XCrypto: a cryptographic ISE for RISC-V

Acting as a component part of the wider SCARV project, XCrypto is a general-purpose Instruction Set Extension (ISE) for RISC-V that supports software-based cryptographic workloads.

Overview

A given cryptographic workload is commonly expected to satisfy a challenging and diverse range of traditional design metrics, including some combination of high-throughput, low-latency, low-footprint, power-efficiency, and high-assurance, while executing in what is potentially an adversarial environment. A large design space of options can be drawn from when developing a concrete implementation: these options span a spectrum, between those entirely based on hardware (e.g., a dedicated IP core) and those entirely based on software. ISEs can be viewed as representing a hybrid option, in the sense they alter a general-purpose processor core with special-purpose hardware and associated instructions; such targeted alterations then help to improve a software-based implementation wrt. some design metric (e.g., latency).

As an ISE, we pitch XCrypto as a solution (vs. the solution) within the wider design space of options. For example, it offers as an alternative to the solution being proposed by the RISC-V cryptography extensions group (see, e.g., their presentation: the design extends the RISC-V vector ISE). The idea is to leverage extensive existing literature and hence experience wrt. cryptographic ISEs (see, e.g., published work at the CHES conference), translating and applying it to RISC-V. Although potentially less performant than alternatives, we expect implementations using XCrypto to be more lightweight and flexible; as a result, we view it as representing an attractive solution in the context of micro-controller class cores.

Organisation

├── bin                    - scripts (e.g., environment configuration)
├── build                  - working directory for build
├── doc                    - documentation
├── extern                 - external resources (e.g., submodules)
│   ├── libscarv             - submodule: scarv/libscarv
│   ├── riscv-opcodes        - submodule: scarv/riscv-opcodes
│   ├── texmf                - submodule: scarv/texmf
│   └── wiki                 - submodule: scarv/xcrypto.wiki
├── pdf                    - PDFs, e.g., presentation slides
├── rtl                    - source code for re-usable hardware modules
└── src
    ├── docker             - source code for containers
    ├── helloworld         - source code for example program
    ├── test               - source code for test    program(s)
    └── toolchain          - source code for tool-chain

Note that:

• ${REPO_HOME}/doc houses the XCrypto specification: this document captures the ISE itself, acting as both a) a definition of additional architectural state (e.g., register file and CSRs) and instructions (i.e., their semantics and encoding), and b) a design document. Pre-built versions accompany each releases of XCrypto.

• ${REPO_HOME}/rtl houses a library of re-usable hardware components (e.g., for arithmetic operations), which could be used in an implementation of XCrypto.

• Per the above, the content of this repository is non-specific to an implementation of XCrypto within any given processor core. That said, the associated repository scarv/scarv specifically houses such an implementation: the SCARV processor core (and associated SoC) offer an integrated implementation of components from the entire SCARV project, XCrypto included.

Quickstart (with more detail in the wiki)

1. Execute

   git clone https://github.com/scarv/xcrypto.git ./xcrypto
   cd ./xcrypto
   git submodule update --init --recursive
   source ./bin/conf.sh
   

   to clone and initialise the repository, then configure the environment; for example, you should find that the REPO_HOME environment variable is set appropriately.

2. Use targets in the top-level Makefile to drive a set of common tasks, e.g.,

   Command	Description
   make build-doc	build the LaTeX-based documentation
   make clone-toolchain	clone the tool-chain
   make build-toolchain	build the tool-chain
   make doxygen	build the Doxygen-based documentation
   make spotless	remove everything built in ${REPO_HOME}/build

Questions?

• read the wiki,
• raise an issue,
• raise a pull request,
• drop us an email.

Publications and presentations

• Some slides presented at the RISC-V meetup in Bristol, April 2019.
• A poster presented at the RISC-V Workshop in Zurich, June 2019.

Acknowledgements

This work has been supported in part by EPSRC via grant EP/R012288/1 (under the RISE programme).