@Override
    public byte[] extract(byte[] salt, byte[] labeledIkm) {
        return ((salt.length != 0) ? HKDF.extractedFrom(salt, labeledIkm)
                                   : HKDF.unsaltedExtractedFrom(labeledIkm))
                .pseudoRandomKey();
    }
KDF implementation using HKDF-SHA256

@author vekterli
security-utils/src/main/java/com/yahoo/security/hpke/HkdfSha256.java