CONG

CONG (COre Next Generation) is the name of the project redesigns the CORE service. Here we document the design decisions and parts that are about to change.

Design goals

TODO

Key exchange

While we are at it we may as well improve the key exchange. Currently, we are using our own ECDHE key exchange that derives 2x2 keys. 2 keys for each direction (sending/receiving). Each direction uses two 256-bit symmetric encryption keys derived through the ECDH exchange. Each payload is encrypted using AES(kA, Twofish(kB, payload)) both in CFB mode (!).

For CONG, we should double-check the security of your ECDHE construction and then potentially move away from AES/Twofish, possible towards ChaCha20 or XSalsa20 (Needs discussion).

Proposal:

Peer IDs

Peer ids stop to be unique for the lifetime of a peer, but change each time a peer’s addresses change. This includes gaining or losing an address.

It is important to note that this design choice only increases the cost of network location tracking and does not fully prevent it. For this feature onion routing on top of CADET is envisioned.

At this point it seems like one has to weigh privacy versus performance when it comes to this design decision.

Reasoning

This change was introduced in order to stop tracking of more mobile peers. For example a more mobile peer (laptop) that logs into the network at different places can be easily tracked by everyone just by recording the different addresses that are tied to that peer id over time.

Attacker Model

An attacker observes the hellos (containing ip addresses) published under a peer id and is thus capable of tracking locations and thus obtaining a movement profile of this peer.

With the proposed changes to the peer id an attacker can only see a peer id and its connected set of addresses. A movement profile can not be obtained in the previous way.

Tracking of addresses/locations might still be possible in the scenario that a mobile client uses mobile broadband and wifi uplinks and uses them in an ‘overlapping’ manner. (Switching on mobile broadband before leaving the range of a wifi hotspot.) The overlap can be used to link the ‘before’ and ‘after’ address and - in extreme scenarios - obtain the full movement profile again. Note that this does not work on all, not all the time and requires work for the correlation.

A way to circumvent this tracking mechanism would be for an attacker to exploit the means for consistently connect to the same peer. For example gns. With the knowledge of the gns entry, its peer ids and thus its addresses can be fully tracked. ..

TODO explain that this limits the attack, is all we can do on this level and that onion routing/mix networking is supposed to circumvent this

Implications

Here we spell out the implications for different parts of the framework. This should help get a grasp of the implications of the change. .. TODO poor phrasing?

DHT

The DHT uses the peer id such that it determines which buckets a peer is responsible for. So each time the peer id changes, the peer becomes responsible for different data.

Scope of Peer ID for higher Layers

When peer ids stop to be unique over time, the framework is in lack of a globally unique identifier. Higher layers may rely (have reliede) on the uniqueness of the ids. This means gnunet has to use other means for this purpose. The Reconnects section below is concerned with the specific impact on reconnects for different higher-layer services. In general gns/identity offers this functionality.

As peer ids cease to be unique over time, this might be a good point to review the scope of its and other elements’ usage and terminology. (See Open_Design_Questions)

FIXME This section overlaps in scope with the next section.

Reconnects

When addresses (and with those the peer id) of a peer change, all core connections need to be torn down and with them all higher-layer connections. This affects the layers above CADET as follows:

TODO lookup rst lists!

  • Revocation: Is not really affected as it is only connected to direct CADET neighbors and makes no use of CADET’s routing, only of its flow and congestion control.

  • File sharing: Only the non-anonymous filesharing uses CADET connections. This is not significantly affected by a reconnect as it only looks up peer and port in the DHT, so in the meanwhile it’s looking for other peers. TODO this is very unclear to me!

  • Messenger: All CADET connections would break and the peer might assume that all previously connected peers went offline. So it would require a mechanism to reconnect to peers with a known peer identity which offer routing capability (open port via CADET to connect to). In case the peer itself is providing such capability, it would help to know about peer ID changes ahead of time to communicate a switch between IDs to other peers. For other reconnections via GNS lookups are required.

  • Conversation: The call would be interrupted until the new peer id of the other has been found via GNS.

TODO: question: is gns needed for a reconnect? couldn’t the peer with the new id simply ‘call back’ the other peer?

See Open_Design_Questions for thoughts on good designs to handle address changes more smoothly.

Messenger

The current implementation of messenger heavily relies on a globally unique peer id. The change requires messenger to account for peer id changes.

Details on how

Open questions

Core’s Ownership of Peer IDs

When the peer id was static, all parts of gnunet had a simple way to interface with it. Once it becomes dynamic, it makes a lot of sense that a single part takes control/responsibility for it. Core is the most suitable layer for this.

Implications

Core needs to take ownership. It is responsible for generating, changing and publishing the peer id to the peersore (?). As other parts still rely on the usage of the peer id, it needs to provide an interface for those systems: It needs to inform about current id, inform about changes and sign data on request. To be gentle on the ipc, it should not sign big amounts of data - if applicable rather hashes of data or such.

Transport

Details on how

Protocol-Versioning

TODO

libp2p Underlay

TODO

Open Design Questions

In this section we list design question that are not decided on, yet.

Peer ID changes and connectivity

In case a peer’s addresses change, it gets a new peer id and therefor needs to reconnect. The challenge is to reconnect as fast as possible. The main problem is that a peer cannot know its next peer id in all cases. Connections that have dedicated peers at its endpoints will probably look up the new peer id of the other peer in a higher-layer service, most probably gns.

In the case in which a peer just gains an additional address, that peer can pre-calculate its next peer id, signal it via still open connections on the old peer id and finally switch to using the new peer id.

Other more evolved ideas include using multiple peer ids per peer: Either an additional address-independent peer id that will ‘survive’ address changes and serve as means to link to the address-based peer id after a change. It would just be sent to connected peers and reset once all connections have been re-established. Alternatively (maybe in addition) peers could use multiple address-based peer ids - one per address. Thus some peer ids might stay unchanged while others go offline.

Another idea to address this challenge is to keep peer ids in use on connections which are still in use, but don’t publish those ids anymore.

Terminology-wise we might add another perspective and say that we selectively and deliberately provide tracking capability to peers which we want to stay in touch with.

Requirements

In the discussions we seem to have lost partial oversight over things. In this section we figure out the requirements for core (and possibly other components) it’s mechanisms.

TODO: what requirements exactly did we want to document here?

Use Cases and Scenarios

This section is supposed to help with the understanding of the use of elements and structures by imagined examples.

Peer ID

The peer id has been used throughout gnunet as a very convenient means for many purposes. Here we collect the intended purposes and use cases and also point out some uses for which it was not intended and point to other means to achieve it.

GNS

TODO

CADET

TODO

Messenger

TODO