code: func (api *PublicWhisperAPI) AddSymKey(ctx context.Context, key hexutil.Bytes) (string, error) {
  	return api.w.AddSymKeyDirect([]byte(key))
  }
docs: // AddSymKey import a symmetric key.
  // It returns an ID that can be used to refer to the key.
  // Can be used encrypting and decrypting messages where the key is known to both parties.