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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)

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.

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).