private SecretKeySpec deriveHmacKeySpec(byte[] keyMaterial) throws GeneralSecurityException {
    return new SecretKeySpec(keyMaterial, keySizeInBytes, HMAC_KEY_SIZE_IN_BYTES, tagAlgo);
  }
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