I already read the API documentation of Picasa. I can see the normal albums, but not the shared ones. The documentation says:
If you are logged in as the owner of the content, you can retrieve all of the photos you've uploaded and albums you've created in Google Photos and Picasa Web Albums via the Picasa Web Albums Data API. There are some exceptions. For example, Google Photos "Shared Albums" are not available via this API. There are no plans to add support for all Google Photos features to the Picasa Web Albums Data API.
Okay so there is no way with the Picasa API, is there an alternative API which grant the access?
Update: Google has launched a new Google Photos API.
Google Photos (and previously Google+ Photos) does not have an API. The photo data that is accessible is only available through the Picasa API.