About GNUnet#
GNUnet in its current version is the result of almost 20 years of work from many contributors. So far, most contributions were made by volunteers or people paid to do fundamental research. At this stage, GNUnet remains an experimental system where significant parts of the software lack a reasonable degree of professionalism in its implementation. Furthermore, we are aware of a significant number of existing bugs and critical design flaws, as some unfortunate early design decisions remain to be rectified. There are still known open problems; GNUnet remains an active research project.
The project was started in 2001 when some initial ideas for improving Freenet’s file-sharing turned out to be too radical to be easily realized within the scope of the existing Freenet project. We lost our first contributor on 11.9.2001 as the contributor realized that privacy may help terrorists. The rest of the team concluded that it was now even more important to fight for civil liberties. The first release was called “GNet” – already with the name GNUnet in mind, but without the blessing of GNU we did not dare to call it GNUnet immediately. A few months after the first release we contacted the GNU project, happily agreed to their governance model and became an official GNU package.
Within the first year, we created GNU libextractor, a helper library for meta data extraction which has been used by a few other projects as well. 2003 saw the emergence of pluggable transports, the ability for GNUnet to use different mechanisms for communication, starting with TCP, UDP and SMTP (support for the latter was later dropped due to a lack of maintenance). In 2005, the project first started to evolve beyond the original file-sharing application with a first simple P2P chat. In 2007, we created GNU libmicrohttpd to support a pluggable transport based on HTTP. In 2009, the architecture was radically modularized into the multi-process system that exists today. Coincidentally, the first version of the ARM service (ARM: Automatic Restart Manager) was implemented a day before systemd was announced. From 2009 to 2014 work progressed rapidly thanks to a significant research grant from the Deutsche Forschungsgesellschaft. This resulted in particular in the creation of the R5N DHT, CADET, ATS and the GNU Name System. In 2010, GNUnet was selected as the basis for the secushare online social network, resulting in a significant growth of the core team. In 2013, we launched GNU Taler to address the challenge of convenient and privacy-preserving online payments. In 2015, the pretty Easy privacy (pEp) project announced that they will use GNUnet as the technology for their meta-data protection layer, ultimately resulting in GNUnet e.V. entering into a formal long-term collaboration with the pEp Foundation. In 2016, Taler Systems SA, a first startup using GNUnet technology, was founded with support from the community.
GNUnet is not merely a technical project, but also a political mission: like the GNU project as a whole, we are writing software to achieve political goals with a focus on the human right of informational self-determination. Putting users in control of their computing has been the core driver of the GNU project. With GNUnet we are focusing on informational self-determination for collaborative computing and communication over networks.
The Internet is shaped as much by code and protocols as it is by its associated political processes (IETF, ICANN, IEEE, etc.). Similarly its flaws are not limited to the protocol design. Thus, technical excellence by itself will not suffice to create a better network. We also need to build a community that is wise, humble and has a sense of humor to achieve our goal to create a technical foundation for a society we would like to live in.
Project governance#
GNUnet, like the GNU project and many other free software projects, follows the governance model of a benevolent dictator. This means that ultimately, the GNU project appoints the GNU maintainer and can overrule decisions made by the GNUnet maintainer. Similarly, the GNUnet maintainer can overrule any decisions made by individual developers. Still, in practice neither has happened in the last 20 years for GNUnet, and we hope to keep it that way.
The current maintainers of GNUnet are:
The GNUnet project is supported by GNUnet e.V., a German association where any developer can become a member. GNUnet e.V. serves as a legal entity to hold the copyrights to GNUnet. GNUnet e.V. may also choose to pay for project resources, and can collect donations as well as choose to adjust the license of the software (with the constraint that it has to remain free software). In 2018 we switched from GPL3 to AGPL3, in practice these changes do not happen very often.
Philosophy#
The primary goal of the GNUnet project is to provide a reliable, open, non-discriminating and censorship-resistant system for information exchange. We value free speech above state interests and intellectual monopoly. GNUnet’s long-term goal is to serve as a development platform for the next generation of Internet protocols.
Participants are encouraged to contribute at least as much resources (storage, bandwidth) to the network as they consume, so that their participation does not have a negative impact on other users.
Design Principles#
These are the GNUnet design principles, in order of importance:
GNUnet must be implemented as Free Software — This means that you have the four essential freedoms: to run the program, to study and change the program in source code form, to redistribute exact copies, and to distribute modified versions. (https://www.gnu.org/philosophy/free-sw.html).
