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Yawning Angel wrote:

[Folding in the 2nd reply]

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If you know some details of how I2P resists blocking, please add them to this wiki page: https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports

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It doesn't do anything special.

Yet (see below).

There are several reason for I2P not being blocked as much as Tor:

* Traffic - I2P is used by fewer "clients" (tens of thousands) than Tor (hundreds of thousands to millions). * Content - I2P doesn't provide easy access to clearnet resources like Tor does. * Easiness - non-bridged Tor uses TLS as its outside protocol, and can therefore be easily identified using standard TLS identifiers for firewalls. Identification of I2P traffic requires special-purpose identifiers (which do exist, but add to the maintenance burden of the firewall).

Thus, blocking I2P currently provides less value-for-effort than blocking Tor.

The old TCP based protocol (NTCP) is trivially identifiable.

We have been (slowly) working on NTCP2, a replacement for NTCP, for nearly 2 years now. We have two different proposals on the table [0], and are also considering the benefits of the mutual-authentication variant of Ace [1,2]. Anyone here interested in protocol design is welcome to add to the discussion!

Blocking the more modern UDP protocol (SSU) would require looking for high-entropy UDP or doing statistical attacks IIRC. Active probing is possible if they run a node that's part of the network and obtain enough key material (But, I'd need to look at the floodfill system again to figure out how many nodes for how long is realistically required).

Active probing of this kind essentially requires that the adversary enumerate the entire netDB, because geographical information cannot be used to index into the hash table. We estimate there to be about 750 floodfills at present, so it's not outside the realms of possibility. However, in practice it isn't really viable:

* By default, routers in China (and several other countries, picked based on the Press Freedom Index [3]) start in hidden mode, meaning they don't publish their details in the netDB. So harvesting attacks will not pick any of those up, and therefore won't be useful for matching the source of UDP packets.

* I2P routers actively measure the performance of their peers, and make routing decisions based on their measurements. If an adversary dropped UDP packets based on matching their destination to the harvested netDB, the router would simply use other routers that aren't being blocked.

The most vulnerable point in the system is actually the reseed servers - - blanket blocks on them can prevent a router from finding any initial peers. However, if a router can connect to a reseed server, it has a good chance of being able to connect to the network (HTTPS with certificate pinning prevents snooping on or modifying what peers a router is given).

Now that I think of it, the reseed system would be an ideal location to start introducing PTs into I2P. We have been thinking about how to leverage PTs in inter-node communication for several years, but reseeding over PTs would be much easier to implement and would provide noticeable benefits (making blanket bans much harder).

str4d

[0] http://zzz.i2p/topics/1577-ntcp2 [1] https://moderncrypto.org/mail-archive/curves/2014/000151.html [2] https://moderncrypto.org/mail-archive/curves/2014/000163.html [3] https://github.com/i2p/i2p.i2p/blob/master/router/java/src/net/i2p/route r/transport/BadCountries.java