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  • From: Ted Anderson <TedAnderson AT mindspring.com>
  • To: Global-Scale Distributed Storage Systems <bluesky AT franklin.oit.unc.edu>
  • Subject: Tangler - a summary and comments
  • Date: Mon, 07 Oct 2002 21:44:03 -0400


Thanks to Wei Dai for the pointer to the IRIS project. While reading the IRIS summary[0] I took a look at Tangler. Here is a short description and a few comments I made while reading it.

"Tangler: A Censorship-Resistant Publishing System Based On Document
Entanglements" by Marc Waldman and David Mazières dated December 8,
2001. This system contains an interesting combination of features that
could make a very useful publishing paradigm. The name comes from the
idea of using Shamir secret sharing to entangle each data block being
with two other randomly selected blocks from the storage pool. They
propose using 3 of 4 sharing so that each data block is represented by 4
server blocks, any three of which are needed to reconstruct the original
data. Each block of data appears completely random in isolation.
Server blocks are indexed by the SHA-1 hash of their contents. Each
data block is then identified by a set of four SHA-1 hash values. Each
file consists of a data block similar to an inode consisting of the 4
hashes that identify each data block in the file. A collection consists
of a tree of such files and directories assembled recursively using this
entanglement process. The collection root is signed and labeled with
the publisher's public key.

A similar scheme described by David Madore[2], called Random Pads[3],
involves XORing multiple large blocks of data with several existing
random pads and storing the result as another random pad. This approach
is considerable cheaper than Shamir secret sharing, the main difference
being that all random pads must be located to reconstruct the original.
Because of this there is no threshold of tolerable loss, so this faster
method is considerably more fragile. My recollection is that I saw this
suggested on the Freenet mailing list. Mojonation and other systems
also use n-of-m sharing, but I don't know if the performance or other
characteristics are similar to that used by Tangler.

An interesting consequence of this entanglement is that each publisher
has an interest in preservation of the blocks needed to reconstruct the
content of other publishers. A file cannot be removed from the system
without also removing blocks needed by other files. This furthers the
cause of censor-resistance.

Unlike other peer to peer systems with very many nodes, Tangler is
designed to operate with a modest number of server block storage nodes
each of which knows of the others. The storage network uses credits and
receipts to validate the behavior of the servers. A server's operation
is implicitly audited during ordinary use and it can be ejected from the
system for non-performance. I must say, however, that I found the
paper's description of the server algorithm difficult to understand.

Ted Anderson

[0] http://iris.lcs.mit.edu/proposal.html
[1] http://citeseer.nj.nec.com/waldman01tangler.html
[2] http://www.eleves.ens.fr:8080/home/madore/
[3] http://reactor-core.org/security/random-pads.html




  • Tangler - a summary and comments, Ted Anderson, 10/07/2002

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