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I have been contemplating how to mark broadcast messages as being 'for' someone. To foil traffic analysis, you don't want to include their nym or key-id, for the sake of the your poor CPU, you want to avoid the need to attempt decryption on everything that passes through.
The main problem is how to avoid decrypting _most_ of the traffic, without giving away significant information about the recipient. One approach is to do something some political users have been asking for - implement support for very short keyids (e.g. 4 bits instead of 24-32), so that the keyid isn't a good identifier for the user. Another approach is to include a tag in the Subject: with either a hash of the key (substantially reducing the number of bits), or simply the last hex or two of the keyid - that lets you ignore 15/16th or 255/256th of the traffic, without giving away much.
I am not completely clear on what sort of communication you are trying to protect, and what your threat model is. Are you worried about an attacker noticing that an anonymous ID is getting a lot of messages? If you are using PGP and a message pool, any attacker can decrypt all the messages, and see which correspond to which key, and therefor to which anonymous ID. The only way around this is to use private key crypto. If you are doing that, then you can also use a shared secret to generate a stream of one use message IDs. -----BEGIN PGP SIGNATURE----- Version: 2.6 iQCVAwUBLu09tlVkk3dax7hlAQHLkAP+L8j+9eLcwC7oPpq+OPxDb+C6QJ/H0OX5 3O7uQnU8OZY9YgHsMETh6AY7aTMZYrm9+p3wJu9znFYOwXRIzF+spfyxDDzLVuE1 kQBwGKQt/5YQd6i/jc1Jias6rb/GOBvckYcHKERjSBL638Gi65cC4OFEff5k6ujQ YkkQXkh3JWg= =o5nF -----END PGP SIGNATURE----- -------------------------------------------------- Lance Cottrell who does not speak for CASS/UCSD loki@nately.ucsd.edu PGP 2.6 key available by finger or server. Encrypted mail welcome. Home page http://nately.ucsd.edu/~loki/ Home of "chain" the remailer chaining script. For anon remailer info, mail remailer@nately.ucsd.edu Subject: remailer-help "Love is a snowmobile racing across the tundra. Suddenly it flips over, pinning you underneath. At night the ice weasels come." --Nietzsche