private byte[] extractAndExpand(byte[] dhSharedSecret, byte[] kemContext)
      throws GeneralSecurityException {
    byte[] kemSuiteId = HpkeUtil.kemSuiteId(HpkeUtil.X25519_HKDF_SHA256_KEM_ID);
    return hkdf.extractAndExpand(
        /* salt= */ null,
        dhSharedSecret,
        "eae_prk",
        kemContext,
        "shared_secret",
        kemSuiteId,
        hkdf.getMacLength());
  }
Construct X25519-HKDF HPKE KEM using {@code hkdf}.
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/X25519HpkeKem.java