) and private key (secure from both the helper and the adversary). We characterize the relationship between the key rates in our model and the existing models with a passive adversary. We characterize conditions under which the generated key rates in this model are non-zero, and further provide an authentication scheme to show that key capacities, if are non-zero, are the same as those for the scenario with a passive adversary.

DOI
10.1109/cns.2019.8802730

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In this paper, the problem of key generation over an unauthenticated public channel in a network setup is considered. The considered model consists of four terminals: two legitimate terminals who need to share a key, a helper and an active adversary. The legitimate terminals and the helper are allowed to exchange messages over a public noiseless channel, to which the adversary has full access and may modify messages exchanged through this public channel. We investigate two types of keys: secret key (secure from the adversary only) and private key (secure from both the helper and the adversary). We characterize the relationship between the key rates in our model and the existing models with a passive adversary. We characterize conditions under which the generated key rates in this model are non-zero, and further provide an authentication scheme to show that key capacities, if are non-zero, are the same as those for the scenario with a passive adversary.
Encrypting information with pre-agreed key is an effective method to ensure information security. With the increasing requirement for key rate, this paper investigates the problem of simultaneously generating a secret key (SK) and a private key (PK) over three legitimate terminals with a trusted helper via public discussion that a passive eavesdropper completely access to. The legitimate terminals and the helper observe the correlated source sequences from the outputs of discrete memoryless source (DMS). The legitimate terminals wish to generate an SK that is concealed from the eavesdropper, and two designated legitimate terminals want to generate a PK that needs to be additionally keep secret from the third legitimate terminal. Under this model, the SK-PK capacity region is fully characterized, and the source coding scheme is further provided to show that the region is achievable. The security performance of the generated keys is analyzed in terms of key leakage rate.
We consider the problem of keyless message authentication over noisy channels in the presence of an active adversary. Different from the existing models, in our model, the legitimate users do not have any pre-shared key for authentication. Instead, we use the noisy channel connecting the legitimate users for authentication. The main idea is to utilize the noisy channel connecting the legitimate users to distinguish a legitimate message from a fake message, by generating an output at the receiver that is difficult for the adversary to replicate through its noisy channel. By interpreting the message authentication as a hypothesis testing problem, we investigate the authentication exponent and the authenticated channel capacity of the noisy channel. In the authentication exponent problem, for a given message rate, we investigate the speed at which the optimal successful attack probability can be driven to zero. We fully characterize the authentication exponent for the zero-rate message case and provide both an upper bound and a lower bound on the exponent for the non-zero message rate case. In the authenticated capacity problem, we study the largest data transmission rate under which the attacker's optimal successful attack probability can still be made arbitrarily small. We establish an all or nothing result. In particular, we show that the authenticated channel capacity is the same as the classic channel capacity if a simulatability condition is not satisfied, while the authenticated capacity will be zero if this condition is satisfied. We also provide efficient algorithms to check this condition. We further show that our results are robust to modeling uncertainties about the eavesdropper's channels.
Physical layer security, also called information theoretic security, is an emerging field which exploits physical layer properties of communication channels to secure future generations of communication systems. The approach to generating the secret key at the physical layer is proposed in the source model by the pioneer works. Different from the key generation process in the source model, the legitimate users typically observe no sources in advance, but they can obtain the sources via estimating the channel gain. This chapter discusses the problem of key generation under active attacks. It first reviews the basic scenarios for key generation in the presence of a passive adversary. Then, it considers the scenario where the attacker attacks the public discussion, reviews the relationship between the key rate obtained from the case with an active attacker with the corresponding key rate obtained from the passive case and discusses the condition which decides this relationship.
This is the second part of a two-part paper on information-theoretically secure secret key agreement. This part covers the secret key capacity under the channel model. In this model, multiple terminals wish to create a shared secret key that is secure from an eavesdropper with unlimited computational resources. The terminals are all connected to a noiseless and authenticated but insecure channel, called the “public channel.” Furthermore, the terminals have access to a secure but noisy discrete memoryless broadcast channel (DMBC). The first terminal can choose a sequence of inputs to the DMBC, which has outputs at the other terminals and at the eavesdropper. After each channel use, the terminals can engage in arbitrarily many rounds of interactive authenticated communication over the public channel. At the end, each legitimate terminal should be able to generate the secret key. In this paper, we derive new lower and upper bounds on the secrecy capacity. In each case, an example is provided to show that the new bound represents a strict improvement over the previously best known bound. This part of the paper is not standalone, and is written under the assumption that the reader has access to Part I, which is published in the same issue.
Shannon-theoretic secret key generation by several parties is considered for models in which a secure noisy channel with one input terminal and multiple output terminals and a public noiseless channel of unlimited capacity are available for accomplishing this goal. The secret key is generated for a setAAAof terminals of the noisy channel, with the remaining terminals (if any) cooperating in this task through their public communication. Single-letter characterizations of secrecy capacities are obtained for models in which secrecy is required from an eavesdropper that observes only the public communication and perhaps also a set of terminals disjoint fromAAA. These capacities are shown to be achievable with noninteractive public communication, the channel input terminal sending no public message and each output terminal sending at most one public message, not using randomization. Moreover, when the input terminal belongs to the setAAA, it can generate the secret key at the outset and transmit it over the noisy channel, suitably encoded, whereupon the output terminals inAAAsecurely recover this key using public communication as above. For models in which the eavesdropper also possesses side information that is not available to any of the terminals cooperating in secrecy generation, an upper bound for the secrecy capacity and a sufficient condition for its tightness are given.
A source model including four terminals is considered, where three simultaneously generating three types of keys are intended. Terminals 1, 2 and 3 wish to share a common key, the secret key, which should be kept secret from terminal 4 and simultaneously terminals 1 and 2 intend to share a private key with terminal 3, which should be kept secret from each other. Also, all the keys should be concealed from terminal 4 (the external wiretapper). The authors assume that all terminals including the external wiretapper have access to distinct correlated i.i.d. sources; there is also a noiseless public channel with unlimited capacity among the terminals. The authors have investigated the model on two scenarios of key sharing depending on the direction of the public channel. Rate regions of the keys are derived. It is shown that in some special cases the inner and outer bounds of the capacity regions coincide and the capacity regions are derived.