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 identitys owner learns all the private key shares, and can therefore construct the private key, while all participants can obtain any users 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.
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