Simple AEAD Hardware Interface (SÆHI) in a SoC: Implementing an On-Chip Keyak/WhirlBob Coprocessor

Abstract

Simple AEAD Hardware Interface (SÆHI) is a hardware cryptographic interface aimed at CAESAR Authenticated Encryption with Associated Data (AEAD) algorithms. Cryptographic acceleration is typically achieved either with a coprocessor or via instruction set extensions. ISA modifications require re-engineering the CPU core, making the approach inapplicable outside the realm of open source processor cores. At minimum, we suggest implementing CAESAR AEADs as universal memory-mapped cryptographic coprocessors, synthesizable even on low end FPGA platforms. AEADs complying to SÆHI must also include C language API drivers targeting low-end MCUs that directly utilize the memory mapping in a ``bare metal'' fashion. This can also be accommodated on MMU-equipped mid-range CPUs. Extended battery life and bandwidth resulting from dedicated cryptographic hardware is vital for currently dominant computing and communication devices: mobile phones, tablets, and Internet-of-Things (IoT) applications. We argue that these should be priority hardware optimization targets for AEAD algorithms with realistic payload profiles. We demonstrate a fully integrated implementation of WhirlBob and Keyak AEADs on the FPGA fabric of Xilinx Zynq 7010. This low-cost System-on-Chip (SoC) also houses a dual-core Cortex-A9 CPU, closely matching the architecture of many embedded devices. The on-chip coprocessor is accessible from user space with a Linux kernel driver. An integration path exists all the way to end-user applications.