Module kdf 

Available on crate features kdf-hkdf or kdf-argon2 only.

Key Derivation Functions (KDF).

Two algorithms ship in 0.6.0, addressing different needs:

Algorithm	Purpose	Speed	Feature
HKDF-SHA256	Derive one-or-many subkeys from a high-entropy IKM	Fast (µs)	kdf-hkdf
HKDF-SHA512	Same, wider underlying digest	Fast (µs)	kdf-hkdf
Argon2id	Derive a key from a password (low-entropy input)	Slow (~100ms)	kdf-argon2

Which one do I want?

• HKDF (RFC 5869) for deriving subkeys from a master key, a shared secret from a key exchange, or anything else already high-entropy. HKDF does not protect against weak inputs — feeding it a password is a security mistake.

• Argon2id (RFC 9106) for deriving a key from a password. The memory-hardness and tuneable cost are what protect against brute-force attempts; the slowness is the point.

Examples

Deriving a 32-byte subkey from a master:

use crypt_io::kdf;
let master = [0x42u8; 32];
let subkey = kdf::hkdf_sha256(&master, Some(b"salt"), b"app:session:v1", 32)?;
assert_eq!(subkey.len(), 32);

Hashing and verifying a password:

use crypt_io::kdf;
let phc = kdf::argon2_hash(b"correct horse battery staple")?;
assert!(kdf::argon2_verify(&phc, b"correct horse battery staple")?);
assert!(!kdf::argon2_verify(&phc, b"wrong guess")?);

Structs

Argon2Paramskdf-argon2

Tuneable Argon2id parameters.

Constants

ARGON2_DEFAULT_OUTPUT_LENkdf-argon2

Default Argon2id output length, in bytes. Equal to 32 (256 bits).

ARGON2_DEFAULT_SALT_LENkdf-argon2

Default Argon2id salt length, in bytes. Equal to 16 (128 bits, the PHC-recommended minimum).

HKDF_MAX_OUTPUT_SHA256kdf-hkdf

Maximum HKDF-SHA256 output length, in bytes. Equal to 255 * 32 = 8160.

HKDF_MAX_OUTPUT_SHA512kdf-hkdf

Maximum HKDF-SHA512 output length, in bytes. Equal to 255 * 64 = 16320.

Functions

argon2_hashkdf-argon2

Hash password with Argon2id using the default parameter set and a fresh random salt. Returns the PHC-encoded hash string.

argon2_hash_with_paramskdf-argon2

Like argon2_hash but uses caller-supplied parameters.

argon2_verifykdf-argon2

Verify password against a PHC-encoded Argon2 hash string.

hkdf_sha256kdf-hkdf

Derive len bytes of output keying material via HKDF-SHA256.

hkdf_sha512kdf-hkdf

Derive len bytes of output keying material via HKDF-SHA512.