###################
Security Principles
###################

Security is one of the primary project goals. This means that it should
not be possible for an attacker to join a cluster uninvited, and it
should not be possible to extract private information from intercepted
traffic. Currently this is implemented as follows.

All traffic is protected by TLS. To prevent uninvited nodes from joining
a cluster, the certificate fingerprint of each node is compared to a
preset list of acceptable nodes at connection establishment. The
fingerprint is computed as the SHA-256 hash of the certificate and
displayed in BASE32 encoding to form a reasonably compact and convenient
string.

Incoming requests for file data are verified to the extent that the
requested file name must exist in the local index and the global model.

For information about ensuring you are running the code you think you
are and for reporting security vulnerabilities, please see the official
`security page <http://syncthing.net/security.html>`__.

Information Leakage
===================

Global Discovery
----------------

When global discovery is enabled, Syncthing sends an announcement packet
every 30 minutes to the global discovery server, so that it can keep a
mapping between your device ID and external IP. Also, when connecting to
other devices that have not been seen on the local network, a query is
sent to the global discovery server containing the device ID of the
requested device. The discovery server is currently **hosted by
@calmh**. Global discovery defaults to **on**.

When turned off, devices with dynamic addresses not on the local network
cannot be found and connected to.

If a different global discovery server is configured, no data is sent to
the default global discovery server.

Local Discovery
---------------

When local discovery is enabled, Syncthing sends broadcast (IPv4) and
multicast (IPv6) packets to the local network every 30 seconds. The
packets contain the device ID and listening port. Local discovery
defaults to **on**.

An eavesdropper on the local network can deduce which machines are
running Syncthing with local discovery enabled, and what their device
IDs are.

When turned off, devices with dynamic addresses on the local network
cannot be found and connected to.

Upgrade Checks
--------------

When automatic upgrades are enabled, Syncthing checks for a new version
at startup and then once every twelve hours. This is by an HTTPS request
to the download site for releases, currently **hosted at GitHub**.
Automatic upgrades default to **on** (unless Syncthing was compiled with
upgrades disabled).

Even when automatic upgrades are disabled in the configuration, an
upgrade check as above is done when the GUI is loaded, in order to show
the "Upgrade to ..." button when necessary. This can be disabled only by
compiling syncthing with upgrades disabled.

In effect this exposes the majority of the Syncthing population to
tracking by the operator of the download site (currently GitHub). That
data is not available to outside parties (including @calmh etc), except
that download counts per release binary are available in the GitHub API.
The upgrade check (or download) requests *do not* contain any
identifiable information about the user, device, Syncthing version, etc.

Usage Reporting
---------------

When usage reporting is enabled, Syncthing reports usage data at startup
and then every 24 hours. The report is sent as an HTTPS POST to the
usage reporting server, currently **hosted by @calmh**. The contents of
the usage report can be seen behind the "Preview" link in settings.
Usage reporting defaults to **off** but the GUI will ask once about
enabling it, shortly after the first install.

The reported data is protected from eavesdroppers, but the connection to
the usage reporting server itself may expose the client as running
Syncthing.

Sync Connections (BEP)
----------------------

Sync connections are attempted to all configured devices, when the
address is possible to resolve. The sync connection is based on TLS 1.2.
The TLS certificates are sent in clear text (as in HTTPS etc), meaning
that the certificate Common Name (by default ``syncthing``) is visible.

An eavesdropper can deduce that this is a Syncthing connection and
calculate the device ID:s involved based on the hashes of the sent
certificates.

Likewise, if the sync port (default 22000) is accessible from the
internet, a port scanner may discover it, attempt a TLS negotiation and
thus obtain the device certificate. This provides the same information
as in the eavesdropper case.

Web GUI
-------

If the web GUI is accessible, it exposes the device as running
Syncthing. The web GUI defaults to being reachable from the **local host
only**.

In Short
========

Parties doing surveillance on your network (whether that be corporate
IT, the NSA or someone else) will be able to see that you use Syncthing,
and your device ID's `are OK to share
anyway <http://docs.syncthing.net/users/faq.html#should-i-keep-my-device-ids-secret>`__,
but the actual transmitted data is protected as well as we can. Knowing
your device ID can expose your IP address, using global discovery.
