Class Kdf

java.lang.Object

zeroecho.sdk.util.Kdf

public final class Kdf
extends Object

HKDF utilities for deriving symmetric keys from a KEM shared secret. This class provides an RFC 5869 compliant HKDF-SHA256 implementation.

Notes

The function implements HKDF-Extract followed by HKDF-Expand with SHA-256. If salt is null or empty, a zero-filled salt of length 32 is used as recommended by RFC 5869. If info is null, it is treated as an empty byte array.

// Example: derive 32 bytes for AES-256 or ChaCha20-Poly1305
byte[] ikm   = kemSharedSecret;          // input keying material from KEM
byte[] salt  = null;                     // or a KEM-specific salt
byte[] info  = "ZeroEcho-KEM".getBytes(StandardCharsets.US_ASCII);
int    len   = 32;
byte[] key   = Kdf.hkdfSha256(ikm, salt, info, len);

Method Summary

Modifier and Type	Method	Description
static byte[]	hkdfSha256(byte[] ikm, byte[] salt, byte[] info, int length)	Derives length bytes of key material using HKDF-SHA256 as specified in RFC 5869.

Methods inherited from class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

hkdfSha256

public static byte[] hkdfSha256(byte[] ikm,
 byte[] salt,
 byte[] info,
 int length)
                         throws GeneralSecurityException

Derives length bytes of key material using HKDF-SHA256 as specified in RFC 5869.

HKDF-Extract: PRK = HMAC(salt, IKM) where salt defaults to HashLen zeros if salt is null or empty.

HKDF-Expand: T(0)=empty, T(i)=HMAC(PRK, T(i-1) || info || i), and the output key material is the first length bytes of T(1)||...||T(N), where N = ceil(length / HashLen) and i is a single octet counter.

Parameters:

ikm - input keying material; must not be null or empty

salt - optional salt; if null or empty, a zero salt of 32 bytes is used

info - optional context/application info; if null, treated as empty

length - number of output bytes to produce; must be in range 1..(255*32)

Returns:

derived keying material of exactly length bytes

Throws:

GeneralSecurityException - if the MAC operation fails

IllegalArgumentException - if ikm is empty or length is out of range

Since:

1.0