About GNUnet

GNUnet in its current version is the result of over 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

GNUnet is an alternative network stack for building secure, decentralized and privacy-preserving distributed applications. Our goal is to replace the old insecure Internet protocol stack. Starting from an application for secure publication of files, it has grown to include all kinds of basic protocol components and applications towards the creation of a GNU internet.

Today, the actual use and thus the social requirements for a global network differs widely from those goals of 1970. While the Internet remains suitable for military use, where the network equipment is operated by a command hierarchy and when necessary isolated from the rest of the world, the situation is less tenable for civil society.

Due to fundamental Internet design choices, Internet traffic can be misdirected, intercepted, censored and manipulated by hostile routers on the network. And indeed, the modern Internet has evolved exactly to the point where, as Matthew Green put it, “the network is hostile”.

We believe liberal societies need a network architecture that uses the anti-authoritarian decentralized peer-to-peer paradigm and privacy-preserving cryptographic protocols. The goal of the GNUnet project is to provide a Free Software realization of this ideal.

Specifically, GNUnet tries to follow the following design principles, in order of importance:

GNUnet must be implemented as Free Software.

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.

Architecture

GNUnet consists of a set of services. In order to realize a peer-to-peer network stack, a subset of GNUnet subsystems emulate what can be found in the ISO/OSI layer of the Internet. (TODO insert image here)

Peer Identities

In GNUnet, the identity of a host is its public key called Peer Identity. 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).

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-core.

Almost all peer-to-peer communications in GNUnet are between mutually authenticated peers. 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)

Security goals and threat model

GNUnet is designed as to subsist in the face of a strong adversaries (malicous, bad actors). This includes, in decending strength, malicous

nation states,

network operators,

peer operators,

and GNUnet users.

External, network-level adversaries may attempt to identify GNUnet traffic and throttle or otherwise impair its use. To prevent easy identification, GNUnet communication is cryptographically and steganographically obfuscated. For example, GNUnet traffic can be made to look like QUIC or HTTP/3 traffic or even look like random noise on the network. Further, through the use of multiple communication protocols at the same time (e.g. QUIC and Ad-hoc WiFi), loss of a single communication method does not cause complete communication breakdown. Adversaries 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 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.

GNUnet attemtps to satisfy the following security goals in the face of those adversaries:

Censorship resistance
Confidential communication
Anonymity (where possible)

From the lowest layer to the applications layer, the securty goals and associated subsystems are:

Base layer (Communicators/TRANSPORT): This layer optionally provides steganographic and ad-hoc security guarantees against external adversaries that largely depend on the communicator(s) used. For example, use of the HTTP3/QUIC communicator will use TLS and try to validate a certificate signed by the peer we want to connect to. Other communicators may not provide the same properties.

QUIC Communicator: Appears to be a regular TLS connection (EdDSA/X25519).

TCP/UDP Communicator: Uses Diffie-Hellman with Elligator (LSD 0011) to look like random noise.

Peer connectivity and routing layer (CORE, R5N): This layer provides a secure channel between two (physically) connected peers. Peers are mutually authenticated and a secure cryptographic channel is established, but there is no particular trust required between the communication partners. It does not assume any security guarantees from the previous layer. It provides confidential communication in the face of an external adversary. The R5N uses this layer to establish an overlay network (DHT).

CORE: DTLS-style KEMTLS called CAKE with EdDSA and X25519. Specification: LSD 0012

R5N: EdDSA signatures for route recording: LSD 0004

Peer connectivity layer (CADET): This layer provides an end-to-end secure secure cryptographic channel between two peers. It is assumed that this channel is established between to peers that share a strong trust relationship.

CADET: Axolotl 3DH with EdDSA Peer Identities to provide perfect forward secrecy, post-compromise security, secure out-of-order delivery and participant repudication (deniability).

Application layer: Each subsystem of GNUnet incorporates its own security mechanism taking the existing baseline of the GNUnet network as well as the adversary model into account. See the respective section in the User handbook.

GNS: Resource records are signed using EDDSA (or EdDSA) for data origin authentication and encrypted using AES-CTR (or EdDSA+XSalsa20-Poly1305) to achieve data confidentiality against certain adversaries. Public keys are blinded to prevent censorship. Specification: LSD 0001

Cryptography

Cryptographic Inventory

GNUnet makes heavy use of standard, well-tested cryptographic primitives to implement its protocols. The primary primtives are:

Digital signatures: For Peer Identities and general (data origin) authentication. Scheme: EdDSA.
Key exchange and KEMs: For handshakes. Schemes: X25519 (with Ed25519-to-Curve25519 transformations where necessary, such as CORE).
Blindable signature keys: For the GNU Name System. Schemes: EDDSA and EdDSA.
Blind signatures: For blind signing. Primarily used by GNU Taler. Scheme: RSA-FDH.
Symmetric encryption: For secure communication. Schemes: XSalsa20-Poly1305, AES (to be phased out in favor of AEGIS where possible).
Public-key encryption: To send encrypted messages. Schemes: HPKE (RFC 9180), only DHKEM(X25519, HKDF-SHA256), HKDF-SHA256, ChaCha20-Poly1305.
Hash functions and KDF: GNUnet primarily uses SHA(-512) and HKDF.

Currently, no clear path to post-quantum primitives has been laid out. This is mostly due to open research questions in the areas of key blinding and blind signatures.

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. The current primary use for Egos are in the GNU Name System as zone keys.

Zones in the GNU Name System

Egos are used as 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 RFC 9498.

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 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)