
@cindex Key Concepts
@node Key Concepts
@chapter Key Concepts

In this section, the fundamental concepts of GNUnet are explained.
@c FIXME: Use @uref{https://docs.gnunet.org/bib/, research papers}
@c once we have the new bibliography + subdomain setup.
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.

@menu
* Authentication::
* Accounting to Encourage Resource Sharing::
* Confidentiality::
* Anonymity::
* Deniability::                       
* Peer Identities::
* Zones in the GNU Name System (GNS Zones)::
* Egos::
@end menu

@cindex Authentication
@node Authentication
@section 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 (@uref{http://en.wikipedia.org/wiki/ECDSA, ECDSA}).
The shared secret from ECDHE is used to create a pair of session keys
@c FIXME: Long word for HKDF. More FIXMEs: Explain MITM etc.
(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.

@c FIXME: A while back I got the feedback that I should try and integrate
@c explanation boxes in the long-run. So we could explain
@c "man-in-the-middle" and "man-in-the-middle attacks" and other words
@c which are not common knowledge. MITM is not common knowledge. To be
@c selfcontained, we should be able to explain words and concepts used in
@c a chapter or paragraph without hinting at Wikipedia and other online
@c sources which might not be available or accessible to everyone.
@c On the other hand we could write an introductionary chapter or book
@c that we could then reference in each chapter, which sound like it
@c could be more reusable.
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).
@c FIXME: "IP protocol" feels wrong, but could be what people expect, as
@c IP is "the number" and "IP protocol" the protocol itself in general
@c knowledge?

@c NOTE: For consistency we will use @code{HELLO}s throughout this Manual.
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 @code{HELLO}s or @code{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.
(@uref{https://gnunet.org/git/bibliography.git/plain/docs/transport.pdf, https://gnunet.org/git/bibliography.git/plain/docs/transport.pdf})

@cindex Accounting to Encourage Resource Sharing
@node Accounting to Encourage Resource Sharing
@section 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 @command{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 (@code{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.
(@uref{https://gnunet.org/git/bibliography.git/plain/docs/ebe.pdf, https://gnunet.org/git/bibliography.git/plain/docs/ebe.pdf})

@cindex Confidentiality
@node Confidentiality
@section 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 @pxref{Anonymity}, @pxref{How file-sharing achieves Anonymity},
and @pxref{Deniability}.

@cindex Anonymity
@node Anonymity
@section Anonymity

@menu
* How file-sharing achieves Anonymity::
@end menu

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.
(@uref{https://gnunet.org/git/bibliography.git/plain/docs/article-89.pdf, https://gnunet.org/git/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 the amount of cover traffic available to hide the
request.
While this metric is not as good as, for example, the theoretical metric
given in scientific metrics,
it is probably the best metric available to a peer with a purely local
view of the world that does not rely on unreliable external information.
The default anonymity level is @code{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
@code{0} allows GNUnet to use more efficient, non-anonymous routing.

@cindex How file-sharing achieves Anonymity
@node How file-sharing achieves Anonymity
@subsection 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.
(@uref{https://gnunet.org/git/bibliography.git/plain/docs/aff.pdf, https://gnunet.org/git/bibliography.git/plain/docs/aff.pdf})

@cindex Deniability
@node Deniability
@section 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 @command{gnunet-publish}, @command{gnunet-download},
@command{gnunet-search} and @command{gnunet-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.
(@uref{https://gnunet.org/git/bibliography.git/plain/docs/ecrs.pdf, https://gnunet.org/git/bibliography.git/plain/docs/ecrs.pdf})

@cindex Peer Identities
@node Peer Identities
@section 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 the peer is started for the first time.
While the identity is binary data, it is often expressed as ASCII string.
For example, the following is a peer identity as you might see it in
various places:

@example
UAT1S6PMPITLBKSJ2DGV341JI6KF7B66AC4JVCN9811NNEGQLUN0
@end example

@noindent
You can find your peer identity by running @command{gnunet-peerinfo -s}.

@cindex Zones in the GNU Name System (GNS Zones)
@node Zones in the GNU Name System (GNS Zones)
@section Zones in the GNU Name System (GNS Zones)

@c FIXME: Explain or link to an explanation of the concept of public keys
@c and private keys.
@c FIXME: Rewrite for the latest GNS changes.
GNS (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.
@uref{https://gnunet.org/git/bibliography.git/plain/docs/gns2014wachs.pdf, https://gnunet.org/git/bibliography.git/plain/docs/gns2014wachs.pdf})
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.

@cindex Egos
@node Egos
@section Egos

@c what is the difference between peer identity and egos? It seems
@c like both are linked to public-private key pair.
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.