GNUnet must minimize the amount of personally identifiable information exposed.
GNUnet must be fully distributed and resilient to external attacks and rogue participants.
GNUnet must be self-organizing and not depend on administrators or centralized infrastructure.
GNUnet must inform the user which other participants have to be trusted when establishing private communications.
GNUnet must be open and permit new peers to join.
GNUnet must support a diverse range of applications and devices.
GNUnet must use compartmentalization to protect sensitive information.
The GNUnet architecture must be resource efficient.
GNUnet must provide incentives for peers to contribute more resources than they consume.
Privacy and Anonymity#
The GNUnet protocols minimize the leakage of personally identifiable information of participants and do not allow adversaries to control, track, monitor or censor users activities. The GNUnet protocols also make it as hard as possible to disrupt operations by participating in the network with malicious intent.
Analyzing participant’s activities becomes more difficult as the number of peers and applications that generate traffic on the network grows, even if the additional traffic generated is not related to anonymous communication. This is one of the reasons why GNUnet is developed as a peer-to-peer framework where many applications share the lower layers of an increasingly complex protocol stack. The GNUnet architecture encourages many different forms of peer-to-peer applications.
Practicality#
Wherever possible GNUnet allows the peer to adjust its operations and functionalities to specific use cases. A GNUnet peer running on a mobile device with limited battery for example might choose not to relay traffic for other participants.
For certain applications like file-sharing GNUnet allows participants to trade degrees of anonymity in exchange for increased efficiency. However, it is not possible for any user’s efficiency requirements to compromise the anonymity of any other user.
Key Concepts#
In this section, the fundamental concepts of GNUnet are explained. Most of them are also described in our research papers. First, some of the concepts used in the GNUnet framework are detailed. The second part describes concepts specific to anonymous file-sharing.
Authentication#
Almost all peer-to-peer communications in GNUnet are between mutually authenticated peers. The authentication works by using ECDHE, that is a DH (Diffie—Hellman) key exchange using ephemeral elliptic curve cryptography. The ephemeral ECC (Elliptic Curve Cryptography) keys are signed using ECDSA. The shared secret from ECDHE is used to create a pair of session keys (using HKDF) which are then used to encrypt the communication between the two peers using both 256-bit AES (Advanced Encryption Standard) and 256-bit Twofish (with independently derived secret keys). As only the two participating hosts know the shared secret, this authenticates each packet without requiring signatures each time. GNUnet uses SHA-512 (Secure Hash Algorithm) hash codes to verify the integrity of messages.
In GNUnet, the identity of a host is its public key. For that reason, man-in-the-middle attacks will not break the authentication or accounting goals. Essentially, for GNUnet, the IP of the host has nothing to do with the identity of the host. As the public key is the only thing that truly matters, faking an IP, a port or any other property of the underlying transport protocol is irrelevant. In fact, GNUnet peers can use multiple IPs (IPv4 and IPv6) on multiple ports — or even not use the IP protocol at all (by running directly on layer 2).
GNUnet uses a special type of message to communicate a binding between public (ECC) keys to their current network address. These messages are commonly called HELLOs or peer advertisements. They contain the public key of the peer and its current network addresses for various transport services. A transport service is a special kind of shared library that provides (possibly unreliable, out-of-order) message delivery between peers. For the UDP and TCP transport services, a network address is an IP and a port. GNUnet can also use other transports (HTTP, HTTPS, WLAN, etc.) which use various other forms of addresses. Note that any node can have many different active transport services at the same time, and each of these can have a different addresses. Binding messages expire after at most a week (the timeout can be shorter if the user configures the node appropriately). This expiration ensures that the network will eventually get rid of outdated advertisements.
For more information, refer to the following paper:
Ronaldo A. Ferreira, Christian Grothoff, and Paul Ruth. A Transport Layer Abstraction for Peer-to-Peer Networks Proceedings of the 3rd International Symposium on Cluster Computing and the Grid (GRID 2003), 2003. (https://git.gnunet.org/bibliography.git/plain/docs/transport.pdf)
Accounting to Encourage Resource Sharing#
Most distributed P2P networks suffer from a lack of defenses or precautions against attacks in the form of freeloading. While the intentions of an attacker and a freeloader are different, their effect on the network is the same; they both render it useless. Most simple attacks on networks such as Gnutella involve flooding the network with traffic, particularly with queries that are, in the worst case, multiplied by the network.
