features.md

Features

Select the smallest feature set that exposes the primitives you use. Cargo.toml is the complete, authoritative feature graph.

Start here

The default feature is std; std enables alloc. Disable defaults for no_std, then name every required primitive:

# no_std SHA-2
rscrypto = { version = "0.9", default-features = false, features = ["sha2"] }

# Full API with OS randomness
rscrypto = { version = "0.9", features = ["full", "getrandom"] }

Umbrella features trade build size for convenience:

Feature	Includes
checksums	CRC-16, CRC-24, CRC-32, and CRC-64
crypto-hashes	SHA-2, SHA-3, BLAKE2, BLAKE3, and Ascon hash
fast-hashes	XXH3 and RapidHash
hashes	Cryptographic and fast hashes
auth	MACs, KDFs, password hashing, signatures, and key exchange
aead	Every AEAD implementation
full	Checksums, hashes, authentication, and AEADs

Prefer leaf features such as sha2, blake3, aes-gcm, ed25519, or ml-kem in libraries and constrained builds.

Capability features

Feature	Effect
alloc	Enables APIs that own dynamic memory.
std	Enables runtime CPU detection and standard-library integrations; implies alloc.
getrandom	Enables fallible helpers that obtain keys, nonces, salts, or seeds from the OS.
parallel	Enables Rayon-backed BLAKE3 and Argon2 work; implies std, blake3, and argon2.
serde	Serializes public types.
serde-secrets	Also serializes secret keys and shared secrets; use only at an explicit key-storage boundary.
portable-only	Makes runtime capability detection report no SIMD or ASM capabilities.
diag	Exposes unstable diagnostic and evidence hooks; do not use it as application API.

getrandom changes entropy acquisition, not algorithm availability. APIs that accept caller-provided entropy remain available without it.

portable-only affects dispatchers that consult platform::caps(). It does not remove accelerated code from the binary or override backends selected by compile-time target_feature settings. See platforms.md.

Verify a selection

cargo check --no-default-features --features sha2
just plan
just check
just feature-contracts compile

just check asks Cargo Rail for the affected feature groups and runs only the compile profiles whose resolved Cargo feature graph includes those groups. just validate also runs selected runtime capability profiles. The explicit just feature-contracts compile command remains the full compile contract.

Use docs.rs to see which items each feature exposes.