Abstract. The need for protecting the privacy of relationships in social networks has recently been stressed in the literature. Conventional protection mechanisms in those networks deal with the protection of resources and data, i.e. with deciding whether access to resources and data held by a user (owner) should be granted to a requesting user (requestor). However, the relationships between users are also sensitive and need protection: knowing who is trusted by a user and to what extent leaks a lot of confidential information about that user. The use of symmetric key cryptography to implement private relationships in social networks has recently been proposed. We show in this paper how to use public-key cryptography to reduce the overhead caused by private relationships.