RustCrypto: Constant-Time Utilities

Constant-time utility library with selection and equality testing support targeting cryptographic applications. Supports const fn where appropriate. Built on the cmov crate which provides architecture-specific predication intrinsics. Heavily inspired by the subtle crate.

About

This crate contains constant-time equivalents of the bool and Option types (Choice and CtOption respectively), along with traits that can be used in combination with them.

The CtOption type notably provides eagerly evaluated combinator methods (as opposed to the lazily evaluated combinators on Option) which make it possible to write constant-time code using an idiomatic Rust style.

This is an experimental next-generation constant-time library inspired by subtle, but for now we recommend you continue to stick with subtle. We may attempt to get some of the changes in this library incorporated into subtle for a potential v3.0.

What makes this crate different from subtle?

• Pervasive const fn support
  • Almost all constructors/methods on Choice are const fn
  • Choice can be constructed using various const fn predicates on integer types, enabling writing constant-time const fn logic
  • CtOption supports const fn constructors and *_copied methods to access the inner value when it's a Copy type
  • Macros to act as CtOption pseudo-combinators: map! and unwrap_or!
  • Expanded selection of CtOption combinators that more closely mirrors std::option::Option
• Guaranteed constant-time equality testing and conditional selection on x86(_64) and aarch64 using asm! implementations in the cmov crate which call special constant-time CPU instructions with a portable "best effort" fallback on other platforms using bitwise arithmetic and black_box
• No Copy bounds, which means all functionality can work with heap-allocated types in addition to stack-allocated
• Expanded selection of traits: CtFind and CtLookup for arrays and slices

Many features of this crate are extractions from the crypto-bigint crate, where we implement all core logic as const fn and needed solutions for implementing constant-time code despite the unique constraints it imposes.

⚠️ Security Warning

The implementation contained in this crate has never been independently audited!

USE AT YOUR OWN RISK!

Minimum Supported Rust Version (MSRV) Policy

MSRV increases are not considered breaking changes and can happen in patch releases.

The crate MSRV accounts for all supported targets and crate feature combinations.

License

Licensed under either of:

• Apache License, Version 2.0
• MIT license

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.