This paper describes a high performance implementation of the Rijndael encryption algorithm using the UltraSONIC reconfigurable platform. We show how the UltraSONIC design methodology allowed us to develop a complete hardware/software solution in a matter of weeks. Reconfiguration is exploited to maximise the use of the available hardware resources. A modular implementation allows the trade off of encryption security against hardware resources. The flexibility of the UltraSONIC architecture allows a single implementation to process data from either the host computer or external real- time video sources. We are able to meet the requirements for processing high definition in real-time, achieving a throughput of 2.1 Gbit/sec. In software acceleration mode we are able to achieve more than 4 times speed up compared with a 1GHz Pentium III system.