Preserving Privacy versus Data Retention

The retention of communication data has recently attracted much public interest, mostly because of the possibility of its misuse. In this paper, we present protocols that address the privacy concerns of the communication partners. Our data retention protocols store streams of encrypted data items, some of which may be flagged as critical (representing misbehavior). The frequent occurrence of critical data items justifies the self-decryption of all recently stored data items, critical or not. Our first protocol allows the party gathering the retained data to decrypt all data items collected within, say, the last half year whenever the number of critical data items reaches some threshold within, say, the last month. The protocol ensures that the senders of data remain anonymous but may reveal that different critical data items came from the same sender. We call this the affiliation of critical data. Our second, computationally more complex scheme obscures the affiliation of critical data with high probability.