In order to ensure that freeloaders or attackers have a minimal impact on the network, GNUnet’s file-sharing implementation (FS) tries to distinguish good (contributing) nodes from malicious (freeloading) nodes. In GNUnet, every file-sharing node keeps track of the behavior of every other node it has been in contact with. Many requests (depending on the application) are transmitted with a priority (or importance) level. That priority is used to establish how important the sender believes this request is. If a peer responds to an important request, the recipient will increase its trust in the responder: the responder contributed resources. If a peer is too busy to answer all requests, it needs to prioritize. For that, peers do not take the priorities of the requests received at face value. First, they check how much they trust the sender, and depending on that amount of trust they assign the request a (possibly lower) effective priority. Then, they drop the requests with the lowest effective priority to satisfy their resource constraints. This way, GNUnet’s economic model ensures that nodes that are not currently considered to have a surplus in contributions will not be served if the network load is high.
For more information, refer to the following paper: Christian Grothoff. An Excess-Based Economic Model for Resource Allocation in Peer-to-Peer Networks. Wirtschaftsinformatik, June 2003. (https://git.gnunet.org/bibliography.git/plain/docs/ebe.pdf)
Confidentiality#
Adversaries (malicious, bad actors) outside of GNUnet are not supposed to know what kind of actions a peer is involved in. Only the specific neighbor of a peer that is the corresponding sender or recipient of a message may know its contents, and even then application protocols may place further restrictions on that knowledge. In order to ensure confidentiality, GNUnet uses link encryption, that is each message exchanged between two peers is encrypted using a pair of keys only known to these two peers. Encrypting traffic like this makes any kind of traffic analysis much harder. Naturally, for some applications, it may still be desirable if even neighbors cannot determine the concrete contents of a message. In GNUnet, this problem is addressed by the specific application-level protocols. See for example the following sections: Anonymity, see How file-sharing achieves Anonymity, and see Deniability.
Anonymity#
Providing anonymity for users is the central goal for the anonymous file-sharing application. Many other design decisions follow in the footsteps of this requirement. Anonymity is never absolute. While there are various scientific metrics (Claudia Díaz, Stefaan Seys, Joris Claessens, and Bart Preneel. Towards measuring anonymity. 2002. (https://git.gnunet.org/bibliography.git/plain/docs/article-89.pdf)) that can help quantify the level of anonymity that a given mechanism provides, there is no such thing as “complete anonymity”.
GNUnet’s file-sharing implementation allows users to select for each operation (publish, search, download) the desired level of anonymity. The metric used is based on the amount of cover traffic needed to hide the request.
While there is no clear way to relate the amount of available cover traffic to traditional scientific metrics such as the anonymity set or information leakage, it is probably the best metric available to a peer with a purely local view of the world, in that it does not rely on unreliable external information or a particular adversary model.
The default anonymity level is 1, which uses anonymous routing but imposes no minimal requirements on cover traffic. It is possible to forego anonymity when this is not required. The anonymity level of 0 allows GNUnet to use more efficient, non-anonymous routing.
How file-sharing achieves Anonymity#
Contrary to other designs, we do not believe that users achieve strong anonymity just because their requests are obfuscated by a couple of indirections. This is not sufficient if the adversary uses traffic analysis. The threat model used for anonymous file sharing in GNUnet assumes that the adversary is quite powerful. In particular, we assume that the adversary can see all the traffic on the Internet. And while we assume that the adversary can not break our encryption, we assume that the adversary has many participating nodes in the network and that it can thus see many of the node-to-node interactions since it controls some of the nodes.
The system tries to achieve anonymity based on the idea that users can be anonymous if they can hide their actions in the traffic created by other users. Hiding actions in the traffic of other users requires participating in the traffic, bringing back the traditional technique of using indirection and source rewriting. Source rewriting is required to gain anonymity since otherwise an adversary could tell if a message originated from a host by looking at the source address. If all packets look like they originate from one node, the adversary can not tell which ones originate from that node and which ones were routed. Note that in this mindset, any node can decide to break the source-rewriting paradigm without violating the protocol, as this only reduces the amount of traffic that a node can hide its own traffic in.
