tahoe-lafs/docs/about.rst

======================
Welcome to Tahoe-LAFS!
======================

Welcome to `Tahoe-LAFS <http://tahoe-lafs.org>`_, the first 
decentralized storage system with *provider-independent security*.

What is "provider-independent security"?
========================================

Every seller of cloud storage services will tell you that their service 
is "secure".  But what they mean by that is something fundamentally 
different from what we mean.  What they mean by "secure" is that after 
you've given them the power to read and modify your data, they try 
really hard not to let this power be abused.  This turns out to be 
difficult!  Bugs, misconfigurations, or operator error can accidentally 
expose your data to another customer or to the public, or can corrupt 
your data.  Criminals routinely gain illicit access to corporate 
servers.  Even more insidious is the fact that the employees themselves 
sometimes violate customer privacy out of carelessness, avarice, or 
mere curiousity.  The most conscientious of these service providers 
spend considerable effort and expense trying to mitigate these risks.

What we mean by "security" is something different.  *The service 
provider never has the ability to read or modify your data in the first 
place -- never.*  If you use Tahoe-LAFS, then all of the threats 
described above are non-issues to you.  Not only is it easy and 
inexpensive for the service provider to maintain the security of your 
data, but in fact they couldn't violate its security if they tried.  
This is what we call *provider-independent security*.

This guarantee is integrated naturally into the Tahoe-LAFS storage 
system and doesn't require you to perform a manual pre-encryption step 
or cumbersome key management.  (After all, having to do cumbersome 
manual operations when storing or accessing your data would nullify one 
of the primary benefits of using cloud storage in the first place -- 
convenience.)

Here's how it works:

.. image:: http://tahoe-lafs.org/~zooko/network-and-reliance-topology.png

A "storage grid" is made up of a number of storage servers.  A storage 
server has direct attached storage (typically one or more hard disks).  
A "gateway" uses the storage servers and provides access to the 
filesystem over HTTP(S) or (S)FTP.

Users do not rely on storage servers to provide *confidentiality* nor 
*integrity* for their data -- instead all of the data is encrypted and 
integrity-checked by the gateway, so that the servers can neither read 
nor modify the contents of the files.

Users do rely on storage servers for *availability*.  The ciphertext is 
erasure-coded and distributed across ``N`` storage servers (the default 
value for ``N`` is 10) so that it can be recovered from any ``K`` of 
these servers (the default value of ``K`` is 3).  Therefore only the 
simultaneous failure of ``N-K+1`` (with the defaults, 8) servers can 
make the data unavailable.

In the typical deployment mode each user runs her own gateway on her 
own machine.  This way she relies on her own machine for the 
confidentiality and integrity of the data.

An alternate deployment mode is that the gateway runs on a remote 
machine and the user connects to it over HTTPS or SFTP.  This means 
that the operator of the gateway can view and modify the user's data 
(the user *relies on* the gateway for confidentiality and integrity), 
but the advantage is that the user can access the filesystem with a 
client that doesn't have the gateway software installed, such as an 
Internet kiosk or cell phone.

Access Control
==============

There are two kinds of files: immutable and mutable.  Immutable files 
have the property that once they have been uploaded to the storage grid 
they can't be modified.  Mutable ones can be modified.  A user can have 
read-write access to a mutable file or read-only access to it (or no 
access to it at all).

A user who has read-write access to a mutable file or directory can 
give another user read-write access to that file or directory, or they 
can give read-only access to that file or directory.  A user who has 
read-only access to a file or directory can give another user read-only 
access to it.

When linking a file or directory into a parent directory, you can use a 
read-write link or a read-only link.  If you use a read-write link, 
then anyone who has read-write access to the parent directory can gain 
read-write access to the child, and anyone who has read-only access to 
the parent directory can gain read-only access to the child.  If you 
use a read-only link, then anyone who has either read-write or 
read-only access to the parent directory can gain read-only access to 
the child.

For more technical detail, please see the `the doc page 
<http://tahoe-lafs.org/trac/tahoe-lafs/wiki/Doc>`_ on the Wiki.

Get Started
===========

To use Tahoe-LAFS, please see `quickstart.rst <quickstart.rst>`_.

License
=======

You may use this package under the GNU General Public License, version 
2 or, at your option, any later version.  See the file `COPYING.GPL 
<../COPYING.GPL>`_ for the terms of the GNU General Public License, 
version 2.

You may use this package under the Transitive Grace Period Public 
Licence, version 1 or, at your option, any later version.  The 
Transitive Grace Period Public Licence has requirements similar to the 
GPL except that it allows you to wait for up to twelve months after you 
redistribute a derived work before releasing the source code of your 
derived work. See the file `COPYING.TGGPL <../COPYING.TGPPL.html>`_ for 
the terms of the Transitive Grace Period Public Licence, version 1.

(You may choose to use this package under the terms of either licence, 
at your option.)