In many content distribution systems it is important both to restrict access to content to authorized users and to protect the identities of these users. We discover that current systems for encrypting content to sets of users are subject to attacks on user privacy. We propose a new mechanism, private broadcast encryption, to protect the privacy of users of encrypted file systems and content delivery systems. We construct a private broadcast scheme, with a strong privacy guarantee against an active attacker, that achieves ciphertext length, encryption time, and decryption time comparable with the non-private schemes currently used in encrypted file systems.
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