If we want to hide our actions in the traffic of other nodes, we must make our traffic indistinguishable from the traffic that we route for others. As our queries must have us as the receiver of the reply (otherwise they would be useless), we must put ourselves as the receiver of replies that actually go to other hosts; in other words, we must indirect replies. Unlike other systems, in anonymous file-sharing as implemented on top of GNUnet we do not have to indirect the replies if we don’t think we need more traffic to hide our own actions.
This increases the efficiency of the network as we can indirect less under higher load. Refer to the following paper for more: Krista Bennett and Christian Grothoff. GAP — practical anonymous networking. In Proceedings of Designing Privacy Enhancing Technologies, 2003. (https://git.gnunet.org/bibliography.git/plain/docs/aff.pdf)
How messaging provided Anonymity#
While the file-sharing tries to achieve anonymity through hiding actions in other traffic, the messaging service provides a weaker form of protection against identification.
The messaging service allows the use of an anonymous ego for the signing and verification process of messages instead of a unique ego. This anonymous ego is a publicly known key pair which is shared between all peers in GNUnet.
Using this ego only ensures that individual messages alone can’t identify its sender inside of a messenger room. It should be clarified that the route of the traffic for each message can still be tracked to identify the senders peer inside of a messenger room if the threat agent controls certain peers hosting the room.
Also opening a room in the messenger service will potentially match your peer identity with the internal member identity from the messenger service. So despite using the anonymous ego you can reveal your peer identity. This means to decrease the chance of being identified, it is recommended to enter rooms but you should not open them for others.
Deniability#
Even if the user that downloads data and the server that provides data are anonymous, the intermediaries may still be targets. In particular, if the intermediaries can find out which queries or which content they are processing, a strong adversary could try to force them to censor certain materials.
With the file-encoding used by GNUnet’s anonymous file-sharing, this problem does not arise. The reason is that queries and replies are transmitted in an encrypted format such that intermediaries cannot tell what the query is for or what the content is about. Mind that this is not the same encryption as the link-encryption between the nodes. GNUnet has encryption on the network layer (link encryption, confidentiality, authentication) and again on the application layer (provided by gnunet-publish, gnunet-download, gnunet-search and gnunet-fs-gtk).
Refer to the following paper for more: Christian Grothoff, Krista Grothoff, Tzvetan Horozov, and Jussi T. Lindgren. An Encoding for Censorship-Resistant Sharing. 2009. (https://git.gnunet.org/bibliography.git/plain/docs/ecrs.pdf)
Peer Identities#
Peer identities are used to identify peers in the network and are unique for each peer. The identity for a peer is simply its public key, which is generated along with a private key when the peer is started for the first time. While the identity is binary data, it is often expressed as an ASCII string. For example, the following is a peer identity as you might see it in various places:
UAT1S6PMPITLBKSJ2DGV341JI6KF7B66AC4JVCN9811NNEGQLUN0
You can find your peer identity by running gnunet-peerinfo -s.
Zones in the GNU Name System (GNS Zones)#
GNS zones are similar to those of DNS zones, but instead of a hierarchy of authorities to governing their use, GNS zones are controlled by a private key. When you create a record in a DNS zone, that information is stored in your nameserver. Anyone trying to resolve your domain then gets pointed (hopefully) by the centralised authority to your nameserver. Whereas GNS, being fully decentralized by design, stores that information in DHT. The validity of the records is assured cryptographically, by signing them with the private key of the respective zone.
Anyone trying to resolve records in a zone of your domain can then verify the signature of the records they get from the DHT and be assured that they are indeed from the respective zone. To make this work, there is a 1:1 correspondence between zones and their public-private key pairs. So when we talk about the owner of a GNS zone, that’s really the owner of the private key. And a user accessing a zone needs to somehow specify the corresponding public key first.
For more information, refer to the following paper:
Matthias Wachs, Martin Schanzenbach, and Christian Grothoff. A Censorship-Resistant, Privacy-Enhancing and Fully Decentralized Name System. In proceedings of 13th International Conference on Cryptology and Network Security (CANS 2014). 2014. https://git.gnunet.org/bibliography.git/plain/docs/gns2014wachs.pdf
Egos#
Egos are your “identities” in GNUnet. Any user can assume multiple identities, for example to separate their activities online. Egos can correspond to “pseudonyms” or “real-world identities”. Technically an ego is first of all a key pair of a public- and private-key.