A Framework for Securing Software Update Systems
------------------------------------------------

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TUF (The Update Framework) helps developers secure their new or existing
software update systems. Software update systems are vulnerable to many
known attacks, including those that can result in clients being
compromised or crashed. TUF helps solve this problem by providing a
flexible security framework that can be added to software updaters.

What Is a Software Update System?
---------------------------------

Generally, a software update system is an application (or part of an
application) running on a client system that obtains and installs
software. This can include updates to software that is already installed
or even completely new software.

Three major classes of software update systems are:

-  **Application updaters** which are used by applications to update
   themselves. For example, Firefox updates itself through its own
   application updater.

-  **Library package managers** such as those offered by many
   programming languages for installing additional libraries. These are
   systems such as Python's pip/easy_install + PyPI, Perl's CPAN,
   Ruby's Gems, and PHP's PEAR.

-  **System package managers** used by operating systems to update and
   install all of the software on a client system. Debian's APT, Red
   Hat's YUM, and openSUSE's YaST are examples of these.

Our Approach
------------

There are literally thousands of different software update systems in
common use today. (In fact the average Windows user has about `two
dozen <http://secunia.com/gfx/pdf/Secunia_RSA_Software_Portfolio_Security_Exposure.pdf>`_
different software updaters on their machine!)

We are building a library that can be universally (and in most cases
transparently) used to secure software update systems.

Overview
--------

At the highest level, TUF simply provides applications with a secure
method of obtaining files and knowing when new versions of files are
available. We call these files, the ones that are supposed to be
downloaded, "target files". The most common need for these abilities is
in software update systems and that's what we had in mind when creating
TUF.

On the surface, this all sounds simple. Securely obtaining updates just
means:

-  Knowing when an update exists.
-  Downloading the updated file.

The problem is that this is only simple when there are no malicious
parties involved. If an attacker is trying to interfere with these
seemingly simple steps, there is plenty they can do.

Background
----------

Let's assume you take the approach that most systems do (at least, the
ones that even try to be secure). You download both the file you want
and a cryptographic signature of the file. You already know which key
you trust to make the signature. You check that the signature is correct
and was made by this trusted key. All seems well, right? Wrong. You are
still at risk in many ways, including:

-  An attacker keeps giving you the same file, so you never realize
   there is an update.
-  An attacker gives you an older, insecure version of a file that you
   already have, so you download that one and blindly use it thinking
   it's newer.
-  An attacker gives you a newer version of a file you have but it's not
   the newest one. It's newer to you, but it may be insecure and
   exploitable by the attacker.
-  An attacker compromises the key used to sign these files and now you
   download a malicious file that is properly signed.

These are just some of the attacks software update systems are
vulnerable to when only using signed files. See
`Security <https://github.com/theupdateframework/tuf/tree/develop/SECURITY.md>`_ for a full list of attacks and updater
weaknesses TUF is designed to prevent.

The following papers provide detailed information on securing software
updater systems, TUF's design and implementation details, attacks on
package managers, and package management security:

-  `Survivable Key Compromise in Software Update
   Systems <https://github.com/theupdateframework/tuf/tree/develop/docs/papers/survivable-key-compromise-ccs2010.pdf?raw=true>`_

-  `A Look In the Mirror: Attacks on Package
   Managers <https://github.com/theupdateframework/tuf/tree/develop/docs/papers/package-management-security-tr08-02.pdf?raw=true>`_

-  `Package Management
   Security <https://github.com/theupdateframework/tuf/tree/develop/docs/papers/attacks-on-package-managers-ccs2008.pdf?raw=true>`_

What TUF Does
-------------

In order to securely download and verify target files, TUF requires a
few extra files to exist on a repository. These are called metadata
files. TUF metadata files contain additional information, including
information about which keys are trusted, the cryptographic hashes of
files, signatures on the metadata, metadata version numbers, and the
date after which the metadata should be considered expired.

When a software update system using TUF wants to check for updates, it
asks TUF to do the work. That is, your software update system never has
to deal with this additional metadata or understand what's going on
underneath. If TUF reports back that there are updates available, your
software update system can then ask TUF to download these files. TUF
downloads them and checks them against the TUF metadata that it also
downloads from the repository. If the downloaded target files are
trustworthy, TUF hands them over to your software update system. See
`Metadata <https://github.com/theupdateframework/tuf/tree/develop/METADATA.md>`_ for more information and examples.

