code: func (c *Client) ServerPublicKeyBlobRef() (blob.Ref, error) {
  	if err := c.condDiscovery(); err != nil {
  		return blob.Ref{}, err
  	}
  
  	if !c.serverPublicKeyBlobRef.Valid() {
  		return blob.Ref{}, ErrNoSigning
  	}
  	return c.serverPublicKeyBlobRef, nil
  }
docs: // ServerPublicKeyBlobRef returns the server's public key blobRef
  // If the server isn't running a sign handler, the error will be ErrNoSigning.