Crate subtle [] [src]

subtle

Pure-Rust traits and utilities for constant-time cryptographic implementations.

This crate represents a “best-effort” attempt, since side-channels are ultimately a property of a deployed cryptographic system including the hardware it runs on, not just of software.

It consists of a Choice type, and a collection of traits using Choice instead of bool which are intended to execute in constant-time. The Choice type is a wrapper around a u8 that holds a 0 or 1.

The traits are implemented using bitwise operations, and should execute in constant time provided that a) the bitwise operations are constant-time and b) the operations are not optimized into a branch.

To prevent this, when using the nightly feature (recommended), the crate attempts to hide the value of a Choice's inner u8 from the optimizer, by passing it through an inline assembly block. For more information, see the About section below.

When not using the nightly feature, there is no protection against b). This is unfortunate, but is no worse than C code.

[dependencies.subtle]
version = "^0.6"
features = ["nightly"]

Features

Documentation

Documentation is available here.

About

This library aims to be the Rust equivalent of Go’s crypto/subtle module.

The optimization barrier in impl From<u8> for Choice was based on Tim Maclean's work on rust-timing-shield, which attempts to provide a more comprehensive approach for preventing software side-channels in Rust code.

Warning

This code is a low-level library, intended for specific use-cases implementing cryptographic protocols. It represents a best-effort attempt to protect against some software side-channels.

USE AT YOUR OWN RISK

Note that docs will only build on nightly Rust until RFC 1990 stabilizes.

Structs

Choice

The Choice struct represents a choice for use in conditional assignment.

Traits

ConditionallyAssignable

A type which can be conditionally assigned in constant time.

ConditionallyNegatable

A type which can be conditionally negated in constant time.

ConditionallySelectable

Select one of two inputs according to a Choice in constant time.

ConditionallySwappable

A type which is conditionally swappable in constant time.

ConstantTimeEq

An Eq-like trait that produces a Choice instead of a bool.