TUF specification document is also available:

-  `The Update Framework Specification <https://github.com/theupdateframework/tuf/tree/develop/docs/tuf-spec.txt?raw=true>`_

TUF Home Page
-------------

The home page for the TUF project can be found at:
https://updateframework.com

Mailing List
------------
Please visit `https://groups.google.com/forum/?fromgroups#!forum/theupdateframework <https://groups.google.com/forum/?fromgroups#!forum/theupdateframework>`_ if you would like to contact the TUF team.  Questions, feedback, and suggestions are welcomed in this low-volume mailing list.

A group feed is available at: https://groups.google.com/forum/feed/theupdateframework/msgs/atom.xml?num=50


Installation
------------

::

    pip - installing and managing Python packages (recommended)

    Installing from Python Package Index (https://pypi.python.org/pypi).
    Note: Please use "pip install --no-use-wheel tuf" if your version
    of pip <= 1.5.6
    $ pip install tuf

    Installing from local source archive.
    $ pip install <path to archive>

    Or from the root directory of the unpacked archive.
    $ pip install . 

Installation of Optional Requirements (after minimal install)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The optional ``tuf[tools]`` can be installed by repository maintainers
that need to generate TUF repository files, such as metadata,
cryptographic keys, and signatures. Whereas the minimal install can only
verify ed25519 signatures and is intended for sofware updater clients,
``tuf[tools]`` provides repository maintainers secure ed25519 key and
signature generation with PyNaCl / libsodium.

TUF tools also enable general-purpose cryptography with PyCrypto.
Software updaters that want to support verification of RSASSA-PSS
signatures should require their clients to install ``tuf[tools]``.

Installing extras does not work if minimal install was a wheel (pip <= 1.5.6.)
`https://github.com/pypa/pip/issues/1885 <https://github.com/pypa/pip/issues/1885>`_

::

    $ pip install --no-use-wheel tuf
    $ pip install tuf[tools]

Instructions for Contributors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Development: `https://github.com/theupdateframework/tuf <https://github.com/theupdateframework/tuf>`_

`Virtualenv <https://virtualenv.pypa.io/en/latest/index.html>`_
is a tool to create isolated Python environments. It also includes
``pip`` and ``setuptools``, Python packages used to install TUF and its
dependencies. All installation methods of virtualenv are outlined in the
`installation
section <https://virtualenv.pypa.io/en/latest/installation.html>`_
and instructions for installing locally from source here:
::

    $ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-1.11.6.tar.gz
    $ tar xvfz virtualenv-1.11.6.tar.gz
    $ cd virtualenv-1.11.6
    $ python virtualenv.py myVE


PyCrypto and PyNaCl (third-party dependencies needed by the repository
tools) require Python and FFI (Foreign Function Interface) development
header files. Debian-based distributions can install these header
libraries with apt (Advanced Package Tool.)
::

    $ apt-get install python-dev
    $ apt-get install libffi-dev

Installation of minimal, optional, development, and testing requirements
can then be accomplished with one command:
::

    $ pip install -r dev-requirements.txt

The Update Framework's unit tests can be executed by invoking
`tox <https://testrun.org/tox/>`_. All supported Python versions are
tested, but must already be installed locally.
::

    $ tox

Using TUF
---------

TUF has four major classes of users: clients, for whom TUF is largely
transparent; mirrors, who will (in most cases) have nothing at all to do
with TUF; upstream servers, who will largely be responsible for care and
feeding of repositories; and integrators, who do the work of putting TUF
into existing projects.

A low-level integration requires importing a single module and calling
particular methods to perform updates.  A high-level integration, on the
other hand, can handle TUF-related updates transparently.  The client
populates a configuration file and the library interposes on urllib calls.
Generating metadata files stored on upstream servers can be handled by the
repository tool, covered in ``Creating a Repository``.


-  `Creating a Repository <https://github.com/theupdateframework/tuf/tree/develop/tuf/README.md>`_

-  `Low-level Integration <https://github.com/theupdateframework/tuf/tree/develop/tuf/client/README.md>`_

-  `High-level Integration <https://github.com/theupdateframework/tuf/tree/develop/tuf/interposition/README.md>`_

Acknowledgements
----------------

This material is based upon work supported by the National Science
Foundation under Grant No. CNS-1345049 and CNS-0959138. Any opinions,
findings, and conclusions or recommendations expressed in this material
are those of the author(s) and do not necessarily reflect the views of
the National Science Foundation.