private SecretKeySpec deriveKeySpec(byte[] salt, byte[] aad) throws GeneralSecurityException {
    byte[] key = Hkdf.computeHkdf(hkdfAlg, ikm, salt, aad, keySizeInBytes);
    return new SecretKeySpec(key, "AES");
  }
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/AesGcmHkdfStreaming.java