Fast implementations of secret-key block ciphers using mixed inner- and outer-round pipelining

Abstract

The new design methodology for secret-key block ciphers, based on introducing an optimum number of pipeline stages inside of a cipher round is presented and evaluated. This methodology is applied to five well-known modern ciphers, Triple DES, Rijndael, RC6, Serpent, and Twofish, with the goal to first obtain the architecture with the optimum throughput to area ratio, and then the architecture with the highest possible throughput. All ciphers are modeled in VHDL, and implemented using Xilinx Virtex FPGA devices. It is demonstrated that all investigated ciphers can operate with similar maximum clock frequencies, in the range from 95 to 131 MHz, limited only by the delay of a single CLB layer and delays of interconnects. Rijndael, RC6, Twofish, and Serpent achieve throughputs in the range from 12.1 Gbit/s to 16.8 Gbit/s; and Triple DES achieves the throughput of 7.5 Gbit/s. Because of the optimum speed to cost ratio, the proposed architecture seems to be very well suited for practical implementations of secret-key block ciphers using both FPGAs and custom ASICs. We also show that using this architecture for comparing hardware performance of secret-key block ciphers, such as AES candidates, operating in non-feedback cipher modes, leads to the more prudent and fairer analysis than comparisons based on other types of pipelined architectures.