Group communication can benefit from IP multicast to achieve scalable exchange of messages. However, there is a challenge of effectively controlling access to the transmitted data. IP multicast by itself does not provide any mechanisms for preventing nongroup members to have access to the group communication. Although encryption can be used to protect messages exchanged among group members, distributing the cryptographic keys becomes an issue. Researchers have proposed several different approaches to group key management. These approaches can be divided into three main classes: centralized group key management protocols, decentralized architectures and distributed key management protocols. The three classes are described here and an insight given to their features and goals. The area of group key management is then surveyed and proposed solutions are classified according to those characteristics.

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A survey of key management for secure group communication

Analysis of technology and application trends reveals a growing imbalance in the peak compute-to-memory-capacity ratio for future servers. At the same time, the fraction contributed by memory systems to total datacenter costs and power consumption during typical usage is increasing. In response to these trends, this paper re-examines traditional compute-memory co-location on a single system and details the design of a new general-purpose architectural building block-a memory blade-that allows memory to be "disaggregated" across a system ensemble. This remote memory blade can be used for memory capacity expansion to improve performance and for sharing memory across servers to reduce provisioning and power costs. We use this memory blade building block to propose two new system architecture solutions-(1) page-swapped remote memory at the virtualization layer, and (2) block-access remote memory with support in the coherence hardware-that enable transparent memory expansion and sharing on commodity-based systems. Using simulations of a mix of enterprise benchmarks supplemented with traces from live datacenters, we demonstrate that memory disaggregation can provide substantial performance benefits (on average 10X) in memory constrained environments, while the sharing enabled by our solutions can improve performance-per-dollar by up to 57% when optimizing memory provisioning across multiple servers.

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Disaggregated memory for expansion and sharing in blade servers

Secure media broadcast over the Internet poses unique security challenges. One problem is access control to a large number of subscribers in a public broadcast. A common solution is to encrypt the broadcast data and to disclose the decryption key to legitimate receivers only. However, how do we securely and efficiently establish a shared secret among the legitimate receivers? And most importantly, how can we efficiently update the group key securely if receivers join or leave? How can we provide reliability for key update messages in a way that scales up to large groups? Recent research makes substantial progress to address these challenges. Current schemes feature efficient key update mechanisms assuming that the key updates are communicated reliably to the receivers. In practice, however the principal impediment to achieve a scalable system is to distribute the key updates reliably to all receivers. We have designed and implemented ELK, a novel key distribution protocol, to address these challenges with the following features: ELK features perfectly reliable, super-efficient member joins; ELK uses smaller key update messages than previous protocols; ELK features a mechanism that allows short hint messages to be used for key recovery allowing a tradeoff of communication overhead with member computation; ELK proposes to append a small amount of key update information to data packets, such that the majority of receivers can recover from lost key update messages; and ELK allows to trade off security with communication overhead.

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ELK, a new protocol for efficient large-group key distribution

We consider the problem of securing communication in large ad hoc networks, i.e., wireless networks with no fixed, wired infrastructure and with multi-hop routes. Such networks, e.g., networks of sensors, are deployed for applications such as microsensing, monitoring and control, and for extending the peer-to-peer communication capability of smaller group of network users. Because the nodes of these networks, which we term pebbles for their very limited size and large number, are resource constrained, only symmetric key cryptography is feasible. We propose a key management scheme to periodically update the symmetric keys used by all pebbles. By combining mobility-adaptive clustering and an effective probabilistic selection of the key-generating node, the proposed scheme meets the requirements of efficiency, scalability and security needed for the survivability of networks of pebbles (pebblenets)

Secure pebblenets

We address the problem of establishing a group key amongst a dynamic group of users over an unreliable, or lossy, network. We term our key distribution mechanisms self-healing because users are capable of recovering lost group keys on their own, without requesting additional transmissions from the group manager thus cutting back on network traffic, decreasing the load on the group manager and reducing the risk of user exposure through traffic analysis. A user must be a member both before and after the session in which a particular key is sent in order to be able to recover the key through self-healing. Binding the ability to recover keys to membership status enables the group manager to use short broadcasts to establish group keys, independent of the group size. In addition, the self-healing approach to key distribution is stateless, meaning that a group member who has been off-line for some time is able to recover new session keys immediately after coming back on-line.

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Self-healing key distribution with revocation

The authors describe a novel approach to scalable group re-keying for secure multicast. Our approach, which we call Kronos, is based upon the idea of periodic group re-keying. We first motivate our approach by showing that if a group is re-keyed on each membership change, as the size of the group increases and/or the rate at which members leave and join the group increases, the frequency of rekeying becomes the primary bottle neck for scalable group re-keying. In contrast, Kronos can scale to handle large and dynamic groups because the frequency of re-keying is independent of the size and membership dynamics of the group. Next, we describe how Kronos can be used in conjunction with distributed key management frameworks such as IGKMP (T. Hardjono et al., 1998) that use a single group-wide session key for encrypting communications between members of the group. Using a detailed simulation, we compare the performance tradeoffs between Kronos and other key management protocols.

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Kronos: a scalable group re-keying approach for secure multicast

In this paper, we present the design and implementation of Dodo, an efficient user-level system for harvesting idle memory in off-the-shelf clusters of workstations. Dodo enables data-intensive applications to use remote memory in a cluster as an intermediate cache between local memory and disk. It requires no modifications to the operating system and/or processor firmware and is hence portable to multiple platforms. Further, the memory recruitment policy used by Dodo is designed to minimize any delays experienced by the owner of desktop machines whose memory is harvested by Dodo. Our implementation of Dodo is operational and currently runs on Linux 2.0.35. For communication, Dodo can use either UDP/IP or U-Net, the low-latency user-level network architecture developed by von Eicken et al. (1995). We evaluated the performance improvements that can be achieved by using Dodo for two real applications and three synthetic benchmarks. Our results show that speedups obtained for an application are highly dependent on its I/O access pattern and data set sizes. Significant speedups (between 2 and 3) were obtained for applications whose working sets are larger than the local memory on a workstation but smaller than aggregate memory available on the cluster and for applications that can benefit from the zero-seek nature of remote memory.

S. Koussih

Papers

Kronos: a scalable group re-keying approach for secure multicast

Dodo: a user-level system for exploiting idle memory in workstation clusters