What Is It?
-----------

A file's capability is derived from two pieces of information on
upload: The content of the file and the upload node's "convergence
secret". By default, the convergence secret is randomly generated by the
node when it first starts up, then stored in the node's base directory
(<Tahoe's node dir>/private/convergence) and re-used after that. So the
same file content uploaded from the same node will always have the same
cap string. Uploading the file from a different node with a different
convergence secret would result in a different cap string—and in a
second copy of the file's contents stored on the grid. If you want files
you upload to converge (also known as "deduplicate") with files uploaded
by someone else, just make sure you're using the same convergence secret
as they are. 

The advantages of deduplication should be clear, but keep in mind that
the convergence secret was created to combat snooping. For example, with
a convergence secret and a file, I can generate a file capability that
would allow me to determine that you have uploaded a file to the grid.
This isn't a big deal for things that are personally produced, but is a
big deal for commonly known things that you want to keep secret that you
know about. Examples would be a Bible in China or The Anarchist's
Cookbook at the High School that I went to. Here's the details of the
origin of the convergence secret: `Convergence Secret Justification`_

.. _`Convergence Secret Justification`:
   https://tahoe-lafs.org/hacktahoelafs/drew_perttula.html

What If I Change My Convergence Secret?
---------------------------------------

All your old file capabilities will still work, but the new data that
you upload will not be deduplicated with the old data. If you upload all
of the same things to the grid, you will end up using twice the space
until garbage collection kicks in, if it's enabled. As one user put it,
it would move the node where you change the convergence secret into a
new deduplication domain.

How To Use It
-------------

To enable deduplication between different clients, **securely** copy
the convergence secret file from one client to all the others.

For example, if you are on host A and have an account on host B and you
have scp installed, run: 

  *scp ~/.tahoe/private/convergence
  my_other_account@B:.tahoe/private/convergence*

If you have two different nodes on a single computer, say one for each
disk, you would do: 

  *cp /tahoe1/private/convergence /tahoe2/private/convergence*