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Subject: Global-Scale Distributed Storage Systems

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  • From: hal AT finney.org
  • To: bluesky AT franklin.oit.unc.edu
  • Subject: Encrypting stored data
  • Date: Fri, 23 Feb 2001 15:03:38 -0800


There seem to be two purposes for encrypting data stored in a globally
shared data space.

One is to allow for "private" data which is only accessible to one person
or a limited group. The extreme case is when the shared data space is
used for personal file backup and only the file owner can decrypt it.
A more widely shared file might be one which is used by a work group.

The other purpose is as an anti censorship measure. In this case the
decryption keys are generally expected to be widely distributed along
with the pointer to the file. The encryption is used specifically to
prevent server owners from being able to know what files they are storing.
Publius [1] does this, as does Freenet [2]. I don't think MojoNation [3]
encrypts stored data, but it does split it up using a cryptographic
technique which provides much the same effect, that a server can't tell
what data it is storing. I think the original Eternity service concept by
Ross Anderson [4] may have included this kind of server blindness as well.

This second purpose is controversial, because it relates to a specific
threat model which may not be realistic in the long term. It prevents
servers from being coerced into removing data based on structural
characteristics of the data. For example, servers might be required to
remove text files which had certain strings in them, or a server operator
might be blamed if an image file showed forbidden material.

What these systems can't prevent is orders to remove data which matches a
specific bit pattern. Even if the data is encrypted or split into pieces,
it will often still be possible for those seeking to suppress the material
to learn the encrypted or split form of the data and order it removed.

A related issue is whether server operators ought to be able to keep
certain material off their systems if they want. Freenet and Publius
intentionally prevent server operators from doing this. Other systems
may take the opposite position and want to allow operators to know what
data they are serving.

Hal

[1] http://www.cs.nyu.edu/~waldman/publius/
[2] http://freenet.sourceforge.net/
[3] http://www.mojonation.net/
[4] http://www.cl.cam.ac.uk/users/rja14/eternity/eternity.html



  • Encrypting stored data, hal, 02/23/2001

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