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Group key exchange protocols allow a group of parties communicating over a public network to come up with a common secret key called a session key . Due to their critical role in building secure multicast channels, a number of group key exchange protocols have been suggested over the years for a variety of settings. Among these is the so-called EKE-M protocol proposed by Byun and Lee for password-based group key exchange in the different password authentication model , where group members are assumed to hold an individual password rather than a common password. While the announcement of the EKE-M protocol was essential in the light of the practical significance of the different password authentication model, Tang and Chen showed that the EKE-M protocol itself suffers from an undetectable on-line dictionary attack. Given Tang and Chen's attack, Byun et al. have recently suggested a modification to the EKE-M protocol and claimed that their modification makes EKE-M resistant to the attack. However, the claim turned out to be untrue. In the current paper, we demonstrate this by showing that Byun et al.'s modified EKE-M is still vulnerable to an undetectable on-line dictionary attack. Besides reporting our attack, we also figure out what has gone wrong with Byun et al.'s modification and how to fix it.

A priority-driven scheduling algorithm is said to be start time (finish time) predictable if the start time (finish time) of jobs in the schedule where each job executes for its actual execution time is bounded by the start times (finish times) of jobs in the schedules where each job executes for its maximum/minimum execution time. In this paper, we study the predictability of a job-level dynamic priority algorithm, LLF (Least Laxity First), on multiprocessor real-time systems. We present a necessary and sufficient condition for a priority-driven algorithm to be start time (finish time) predictable. Then, in LLF scheduling, we show that both the start time and the finish time are predictable if the actual execution times cannot be known. However, solely the finish time is predictable if the actual execution times can be known.

The Stack Resource Policy (SRP) is a real-time synchronization protocol suitable for embedded systems for its simplicity. However, if SRP is applied to dynamic priority scheduling, the runtime overhead of job selection algorithms could affect the performance of the system seriously. To solve the problem, a job selection algorithm was proposed that uses a selection tree as a scheduling queue structure. The proposed algorithm selects a job in O(⌈log2n⌉) time, resulting in significant reduction in the run-time overhead of scheduler. In this paper, the correctness of the job selection algorithm is presented. Also, the job selection algorithm was implemented in GSM/GPRS handset with ARM7 processor to see its effectiveness on embedded systems. The experiments performed on the system show that the proposed algorithm can further utilize the processor by reducing the scheduling overhead.

Real-time scheduling on multiprocessor involves many complex issues. One of them is tie-breaking: the way how ties are broken among tasks with same priority. Unlike on uniprocessor, the feasibility of a task set on multiprocessor depends on a tie-breaking policy as well as on a scheduling algorithm. In this paper, we suggest and compare several tie-breaking policies for deadline-based schedule algorithms on multiprocessor. An effective tie-breaking policy for different scheduling algorithm is identified by simulation.