I found the following definition [here](http://plato.stanford.edu/entries/it-privacy/#IdeMan) : 

> 
> From a privacy perspective a better solution would be the use of
>  attribute-based authentication (Goyal et al. 2006) which allows access
>  of online services based on the attributes of users, for example their
>  friends, nationality, age etc
> 
> 
> 

But it's not very clear to me, so: 

1. What is attribute-based authentication?
2. What are its advantages?
3. How can it enhance privacy protection while online?