features.md

Feature Flags

Each primitive family has a leaf feature so applications can select a narrow surface. Umbrella features compose common bundles; full enables the complete public primitive surface.

Default

The default feature set is ["std"]; std implies alloc. Set default-features = false for a no_std build, then enable every required algorithm feature explicitly.

Quick picks

# One algorithm, no_std.
rscrypto = { version = "0.7.8", default-features = false, features = ["sha2"] }

# RSA public-key import and verification, no_std + alloc.
rscrypto = { version = "0.7.8", default-features = false, features = ["rsa"] }

# RSA key generation, signing, encryption, and private-operation blinding.
rscrypto = { version = "0.7.8", default-features = false, features = ["rsa", "getrandom"] }

# ECDSA P-256/SHA-256 and P-384/SHA-384 signing and verification.
rscrypto = { version = "0.7.8", default-features = false, features = ["ecdsa"] }

# FIPS 203 ML-KEM-512/768/1024 KEM APIs with caller-supplied randomness.
rscrypto = { version = "0.7.8", default-features = false, features = ["ml-kem"] }

# Everything.
rscrypto = { version = "0.7.8", features = ["full", "getrandom"] }

# Everything, with parallel BLAKE3 / Argon2 lanes via Rayon.
rscrypto = { version = "0.7.8", features = ["full", "parallel", "getrandom"] }

# Audit-constrained: makes runtime dispatch ignore host SIMD/ASM capabilities.
rscrypto = { version = "0.7.8", features = ["full", "portable-only"] }

Complete feature index

Core features

Feature	Pulls in	Use
default	std	Normal server, CLI, and app builds.
std	alloc	Runtime CPU detection and std::io adapters.
alloc	--	Allocating APIs such as PHC string encoding and Vec-returning digest, MAC, AEAD, and signature helpers.

Umbrella features

Feature	Pulls in
full	checksums, hashes, auth, aead
checksums	crc16, crc24, crc32, crc64
hashes	crypto-hashes, fast-hashes
crypto-hashes	sha2, sha3, blake2b, blake2s, blake3, ascon-hash
fast-hashes	xxh3, rapidhash
auth	macs, kdfs, password-hashing, signatures, key-exchange
macs	hmac, hmac-sha3, kmac, poly1305
kdfs	hkdf, pbkdf2
password-hashing	argon2, scrypt, phc-strings
signatures	ecdsa, ed25519, rsa
key-exchange	x25519, ml-kem
aead	aes-gcm, aes-gcm-siv, chacha20poly1305, xchacha20poly1305, aegis256, ascon-aead

Algorithm leaf features

Feature	Pulls in	Enables
crc16	--	CRC-16/IBM and CRC-16/CCITT
crc24	--	CRC-24/OpenPGP
crc32	--	CRC-32/IEEE and CRC-32C
crc64	--	CRC-64/XZ and CRC-64/NVMe
sha2	--	SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/256
sha3	--	SHA3-224/256/384/512, SHAKE128/256, cSHAKE128/256
blake2b	--	BLAKE2b variable output, BLAKE2b-256, BLAKE2b-512
blake2s	--	BLAKE2s-128, BLAKE2s-256
blake3	--	BLAKE3 hash, keyed hash, and XOF
ascon-hash	--	Ascon-Hash256, Ascon-XOF128, Ascon-CXOF128
xxh3	--	XXH3-64 and XXH3-128
rapidhash	--	Portable RapidHash V3-64, streaming, and collection state
hmac	sha2	HMAC-SHA256/384/512
hmac-sha3	sha3	HMAC-SHA3-224/256/384/512
kmac	sha3	KMAC128 and KMAC256
hkdf	hmac	HKDF-SHA256, HKDF-SHA384, and HKDF-SHA512
poly1305	--	Standalone Poly1305 one-time MAC
pbkdf2	hmac	PBKDF2-HMAC-SHA256 and PBKDF2-HMAC-SHA512
phc-strings	alloc	Canonical password-record generation and bounded PHC verification
argon2	blake2b, alloc	Argon2i, Argon2d, Argon2id
scrypt	pbkdf2, alloc	scrypt
ecdsa-p256	hmac	ECDSA P-256/SHA-256 signing and verification
ecdsa-p384	hmac	ECDSA P-384/SHA-384 signing and verification
ecdsa	ecdsa-p256, ecdsa-p384	ECDSA P-256/P-384 signing and verification
ed25519	sha2	Ed25519 signatures
rsa	alloc, sha2	RSA public/private keys, RSA signatures, OAEP, PKCS#1 v1.5, key generation
x25519	--	X25519 key exchange
ml-kem	sha3	ML-KEM-512, ML-KEM-768, and ML-KEM-1024 key encapsulation
aes-gcm	--	AES-128-GCM and AES-256-GCM
aes-gcm-siv	--	AES-128-GCM-SIV and AES-256-GCM-SIV
chacha20poly1305	--	ChaCha20-Poly1305
xchacha20poly1305	--	XChaCha20-Poly1305
aegis256	--	AEGIS-256
ascon-aead	--	Ascon-AEAD128

Auxiliary features

Feature	Effect
getrandom	Adds OS-backed random generation; see below.
serde	Serde for non-secret byte wrappers (nonces, tags, public keys, signatures).
serde-secrets	Serde for secret-key and shared-secret bytes. Implies serde. Use only for controlled key-material storage, not logs or DTOs.
parallel	Rayon-backed BLAKE3 and Argon2 lane parallelism. Requires std, blake3, argon2.
diag	Diagnostic introspection of dispatch decisions and selected benchmark-only component hooks. Requires std; hidden diagnostic symbols are not stable application API.
portable-only	Makes runtime capability detection report no SIMD/ASM capabilities. See below.

getrandom

getrandom enables fallible OS-backed constructors such as try_random() and try_generate(). It also enables RapidRandomState::try_new(), OS-salted Argon2id and scrypt password-record generation, ML-KEM try_generate_keypair() and try_encapsulate(), AEAD random sealing, and RSA key generation, signing salt and blinding, encryption randomness, and private-operation blinding.

Argon2idPassword::hash_password_with and ScryptPassword::hash_password_with accept a fallible entropy-filling closure without enabling getrandom; rscrypto still owns the fixed salt buffer and PHC encoding. Other random-generation APIs retain equivalent byte-filling closures for constrained integrations and deterministic tests. Deterministic ECDSA signing does not use OS randomness. RSA key generation uses OS entropy to seed its HMAC_DRBG; no separate DRBG feature is required.

portable-only

portable-only makes platform::caps() return the empty capability set. Dispatchers that consult runtime capabilities therefore fall through to portable backends instead of invoking host SIMD/ASM kernels. Use it when a deployment requires runtime dispatch to ignore host acceleration.

This flag does not change platform::caps_static(), override a backend selected at compile time, remove accelerated code from the binary, or create a constant-time proof. Restrict target-feature through RUSTFLAGS when the binary must exclude compile-time accelerated paths. Use constant-time.md for release evidence boundaries.

See compliance.md for the FIPS-oriented deployment boundary.