PAX: A Datapath-Scalable Minimalist Cryptographic Processor For Mobile Environments

We describe a datapath-scalable, minimalist cryptographic processor, called PAX, for mobile environments where the communication with the outside world is done on wireless connections. PAX is designed to fully utilize the high data rates of the newest and developing wireless technologies. Today, these rates exceed 2 Mbps for cellular/PCS connections, and 50 Mbps for WLAN connections. Future rates are expected to be about 20 Mbps and 100 Mbps for cellular/PCS and WLAN respectively. In designing PAX, we first select a cipher suite that is suitable for mobile environments. This provides all basic security functions such as confidentiality, data integrity, user authentication, and digital signatures. We then define the PAX instruction set, which contains few new instructions that provide huge speedups for key sections of the algorithms in our cipher suite. We compute the processor speeds required for secure communications at data rates that can be supported by the newest and developing wireless technologies. For bulk encryption and hashing, a 7 MHz 32-bit single-issue PAX processor is sufficient to match the 2.4 Mbps data rate of future 3G cellular networks. To match the 54 Mbps data rate of the IEEE 802.11a/g WLAN connections, the clock rate needs to be 150 MHz. Both figures are significantly under the 400 MHz rate used by the processors in today’s mobile information appliances such as PDAs. Datapath scalability refers to the feature that the same instruction set can be implemented in processors with different word sizes. This feature, first introduced in the PLX multimedia instruction set, provides extra flexibility in balancing the performance and cost of a system. We test the usefulness of datapath scalability by varying the word size from 32 bits to 64 bits to 128 bits. For public-key cryptography and bulk encryption, datapath scalability provides 10× to 20× additional speedup.