This thesis considers the problem of securing communication among dynamic groups of participants without relying on an online group keying service. As a solution, we offer the design and implementation of the Public Key Group Encryption (PKGE) service. It is a cryptography library, written in C, and designed to be shared among all communications applications on any particular system. PKGE imposes low communication overhead and embraces disconnected operation, making it especially appropriate for deployment in low-bandwidth tactical environments. PKGE provides forward-secure confidentiality and authentication among any subset of users using small communication overhead by bringing together a number of modern cryptographic developments, with the piece de resistance being the elliptic curve-based Collusion-Resistant Broadcast Encryption. The focus of this thesis is primarily the engineering and synthesis of known theoretical schemes; we also present novel extensions to the Boneh-Gentry-Waters encryption scheme. 1. Forward secrecy: Add forward secrecy to the scheme at a cost of T private keys for T security epochs. 2. Optimized session protocols: Sidestep the majority of costs in computation and bandwidth. 3. Cheap over-provisioning of system capacity: Support up to 232 users for resource costs proportional only to the number actually registered. 4. Chosen Ciphertext Attack (CCA) Security: Elevate security from CPA to CCA strength. Using PKGE, we have developed a plugin for Gaim2 as a motivating launch application. The plugin both demonstrates the use of PKGE and enables secure conferencing over the range of Gaim-supported protocols, including Jabber, IRC, AIM, and ICQ. PKGE and its Gaim plugin may be run and further developed under MS Windows, Mac OS X, and Linux operating systems.
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