lib-constant-time

Minimal no_std constant-time primitives for EriX security-critical code paths.

This crate exists to provide small, auditable building blocks for byte-level operations where timing behavior must not depend on secret data.

UEFI + x86_64 are current primary targets, but this crate is explicitly portable and architecture-neutral.

Scope

This crate provides:

• constant-time equality for fixed and variable byte slices
• constant-time zero checks
• constant-time conditional selection/masking helpers

This crate does not provide:

• hashing algorithms
• signature algorithms
• key storage or secure element integration
• random number generation

Why this crate exists

EriX verification paths (boot image verification, signature checks, hash comparison) need a shared implementation for constant-time byte operations.

Keeping these primitives in a dedicated crate improves:

• auditability (small API, focused code)
• reuse (lib-bootimg, future lib-ed25519, future kernel-side verifiers)
• consistency (single behavior across components)

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Design constraints

• #![no_std]
• zero external dependencies
• no dynamic allocation required
• deterministic behavior and explicit error handling
• minimal unsafe (ideally none)

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Security notes

"Constant-time" here means branch/data-flow behavior should not depend on secret values for supported operations.

This does not eliminate all side channels (cache hierarchy, power, platform noise), but removes common variable-time bugs in straightforward comparisons.

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Intended consumers

• lib-bootimg (manifest/section hash comparisons)
• future lib-ed25519 (verification helpers)
• bootloader and kernel-side validation code

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Status

Implemented baseline.

Core constant-time primitives are implemented with unit coverage. Remaining work focuses on hardening-oriented review and expanded downstream integration checks.

See ARCHITECTURE.md and ROADMAP.md.