Class Hkdf

java.lang.Object
com.codename1.security.Hkdf
public final class Hkdf extends Object

HMAC-based key derivation (HKDF), per RFC 5869.

HKDF turns input keying material that is not uniformly random -- the shared secret of a KeyAgreement, say -- into one or more keys of any length. It runs in two steps: extract concentrates the input into a pseudorandom key, and expand stretches that into as many bytes as are asked for, bound to an info label so that keys derived for different purposes never coincide.

KeyPair mine = KeyGenerator.ec(256);
KeyPair theirs = KeyGenerator.ec(256);
byte[] shared = KeyAgreement.ecdh(mine.getPrivateKey(), theirs.getPublicKey());
byte[] keys = Hkdf.derive("SHA-256", null, shared, "chat keys".getBytes(), 64);

It is pure Java over Hmac, so it behaves identically on every port and on the server.

  • Method Summary

    Modifier and Type
    Method
    Description
    static byte[]
    derive(String algorithm, byte[] salt, byte[] ikm, byte[] info, int length)
    Extract then expand in one call.
    static byte[]
    expand(String algorithm, byte[] prk, byte[] info, int length)
    The expand step: derives length bytes from a pseudorandom key.
    static byte[]
    extract(String algorithm, byte[] salt, byte[] ikm)
    The extract step: HMAC(salt, ikm).

    Methods inherited from class Object

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

  • Method Details

    • extract

      public static byte[] extract(String algorithm, byte[] salt, byte[] ikm)

      The extract step: HMAC(salt, ikm). A null or empty salt is replaced by a string of zero bytes the length of the hash, as the RFC specifies.

      Parameters

      • algorithm: the hash, any name [Hmac#create(String, byte[])] accepts

      • salt: optional salt

      • ikm: the input keying material

    • expand

      public static byte[] expand(String algorithm, byte[] prk, byte[] info, int length)

      The expand step: derives length bytes from a pseudorandom key.

      Parameters

      • algorithm: the hash

      • prk: the pseudorandom key, usually the result of extract(String, byte[], byte[])

      • info: context label; may be null

      • length: bytes wanted, at most 255 times the hash length

    • derive

      public static byte[] derive(String algorithm, byte[] salt, byte[] ikm, byte[] info, int length)

      Extract then expand in one call.

      Parameters

      • algorithm: the hash, for example "SHA-256"

      • salt: optional salt

      • ikm: the input keying material

      • info: context label; may be null

      • length: bytes wanted