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[WIP] ctutils: constant-time selection and equality testing
Inspired by the `subtle` crate, this is a next-generation constant time utility crate built on the `cmov` crate's constant-time selection/predication and equality comparisons. It proactively uses `const fn` wherever possible. Additionally, it uses `black_box` as a "best effort" optimization barrier whenever accessing its constant time conditional type (called `Choice` as in `subtle`), providing an additional line of defense against possible future compiler optimizations.
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.github/workflows/cmov.yml

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- run: ${{ matrix.deps }}
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- run: cargo test --target ${{ matrix.target }}
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# Test using `cargo careful`
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test-careful:
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runs-on: ubuntu-latest
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steps:
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- uses: actions/checkout@v6
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- uses: dtolnay/rust-toolchain@nightly
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- run: cargo install cargo-careful
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- run: cargo careful test --all-features
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# Cross-compiled tests
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cross:
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test-cross:
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strategy:
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matrix:
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include:

.github/workflows/ctutils.yml

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name: ctutils
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on:
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pull_request:
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paths:
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- ".github/workflows/ctutils.yml"
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- "ctutils/**"
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- "Cargo.*"
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push:
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branches: master
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permissions:
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contents: read
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defaults:
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run:
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working-directory: ctutils
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env:
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CARGO_INCREMENTAL: 0
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RUSTFLAGS: "-Dwarnings"
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# Cancels CI jobs when new commits are pushed to a PR branch
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concurrency:
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group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
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cancel-in-progress: true
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jobs:
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build:
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runs-on: ubuntu-latest
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strategy:
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matrix:
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rust:
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- 1.85.0 # MSRV
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- stable
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target:
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- thumbv7em-none-eabi
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- wasm32-unknown-unknown
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steps:
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- uses: actions/checkout@v5
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- uses: RustCrypto/actions/cargo-cache@master
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- uses: dtolnay/rust-toolchain@master
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with:
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toolchain: ${{ matrix.rust }}
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targets: ${{ matrix.target }}
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- run: cargo build --target ${{ matrix.target }}
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minimal-versions:
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uses: RustCrypto/actions/.github/workflows/minimal-versions.yml@master
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with:
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working-directory: ${{ github.workflow }}
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test:
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runs-on: ubuntu-latest
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strategy:
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matrix:
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rust:
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- 1.85.0 # MSRV
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- stable
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steps:
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- uses: actions/checkout@v5
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- uses: RustCrypto/actions/cargo-cache@master
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- uses: dtolnay/rust-toolchain@master
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with:
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toolchain: ${{ matrix.rust }}
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- run: cargo test
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# Test using `cargo careful`
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test-careful:
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runs-on: ubuntu-latest
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steps:
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- uses: actions/checkout@v6
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- uses: dtolnay/rust-toolchain@nightly
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- run: cargo install cargo-careful
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- run: cargo careful test --all-features

Cargo.lock

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Cargo.toml

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"block-padding",
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"collectable",
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"cmov",
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"ctutils",
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"cpufeatures",
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"dbl",
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"digest-io",
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[profile.dev]
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opt-level = 2
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[patch.crates-io]
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cmov = { path = "cmov" }

cmov/README.md

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[build-image]: https://github.com/RustCrypto/utils/actions/workflows/cmov.yml/badge.svg?branch=master
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[build-link]: https://github.com/RustCrypto/utils/actions/workflows/cmov.yml?query=branch:master
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[//]: # (general links)
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[//]: # (links)
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[RustCrypto]: https://github.com/RustCrypto
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[CMOV family]: https://www.jaist.ac.jp/iscenter-new/mpc/altix/altixdata/opt/intel/vtune/doc/users_guide/mergedProjects/analyzer_ec/mergedProjects/reference_olh/mergedProjects/instructions/instruct32_hh/vc35.htm

ctutils/CHANGELOG.md

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# Changelog
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All notable changes to this project will be documented in this file.
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The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
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and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).

ctutils/Cargo.toml

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[package]
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name = "ctutils"
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description = """
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Constant-time utility library with selection and equality testing support targeting cryptographic
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applications. Supports `const fn` where appropriate. Built on the `cmov` crate which provides
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architecture-specific predication intrinsics. Heavily inspired by the `subtle` crate.
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"""
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version = "0.0.0"
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authors = ["RustCrypto Developers"]
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license = "Apache-2.0 OR MIT"
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homepage = "https://github.com/RustCrypto/utils/tree/master/ctselect"
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repository = "https://github.com/RustCrypto/utils"
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categories = ["cryptography", "no-std"]
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keywords = ["crypto", "intrinsics"]
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readme = "README.md"
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edition = "2024"
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rust-version = "1.85"
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[dependencies]
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cmov = "0.4"

ctutils/LICENSE-APACHE

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ctutils/LICENSE-MIT

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Copyright (c) 2025 The RustCrypto Project Developers
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Permission is hereby granted, free of charge, to any
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person obtaining a copy of this software and associated
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documentation files (the "Software"), to deal in the
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Software without restriction, including without
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the Software, and to permit persons to whom the Software
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is furnished to do so, subject to the following
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The above copyright notice and this permission notice
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shall be included in all copies or substantial portions
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of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
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TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
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CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
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