I wrote about freedom 2.0 mail system:
So these interactive connections are immediately forward-secret, and therefore you have much better protection against subpeona attack. However they are more vulnerable to all-powerful observer attacks who could probably figure out which pseudonym was which by sending lots of unique sized email and then watching traffic patterns flow through the network.
So a couple of other comments: - Ulf Moeller, Anton Stiglic and I published our thoughts about how someone could go about doing the passive adversary traffic analysis attacks on interactive systems such as the freedom anonymous network: Apr 01 - "Traffic Analysis Attacks and Trade-Offs in Anonymity Providing systems", Information Hiding 2001, Adam Back, Ulf Moeller and Anton Stiglic http://www.cypherspace.org/adam/pubs/traffic.pdf - and in fact the version 1 freedom mail system had other issues: the mail was not split up into fixed sized chunks (as it is with mixmaster), so it suffered the same vulnerabilities that type I based nymservers do: it was in addition equally vunlerable to traffic analysis. I'd take this version 1 freedom mail vulnerability to indiate that in essentially all respects version 2 was more secure than version 1; though some of the version 1 design-issues could have been fixed in similar ways that are proposed in the mixminion project. The mixminion project project (aka Type III remailer) design and implementation attempts to avoid these issues by merging reply block functionality into a mixmaster like fixed sized message mix net. Mixminion actually uses Single Use Reply Blocks (SURBs) to in addition reduce vulnerability to flooding attacks (where someone just sends lots of messages to see where they arrive as they flow down the reply block). The recipient I think is expected to send a few SURBs to nyms he communicates with, and to send SURBs to the nymserver to pick up mail from regular internet mail senders (who are not using the mixminion client). If I understand it is also planned that the mixminion / Type III protocol will be implemented within mixmaster as mixmaster version 4. (The current alpha mixminion code is a separate code base, written in python scripting language). The other good thing about mixminion / type III protocol is that finally type I remailers with their traffic analysis issues could be phased out. (Their remaining reason for existance was to support reply-block functionality for nymservers). Adam