code: func (c *Client) ServerKeyID() (string, error) {
  	if err := c.condDiscovery(); err != nil {
  		return "", err
  	}
  	if c.serverKeyID == "" {
  		return "", ErrNoSigning
  	}
  	return c.serverKeyID, nil
  }
docs: // ServerKeyID returns the server's GPG public key ID, in its long (16 capital
  // hex digits) format. If the server isn't running a sign handler, the error
  // will be ErrNoSigning.