typedef void (*SilcIDCacheDestructor)(SilcIDCache cache,
                                          SilcIDCacheEntry entry,
                                          void *context);

    Destructor callback that is called when an cache entry expires or is
    purged from the ID cache. The application must not free cache entry
    because the library will do it automatically. The appliation, however,
    is responsible of freeing any data in the entry.