http://media.ccc.de/browse/congress/2014/31c3_-_6140_-_en_-_saal_1_-_201412291715_-_dp5_pir_for_privacy-preserving_presence_-_ian_goldberg_-_george_danezis_-_nikita_borisov.html

In the wake of the Snowden revelations and the explicit targetting of address book and buddy list information, social service providers may wish to actively avoid learning which of its users are friends. In this talk, we will introduce the workings of a surprising technology called private information retrieval, or PIR. Then, we will describe its use in DP5, a new suite of privacy-preserving presence protocols that allow people to determine when their friends are online (and to establish secure communications with them), without a centralized provider ever learning who is friends with whom.

Ian Goldberg, George Danezis, Nikita Borisov