private byte[] deriveKemSharedSecret(
      byte[] dhSharedSecret, byte[] senderEphemeralPublicKey, byte[] recipientPublicKey)
      throws GeneralSecurityException {
    byte[] kemContext = Bytes.concat(senderEphemeralPublicKey, recipientPublicKey);
    return extractAndExpand(dhSharedSecret, kemContext);
  }
Construct HPKE KEM using {@code curve}.
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/NistCurvesHpkeKem.java