Crate subtle

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

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.

subtle = "2.5"

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.

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 bitwise operations are not recognized as a conditional assignment and optimized back into a branch.

For a compiler to recognize that bitwise operations represent a conditional assignment, it needs to know that the value used to generate the bitmasks is really a boolean i1 rather than an i8 byte value. In an attempt to prevent this refinement, the crate tries to hide the value of a Choice’s inner u8 by passing it through a volatile read. For more information, see the About section below.

Rust versions from 1.66 or higher support a new best-effort optimization barrier (core::hint::black_box). To use the new optimization barrier, enable the core_hint_black_box feature.

Rust versions from 1.51 or higher have const generics support. You may enable const-generics feautre to have subtle traits implemented for arrays [T; N].

Versions prior to 2.2 recommended use of the nightly feature to enable an optimization barrier; this is not required in versions 2.2 and above.

Note: the subtle crate contains debug_asserts to check invariants during debug builds. These invariant checks involve secret-dependent branches, and are not present when compiled in release mode. This crate is intended to be used in release mode.


Documentation is available here.

§Minimum Supported Rust Version

Rust 1.41 or higher.

Minimum supported Rust version can be changed in the future, but it will be done with a minor version bump.


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

Old versions of the optimization barrier in impl From<u8> for Choice were 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.

From version 2.2, it was based on Diane Hosfelt and Amber Sprenkels’ work on “Secret Types in Rust”. Version 2.5 adds the core_hint_black_box feature, which uses the original method through the core::hint::black_box function from the Rust standard library.

subtle is authored by isis agora lovecruft and Henry de Valence.


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. Because side-channel resistance is not a property of software alone, but of software together with hardware, any such effort is fundamentally limited.



  • The Choice struct represents a choice for use in conditional assignment.
  • The CtOption<T> type represents an optional value similar to the Option<T> type but is intended for use in constant time APIs.


  • A type which can be conditionally negated in constant time.
  • A type which can be conditionally selected in constant time.
  • An Eq-like trait that produces a Choice instead of a bool.
  • A type which can be compared in some manner and be determined to be greater than another of the same type.
  • A type which can be compared in some manner and be determined to be less than another of the same type.