directory/directory_transport.go
func (ref dirReference) signaturePath(index int) string {
	return filepath.Join(ref.path, fmt.Sprintf("signature-%d", index+1))
}
func (ref dirReference) signaturePath(index int) string {
	return filepath.Join(ref.path, fmt.Sprintf("signature-%d", index+1))
}
// signaturePath returns a path for a signature within a directory using our conventions.