On 1 Jul 1996, David Wagner wrote:
Someone sniffing the Berkeley 'net can tell when I receive an alt.anonymous.messages message by when I download an article from the NNTP server
So, download every message, all the time, and junk posts that don't interest you offline. You betray yourself as an Evil Anonymous Communicator, but somehow I think they might already know. You give no information about which messages you're actually interested in unless your local workstation is compromised.
Furthermore, even if you run a trusted NNTP server on your local machine, there are still vulnerabilities. Someone sniffing on your subnet can tell when you inject a new message onto alt.anonymous.messages, as can your neighboring NNTP servers.
This is true. You'd have to generate white noise, again betraying yourself, but only in general. "They" would need to track every message. To make it more interesting, encrypt a bunch of messages for bogus PGP keys created for the purpose. -rich