Changes between Version 4 and Version 5 of Convergence Secret


Ignore:
Timestamp:
2013-04-23T17:19:30Z (11 years ago)
Author:
zooko
Comment:

moved

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  • Convergence Secret

    v4 v5  
    1 {{{
    2 #!rst
    3 What Is It?
    4 -----------
    5 
    6 A file's capability is derived from two pieces of information on
    7 upload: The content of the file and the upload node's "convergence
    8 secret". By default, the convergence secret is randomly generated by the
    9 node when it first starts up, then stored in the node's base directory
    10 (<Tahoe's node dir>/private/convergence) and re-used after that. So the
    11 same file content uploaded from the same node will always have the same
    12 cap string. Uploading the file from a different node with a different
    13 convergence secret would result in a different cap string—and in a
    14 second copy of the file's contents stored on the grid. If you want files
    15 you upload to converge (also known as "deduplicate") with files uploaded
    16 by someone else, just make sure you're using the same convergence secret
    17 as they are.
    18 
    19 The advantages of deduplication should be clear, but keep in mind that
    20 the convergence secret was created to combat snooping. For example, with
    21 a convergence secret and a file, I can generate a file capability that
    22 would allow me to determine that you have uploaded a file to the grid.
    23 This isn't a big deal for things that are personally produced, but is a
    24 big deal for commonly known things that you want to keep secret that you
    25 know about. Examples would be a Bible in China or The Anarchist's
    26 Cookbook at the High School that I went to.  This is called the
    27 "Confirm-The-File" attack.
    28 
    29 There is one more way that this can be used.  Let's say that you know
    30 somebody stores their W-2 (a US tax document) on their Tahoe-LAFS grid.
    31 You can, with a little script, try various numbers for identity information
    32 and eventually end up with your full identity and salary.  This is called
    33 the "Learn-The-Remaining-Information" attack.
    34 
    35 Here's the details of the origin of the convergence secret:
    36 `Convergence Secret Justification`_
    37 
    38 .. _`Convergence Secret Justification`:
    39    https://tahoe-lafs.org/hacktahoelafs/drew_perttula.html
    40 
    41 What If I Change My Convergence Secret?
    42 ---------------------------------------
    43 
    44 All your old file capabilities will still work, but the new data that
    45 you upload will not be deduplicated with the old data. If you upload all
    46 of the same things to the grid, you will end up using twice the space
    47 until garbage collection kicks in, if it's enabled. As one user put it,
    48 it would move the node where you change the convergence secret into a
    49 new deduplication domain.
    50 
    51 How To Use It
    52 -------------
    53 
    54 To enable deduplication between different clients, **securely** copy
    55 the convergence secret file from one client to all the others.
    56 
    57 For example, if you are on host A and have an account on host B and you
    58 have scp installed, run:
    59 
    60   *scp ~/.tahoe/private/convergence
    61   my_other_account@B:.tahoe/private/convergence*
    62 
    63 If you have two different nodes on a single computer, say one for each
    64 disk, you would do:
    65 
    66   *cp /tahoe1/private/convergence /tahoe2/private/convergence*
    67 }}}
     1moved to [source:git/docs/convergence-secret.rst]