  Social networking sites supporting federated identities offer a convenient
and increasingly popular mechanism for cross-site authentication.
Unfortunately, they also exacerbate many privacy and tracking risks. We propose
Crypto-Book, an anonymizing layer enabling cross-site authentication while
reducing these risks.
  Crypto-Book relies on a set of independently managed servers that
collectively assign each social network identity a public/private keypair. Only
an identity's owner learns all the private key shares, and can therefore
construct the private key, while all participants can obtain any user's public
key, even if the corresponding private key has yet to be retrieved. Having
obtained an appropriate key set, a user can then leverage anonymous
authentication techniques such as linkable ring signatures to log into
third-party web sites while preserving privacy.
  We have implemented a prototype of Crypto-Book and demonstrate its use with
three applications: a Wiki system, an anonymous group communication system, and
a whistleblower submission system. Our results show that for anonymity sets of
size 100, Crypto-Book login takes 0.56s for signature generation by the client,
0.38s for signature verification on the server, and requires 5.6KB of
communication bandwidth.
