code: func (whisper *Whisper) GetPrivateKey(id string) (*ecdsa.PrivateKey, error) {
  	whisper.keyMu.RLock()
  	defer whisper.keyMu.RUnlock()
  	key := whisper.privateKeys[id]
  	if key == nil {
  		return nil, fmt.Errorf("invalid id")
  	}
  	return key, nil
  }
docs: // GetPrivateKey retrieves the private key of the specified identity.