private byte[] deriveKeyMaterial(byte[] salt, byte[] aad) throws GeneralSecurityException {
    int keyMaterialSize = keySizeInBytes + HMAC_KEY_SIZE_IN_BYTES;
    return Hkdf.computeHkdf(hkdfAlgo, ikm, salt, aad, keyMaterialSize);
  }
Returns the expected size of the ciphertext for a given plaintext. The returned value includes
the header and offset.
java_src/src/main/java/com/google/crypto/tink/subtle/AesCtrHmacStreaming.java