All 
unlisted albums have an authorisation key in the URL; the key is a combination of letters and numbers which make the URL very difficult to guess. Only visitors with the exact web address will be able to see your unlisted album. If anyone attempts to view an unlisted album without this additional code, he'll receive an error message.
Making changes to your album name or Gallery URL will automatically generate a new authorisation key. This will invalidate the existing unlisted URL, making your album inaccessible to anyone that attempts to view the album using the outdated link. If you make changes to these items, make sure you share the new URL with anyone who needs to view the album.
You can also manually generate a new authorisation key at any time for an existing unlisted album. A new key is useful if more people than intended are accessing your unlisted album URL. Here's how you can generate a new key:
- In Picasa Web Album, click on the album from your My Photos page.
- Click Link to this album on the right-hand side.
- Click Reset unlisted link to generate a URL with a new authorisation key.